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Notes on Meteorology Kemp & Young

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T T E R W O R T H I Ν Ε Μ A Ν Ν


B u t t e r w o r t h - H e i n e m a n n Ltd H a l l e y C o u r t , J o r d a n Hill, O x f o r d O X 2 8EJ

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PART OF REED I N T E R N A T I O N A L B O O K S

OXFORD MUNICH

LONDON NEW DELHI

TOKYO

GUILDFORD SINGAPORE

TORONTO

BOSTON SYDNEY

WELLINGTON

First p u b l i s h e d by Stanford M a r i t i m e Ltd 1961 S e c o n d edition 1966 T h i r d e d i t i o n 1971 R e p r i n t e d 1972,1974,1977,1980, 1984,1987 First p u b l i s h e d by B u t t e r w o r t h - H e i n e m a n n Ltd 1989 R e p r i n t e d 1990 © P . Y o u n g 1971 All rights reserved. No part of this publication may be reproduced in any material form (including photocopying or storing in any medium by electronic means and whether or not transiently or incidentally to some other use of this publication) without the written permission of the copyright holder except in accordance with the provisions of the Copyright, Designs and Patents Act 1988 or under the terms of a licence issued by the Copyright Licensing Agency Ltd, 33-34 Alfred Place, London, England WCIE 7DP. Applications for the copyright holder's written permission to reproduce any part of this publication should be addressed to the publishers I S B N 0 7506 0100 0

P r i n t e d a n d b o u n d in G r e a t Britain by Biddies Ltd, Guildford a n d K i n g ' s L y n n


PREFACE TO FIRST EDITION Weather c o n d i t i o n s are ever changing and, in order t o m a k e t h e best use of favour­ able c o n d i t i o n s or t o c o u n t e r unfavourable ones successfully, t h e seafarer should have some theoretical as well as practical k n o w l e d g e of m e t e o r o l o g y . Many of t h e theories of w e a t h e r p h e n o m e n a are revised from t i m e t o t i m e a n d , so as t o keep t h e t e x t concise, we have described t h e m o r e c o m m o n of these theories as simply as possible. A large n u m b e r of diagrams have b e e n i n c l u d e d t o s u p p l e m e n t t h e text a n d t o enable t h e student to m a k e his o w n forecast of t h e likely w e a t h e r in t h e various situations. Although written primarily t o cover t h e needs of t h o s e s t u d y i n g for their e x a m ­ inations, all interested in m e t e o r o l o g y , particularly y a c h t s m e n , will find t h e b o o k of great value. Our t h a n k s are due t o Mr. P.N. Colepeper, F . R . M e t . S . , Master Mariner and First Class Air Navigator, for his painstaking reading of the t e x t and for his helpful suggestions. We are i n d e b t e d t o t h e C o n t r o l l e r of Her Majesty's S t a t i o n e r y Office for permission t o r e p r o d u c e certain drawings of i n s t r u m e n t s from t h e A d m i r a l t y Manual of Seamanship and the m a p of t h e weather forecast areas supplied by t h e Meteorological Office. KENLEY. SURREY May 1961

J . F . KEMP PETER YOUNG

PREFACE TO REVISED EDITION The changeover t o metrication with SI u n i t s has necessitated several alterations in the numerical q u a n t i t i e s in this b o o k b u t , wherever a p p r o p r i a t e , b o t h SI a n d Imperial values are given. It should be n o t e d , however, t h a t t h e imperial u n i t s are n o t necessarily an exact conversion from SI units. T h e o p p o r t u n i t y has also been t a k e n t o revise t h e t e x t and i n t r o d u c e n o t e s on further topics such as facsimile p l o t t i n g a n d w e a t h e r r o u t e i n g so t h a t t h e b o o k m a y c o n t i n u e t o fulfil its original p u r p o s e of providing a basic t e x t on m e t e o r o l o g y for e x a m ­ ination candidates, y a c h t s m e n and all interested in t h e subject.of w e a t h e r . Our t h a n k s are d u e t o t h o s e w h o have read a n d criticised t h e revised t e x t a n d t o Negretti and Z a m b r a (Aviation) Limited, w h o have supplied illustrations.

January 1971

J P - KEMP P E J E R YOUNG


PREFACE TO FIRST EDITION Weather c o n d i t i o n s are ever changing and, in order t o m a k e t h e best use of favour­ able c o n d i t i o n s or t o c o u n t e r unfavourable ones successfully, t h e seafarer should have some theoretical as well as practical k n o w l e d g e of m e t e o r o l o g y . Many of t h e theories of w e a t h e r p h e n o m e n a are revised from t i m e t o t i m e a n d , so as t o keep t h e t e x t concise, we have described t h e m o r e c o m m o n of these theories as simply as possible. A large n u m b e r of diagrams have b e e n i n c l u d e d t o s u p p l e m e n t t h e text a n d t o enable t h e student to m a k e his o w n forecast of t h e likely w e a t h e r in t h e various situations. Although written primarily t o cover t h e needs of t h o s e s t u d y i n g for their e x a m ­ inations, all interested in m e t e o r o l o g y , particularly y a c h t s m e n , will find t h e b o o k of great value. Our t h a n k s are due t o Mr. P.N. Colepeper, F . R . M e t . S . , Master Mariner and First Class Air Navigator, for his painstaking reading of the t e x t and for his helpful suggestions. We are i n d e b t e d t o t h e C o n t r o l l e r of Her Majesty's S t a t i o n e r y Office for permission t o r e p r o d u c e certain drawings of i n s t r u m e n t s from t h e A d m i r a l t y Manual of Seamanship and the m a p of t h e weather forecast areas supplied by t h e Meteorological Office. KENLEY. SURREY May 1961

J . F . KEMP PETER YOUNG

PREFACE TO REVISED EDITION The changeover t o metrication with SI u n i t s has necessitated several alterations in the numerical q u a n t i t i e s in this b o o k b u t , wherever a p p r o p r i a t e , b o t h SI a n d Imperial values are given. It should be n o t e d , however, t h a t t h e imperial u n i t s are n o t necessarily an exact conversion from SI units. T h e o p p o r t u n i t y has also been t a k e n t o revise t h e t e x t and i n t r o d u c e n o t e s on further topics such as facsimile p l o t t i n g a n d w e a t h e r r o u t e i n g so t h a t t h e b o o k m a y c o n t i n u e t o fulfil its original p u r p o s e of providing a basic t e x t on m e t e o r o l o g y for e x a m ­ ination candidates, y a c h t s m e n and all interested in t h e subject.of w e a t h e r . Our t h a n k s are d u e t o t h o s e w h o have read a n d criticised t h e revised t e x t a n d t o Negretti and Z a m b r a (Aviation) Limited, w h o have supplied illustrations.

January 1971

J P - KEMP P E J E R YOUNG


I Instruments In o r d e r t o get as c o m p l e t e a p i c t u r e as possible of t h e w e a t h e r , careful observa足 tions should b e m a d e of t h e individual p h e n o m e n a which go t o m a k e up t h e w e a t h e r . Many of t h e s e observations are m a d e visually; for e x a m p l e , t h e form of clouds, and direction of t h e wind. O t h e r s m u s t b e m a d e by i n s t r u m e n t s ; for instance o n e c a n n o t *fecr t h e pressure or t h e relative h u m i d i t y a l t h o u g h o n e m a y hazard a guess at t h e air temperature. Various i n s t r u m e n t s have been designed t o observe t h e different p h e n o m e n a . The principal o n e s measure pressure, t e m p e r a t u r e and wind velocity, whilst o t h e r s have been designed t o measure sunshine h o u r s and rainfall. T h e r e is n o necessity for a ship t o carry every i n s t r u m e n t , as a b a r o m e t e r and a hygrometer t o g e t h e r with t h e b r o a d c a s t w e a t h e r i n f o r m a t i o n will enable o n e t o m a k e an accurate forecast for the n e x t few h o u r s . T h e Meteorological Office recognises this and usually supplies a precision aneroid or a mercurial b a r o m e t e r , a b a r o g r a p h , a sea t h e r m 足 ometer and a h y g r o m e t e r t o observing ships. P R E S S U R E A N D ITS M E A S U R E M E N T Pressure is force per u n i t area. T h e u n i t of pressure in t h e SI system is t h e BAR which is a p p r o x i m a t e l y equivalent t o a load of 10 t o n n e s weight per square m e t r e (10tf/m2). The average pressure at g r o u n d level is slightly in excess of 1 Bar and t o avoid t h e necessity of having large n u m b e r s of figures after t h e decimal p o i n t in order t o express pressure accurately, t h e bar is divided into 1 0 0 0 p a r t s , each of these p a r t s is a M I L L I B A R , and pressure is expressed in these units. For m a n y years pressure was also expressed in millibars u n d e r t h e Imperial system although it was also, and still m a y b e , expressed as "inches of m e r c u r y " . This is the length of a c o l u m n of m e r c u r y which will balance a c o l u m n of air. As t h e c o l u m n of air exerts a pressure of a p p r o x i m a t e l y 14.7 lbs p e r square inch t h e height of t h e c o l u m n of m e r c u r y (relative density 13.6) necessary t o e x e r t this pressure is a b o u t 30 ins. ISOBAR

A line joining places having equal pressure. O n w e a t h e r c h a r t s these are p l o t t e d at 4 millibar intervals, so t h a t t h e isobars s h o w n are divisible by 4.

ISALLOBAR.

A line j o i n i n g places having an equal change of pressure. A study of these can give an indication of t h e direction of m o v e m e n t of pressure systems.

P R E S S U R E G R A D I E N T . This is t h e difference in pressure in u n i t distance m e a s u r e d at right angles t o t h e isobars. There is a D I U R N A L R A N G E of b a r o m e t r i c pressure which results in t h e baro足 metric pressure being higher t h a n n o r m a l at 1 0 0 0 a n d 2 2 0 0 local t i m e and lower t h a n normal at 0 4 0 0 and 1 6 0 0 local t i m e . T h e semi-diurnal pressure wave is d u e t o t h e at足 mospheric tides which are caused by t h e sun and m o o n . It is possible t h a t t h e r e are other causes as this semi-diurnal change of pressure is still being investigated.


4

N O T E S ON M E T E O R O L O G Y

T h e diurnal range is m o s t m a r k e d in t h e tropics where t h e b a r o m e t e r is frequent­ ly 1.5 millibars above or below normal at t h e times m e n t i o n e d above. It is less notice­ able in higher latitudes w h e r e frequent pressure changes occur d u e t o t h e passage of depressions. However, w h e n t h e pressure gradient is c o n s t a n t and small, this daily range m a y be seen clearly o n a barograph trace although t h e variation is very m u c h less t h a n in low latitudes. MERCURIAL BAROMETER This is c o n s t r u c t e d by filling a t u b e , a b o u t 1 m e t r e ( 3 6 ins.) long with m e r c u r y . T h e e n d of t h e t u b e is temporarily closed and is inverted and placed i n t o a reservoir of m e r c u r y . When t h e closure is removed it will b e seen t h a t t h e level of m e r c u r y falls in t h e t u b e . T h e space above t h e m e r c u r y at t h e t o p of t h e t u b e is k n o w n as a Torricellian vacuum (after Torricelli). If an air b u b b l e were t o get i n t o this space it w o u l d depress t h e m e r c u r y (as t h e v a c u u m w o u l d n o longer b e c o m p l e t e ) a n d an incorrect reading would result. T o p r e v e n t this, an air trap is i n c o φ o r a t e d in t h e t u b e . A f u r t h e r refine­ m e n t in t h e KEW P A T T E R N M A R I N E B A R O M E T E R is t h e capillary t u b e b e t w e e n t h e air t r a p a n d t h e m a r i n e t u b e .

Pressure scale in millibars

Vernier scale Level of mercury

Marine tube

Vernier milled wheel Gymbal support

"Gold" slide and adjusting milled wheel

j - Cistern

J — Capillary tube

Airtrap

I- Cistern

Crown c o p y r i g h t reserved. R e p r o d u c e d by permission of t h e C o n t r o l l e r of H.M. S t a t i o n e r y Office.


NOTES ON METEOROLOGY The mercurial b a r o m e t e r is liable t o error on a c c o u n t of t h e following : — 1. C A P I L L A R I T Y . T h e surface tension of t h e m e r c u r y forms a M E N I S C U S and readings should always be t a k e n at t h e t o p of this. 2. C A P A C I T Y . T h e height of t h e b a r o m e t e r s h o u l d b e t a k e n from t h e t o p of t h e m e r c u r y in t h e cistern t o t h e t o p of t h e niercury in t h e m a r i n e t u b e . If t h e pressure increases, t h e level of t h e m e r c u r y in t h e cistern falls, so t h a t t h e meas­ u r e m e n t s c a n n o t be taken from a fixed p o i n t . This e r r o r is c o m p e n s a t e d by adjusting t h e distance b e t w e e n t h e graduations. On t h e inch b a r o m e t e r t h e bar­ o m e t e r mch will be seen t o be 2 4 / 2 5 t h s of a linear inch. (In t h e F O R T Í N b a r o m e t e r used in l a b o r a t o r y work t h e level of m e r c u r y in t h e cistern is ad­ justable so t h a t t h e readings can always be t a k e n from t h e same level). 3. P U M P I N G . Due t o t h e c o n s t a n t change in height above m e a n sea level of a b a r o m e t e r on a vessel in a seaway, t h e r e will t e n d t o b e a c o n t i n u a l c h a n g e of reading. This m o v e m e n t of t h e m e r c u r y will m a k e it difficult t o get an a c c u r a t e reading. Gusty winds can also cause p u m p i n g . T h e effect of p u m p i n g is con­ siderably r e d u c e d by fitting t h e capillary t u b e above t h e air t r a p . (If p u m p i n g is p r e s e n t t r y t o get a m e a n of t h e hignest a n d lowest readings). 4. H E I G H T . All readings should b e c o r r e c t e d t o sea level. Increase of height m e a n s a decrease of pressure by a p p r o x i m a t e l y 1 millibar for every 10m ( 3 0 t t . ) . This c o r r e c t i o n can be m a d e by tables or by t h e Gold Slide. L A T I T U D E . Due t o t h e earth being s o m e w h a t "flattened** at t h e poles, mer­ 5. cury weighs m o r e at t h e poles t h a n at t h e e q u a t o r . F o r equal a t m o s p h e r i c pressures t h e b a r o m e t e r w o u l d appear t o read less at t h e poles and m o r e at t h e e q u a t o r . Readings should all be c o r r e c t e d for a m e a n l a t i t u d e of 4 5 ^ . T h e c o r r e c t i o n can be m a d e by tables or by t h e Gold Slide. 6. TEMPERATURE. T h e c o l u m n of m e r c u r y will e x p a n d with an increase of t e m p e r a t u r e and c o n t r a c t with a decrease of t e m p e r a t u r e in e x a c t l y t h e same way as does a t h e r m o m e t e r . All readings m u s t be r e d u c e d t o a s t a n d a r d t e m p e r ­ a t u r e which is 2 8 5 i n t h e case of m o s t millibar b a r o m e t e r s a n d 2 8 . 6 ^ F in t h e case of inch b a r o m e t e r s . T h e c o r r e c t i o n can be m a d e by tables or b y t h e Gold Slide. N . B . T h e a t t a c h e d t h e r m o m e t e r should always b e read before t h e b a r o ­ m e t e r as o t h e r w i s e h e a t from t h e observer's b o d y m a y give a false reading. T h e t e m p e r a t u r e at which a b a r o m e t e r reads c o r r e c d y is k n o w n as t h e F I D U C I A L T E M P E R A T U R E . In lat. 4 5 ^ at sea level this is t h e same as t h e s t a n d a r d t e m p ­ e r a t u r e , b u t at sea level in lat. 5 7 ^ t h e fiducial t e m p e r a t u r e w o u l d b e 2 9 1 ^ K , and in lat. 2 1 ° at sea level it w o u l d b e 2 7 3 ° K . Whereas a t 2 0 m (60ft.)above sea level in 4 5 ° i t w o u l d be 2 9 7 ° K . (All t h e foregoing figures are t o t h e nearest degree). 7. OBSERVATIONAL ERRORS. (a) t h e b a r o m e t e r s h o u l d always be u p r i g h t ; it is t h e vertical height of t h e c o l u m n of m e r c u r y t h a t balances t h e c o l u m n of air. If t h e b a r o ­ m e t e r is n o t upright, t o o high a reading is o b t a i n e d . (b) t h e back and front of t h e vernier m u s t be o n t h e same level as t h e observer's eye, otherwise t h e reading will be t o o high. Too high Correct position

Height of barometer

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i


N O T E S ON M E T E O R O L O G Y G O L D SLIDE This takes its n a m e from its inventor Lt. Col. G o l d , and gives a rapid m e a n s of getting t h e l a t i t u d e , height and t e m p e r a t u r e c o r r e c t i o n . T o use t h e slide set the height of t h e b a r o m e t e r above sea level against t h e latitude and read off the correction o p p o s i t e t h e t o p of t h e m e r c u r y in t h e t h e r m o m e t e r .

—Negative corrections in red

'φ Ε 2 o c o

Temperature°absolute

- Milled wheel to set slide Correction shown is - 1 5 mbs

t) - • 2 - • 3 •f 4 -

S

- • β - •

7

C r o w n c o p y r i g h t reserved. R e p r o d u c e d by permission of the controller of Η JVl. Stationery Office. ANEROID BAROMETER. This is a very m u c h m o r e r o b u s t and c o m p a c t i n s t r u m e n t t h a n t h e mercurial barometer. As can be seen from t h e illustration t h e main c o m p o n e n t is a v a c u u m b o x which is partially e x h a u s t e d of air. An increase of a t m o s p h e r i c pressure c o m p r e s s e s this box causing t h e p o i n t e r on t h e dial (via t h e lever s y s t e m ) t o register a higher pressure. The converse occurs with a decrease of pressure.


N O T E S ON M E T E O R O L O G Y Pointer on dial Strong spring

Arrows show movement of lever system for a fall in pressure

^ j /

Adjustment

As t h e r e is n o m e r c u r y in this b a r o m e t e r t h e r e arc n o c o r r e c t i o n s for l a t i t u d e or t e m p e r a t u r e , b u t a h e i ^ t c o r r e c t i o n m u s t b e applied. T h e r e are n o e r r o r s d u e t o cap­ illarity, capacity or p u m p i n g . T h e r e is an a d j u s t m e n t screw o n t h e b a c k of t h e i n s t r u m e n t t o t a k e o u t any i n d e x error. T h e greater t h e area of t h e v a c u u m b o x t h e greater t h e accuracy of t h e instru­ m e n t . It is usual t o give t h e b a r o m e t e r a light t a p before reading; t h i s helps t o free t h e fine chain which m a y stick if pressure changes are only small. PRECISION A N E R O I D BAROMETER This i n s t r u m e n t is n o w supplied, w h e n available, t o all observing ships in place of t h e Kew P a t t e r n mercurial b a r o m e t e r . It is simpler t o t r a n s p o r t a n d t o read w h ü s t t e m ­ p e r a t u r e c o r r e c t i o n is unnecessary. Height c o r r e c t i o n s can b e *built in* b y r e s e t t i n g t h e d a t u m on t h e i n s t r u m e n t . A pressure c h o k e can b e a t t a c h e d if rapid h e i g h t variations, leading t o rapid pressure variations, are e x p e c t e d ; t h i s s m o o t h e s d i e variations t o neg­ ligible a m o u n t s . T h e p h o t o g r a p h shows t h e latest m o d e l (Mark 2 ) of t h i s i n s t r u m e n t .

T h e precision a n e r o i d worics o n t h e s a m e principle as t h a t of a n y a n e r o i d n a m e l y t h e m o v e m e n t of a "Pressure pile", h e r e called a " c a p s u l e " , caused b y variations in air pressure. T h e difference b e t w e e n t h e precision a n e r o i d a n d an o r d i n a r y a n e r o i d is in t h e m e a n s of t r a n s m i t t i n g t h e change in air pressure t o a reading. T h e diagram overleaf shows h o w this is d o n e . This diagram a n d t h e p h o t o g r a p h of t h e i n s t r u m e n t have b e e n kindly supplied by Negretti & Z a m b r a ( A v i a t i o n ) L t d . , w h o m a n u f a c t u r e t h e i n s t r u m e n t .


N O T E S ON M E T E O R O L O G Y

Capsule

Restraining plateJ

- Capsule contact

I Contact a r m

Hairspring

Micrometer lead screw

The m o v e m e n t of the* capsule* causes t h e capsule c o n t a c t t o m o v e t o w a r d s or away from t h e c o n t a c t a r m . If t h e c o n t a c t is b r o k e n t h e * magic eye* shows this and this is t h e instant at which t o t a k e t h e reading. T h e reading can range from 9 0 0 m b t o lOSOmb and t h e accuracy t o which it c a n be t a k e n is O . l m b . A t t h e instant of taking t h e reading t h e capsule is only u n d e r a t m o s p h e r i c pressure as t h e r e is n o load or pressure on it from t h e c o n t a c t a r m . T h e only m a i n t e n a n c e necessary is t h e i n f r e q u e n t renewal ( a b o u t 3-6 m o n t h s ) of t h e small b a t t e r y p o w e r i n g t h e electronic circuit ( n o t illustrated here) w h i c h o p e r a t e s the m i n u t e c a t h o d e ray t u b e or "magic eye**. BAROGRAPH. Drum driven by clockwork should be w o u n d weekly

Adjustment

Arrows show movement of lever system for a fall in pressure


NOTES ON METEOROLOGY

9

This is a recording aneroid b a r o m e t e r . As can b e seen, t h e lever system c o n n e c t s the vacuum pile t o a p e n a r m which m a k e s a m a r k o n t h e c h a r t o n t h e d r u m which is driven by c l o c k w o r k . T h e d r u m is w o u n d a n d t h e c h a r t changed w e e k l y . T h e p r i m e p u φ o s e of t h e barograph is t o record t h e pressure t e n d e n c y , which it w o u l d be impossible t o observe with t h e mercurial or a n e r o i d b a r o m e t e r s unless an observer was detailed t o record t h e pressure every five m i n u t e s or s o . T E M P E R A T U R E A N D ITS M E A S U R E M E N T . The forecasting of weather d e p e n d s as m u c h o n a k n o w l e d g e of t e m p e r a t u r e as of pressure. T h e i n s t r u m e n t s for measuring t e m p e r a t u r e all d e p e n d o n t h e e x p a n s i o n a n d c o n t r a c t i o n of liquids or metals w h e n h e a t e d a n d cooled. T h e t h e r m o m e t e r , in its simplest form, consists of a capillary t u b e o n t h e e n d of which is a b u l b filled with m e r c u r y . This t h e r m o m e t e r is g r a d u a t e d (as are all o t h e r s ) b y placing it in p u r e melting ice a n d m a r k i n g t h e p o s i t i o n of t h e m e r c u r y , t h e n placing it in boiling distilled w a t e r a n d again m a r k i n g t h e p o s i t i o n of t h e m e r c u r y . T h e b a r o m e t r i c pressure in each case should be 7 6 0 m m ( 3 0 ins.) of m e r c u r y . These t w o p o i n t s are k n o w n as t h e fixed p o i n t s of t h e t h e r m o m e t e r . T h e p a r t of t h e t u b e b e t w e e n t h e s e p o i n t s is then divided i n t o a n u m b e r of equal divisions. T h e n u m b e r of divisions d e p e n d s o n t h e scale t o be used. T h e various scales are shown below. Freezing P o i n t Temperature Celsius (C)

0^

Absolute or Kelvin (K) Fahrenheit (F) Reamur (R)

Boiling P o i n t Temperature

Scale Divisions

100^

100

273^

373^

100

32^

212^

180

0^

80^

80

Conversions from o n e scale t o a n o t h e r can be m a d e quickly as : Celsius

=

Fahrenheit

= (Celsius^ χ 9 / 5 ) + 3 2 ^

(FahrenheitO-320)x5/9

Absolute

= CelsiusO+ 2 7 3 ^

Reamur

= Celsius^ χ 4 / 5

It m a y be n o t e d t h a t at - 4 0 ^ t h e Celsius a n d fahrenheit scale readings are t h e same. SPECIFIC H E A T of a substance is t h e n u m b e r of J o u l e s required t o raise 1kg of t h e substance l ^ C . e.g. w a t e r is 4 . 1 8 2 w h e r e a s sand is 0 . 8 4 , w h i c h m e a n s t h a t a given q u a n t i t y of sand will h e a t 5 times as m u c h as t h e same q u a n t i t y of w a t e r p r o v i d e d t h e same a m o u n t of h e a t is applied. It will also cool 5 times as quickly u n d e r similar con­ ditions. I S O T H E R M . A line joining places having equal t e m p e r a t u r e . A I R T E M P E R A T U R E . T h e t h e r m o m e t e r s h o u l d b e placed o u t of t h e direct rays of t h e sun a n d away from local d r a u g h t s o r w a r m air c u r r e n t s .


10

N O T E S ON M E T E O R O L O G Y

G R O U N D T E M P E R A T U R E . This is t a k e n at shore stations only, the t h e r m o m e t e r being placed horizontally 5 0 m m (2 inches) above t h e g r o u n d . SEA T E M P E R A T U R E . This m a y be t a k e n with a t h e r m o m e t e r , specially guarded against breakage, set inside a canvas b u c k e t and trailed in t h e sea. However t h e m o r e c o m m o n m e t h o d is to get a canvas b u c k e t full of t h e surface water and push t h e t h e r m o m e t e r i n t o the b u c k e t after it has been b r o u g h t up on deck. In b o t h cases t h e water m u s t be from the surface a n d preferably well forward so as t o be clear of all discharges. DEW P O I N T T E M P E R A T U R E is t h e t e m p e r a t u r e t o which t h e air has t o be cooled for the water vapour t o c o n d e n s e o u t into water d r o p l e t s . It is also k n o w n as t h e s a t u r a t i o n t e m p e r a t u r e , and is d e p e n d e n t on t h e absolute h u m i d i t y . A B S O L U T E H U M I D I T Y is t h e actual weight of water vapour in a parcel of air and is expressed in grams p e r cubic m e t r e . T h e greater t h e air t e m p e r a t u r e t h e m o r e w a t e r vapour t h a t it can a b s o r b before b e c o m i n g saturated. R E L A T I V E H U M I D I T Y is t h e ratio b e t w e e n t h e a m o u n t of w a t e r v a p o u r in t h e air and t h e a m o u n t t h a t it can c o n t a i n at t h a t t e m p e r a t u r e . It is usually expressed as a p e r c e n t a g e . MAXIMUM T H E R M O M E T E R . Usually used at shore stations in order t o record t h e m a x i m u m daily t e m p erature. As s h o w n , they m a y be of t w o t y p e s , one having a c o n s t r i c t i o n in t h e t u b e (as with a clinical t h e r m o m e t e r ) and t h e o t h e r with an index in t h e b o r e . With this latter t y p e t h e m e r c u r y pushes t h e i n d e x up t h e t u b e w h e n t h e t e m p e r a t u r e rises a n d w h e n it fallí t h e index is left in p o s i t i o n . T h e m a x i m u m t e m p e r a t u r e is read at t h e end of t h e index nearest t o t h e m e r c u r y . Mercury is used as it has a high boiling p o i n t . Read maximum Mercury

^

Range-20°Cto^55C

Reset index ^¡^h magnet

Reset by shaking

MINIMUM T H E R M O M E T E R . As with t h e m a x i m u m t h e r m o m e t e r , this is generally used at shore stations t o record t h e m i n i m u m daily t e m p e r a t u r e . T h e liquid is usually alcohol as this has a low freezing t e m p e r a t u r e . T h e i n d e x is immersed in t h e alcohol and, as t h e t e m p e r a t u r e falls a n d t h e alcohol c o n t r a c t s , t h e surface tension of t h e alcohol draws t h e i n d e x d o w n the t u b e . As t h e t e m p e r a t u r e increases t h e alcohol is free t o flow past t h e i n d e x . T h e m i n i m u m t e m p e r a t u r e is read at t h e e n d of t h e i n d e x nearest t h e o p e n e n d of t h e alcohol.

Alcohol

Read minimum e ^ere • Range -35°C to +40°C


11

N O T E S ON M E T E O R O L O G Y SIXES T H E R M O M E T E R . This is a useful httle in­ s t r u m e n t which incorporates a m a x i m u m and m i n i m u m therm­ o m e t e r and is m u c h used by gardeners. The expansion of t h e alcohol in the r o u n d bulb as t h e t e m p e r a t u r e rises, forces the mer­ cury r o u n d t o w a r d s t h e pear shaped bulb, and in t u r n , forces the index up t h e t u b e . T h e con­ verse occurs when the temper­ ature falls. T h e m a x i m u m and m i n i m u m t e m p e r a t u r e s are read at t h e ends of the indices nearest the mercury.

Alcohol

Alcohol

Minimum temperatures here

\ ^

Maximum temperatures here

Mercury

THERMOGRAPH. This recording t h e r m o m e t e r is n o t often seen a b o a r d ship. A p e n , a t t a c h e d t o a metallic coil which e x p a n d s and c o n t r a c t s , records t h e t e m p e r a t u r e s on a d r u m m o v e d by c l o c k w o r k . P S Y C H R O M E T E R OR H Y G R O M E T E R . Mason's H y g r o m e t e r consists of t w o t h e r m o m e t e r s m o u n t e d side by side in a Stevenson's screen. One is a dry b u l b t h e r m o m e t e r , t h e o t h e r a w e t bulb t h e r m o m e t e r .

Stevenson's screen

Cistern, filled with distilled water

Crown c o p y r i g h t r e s e r v e d . R e p r o d u c e d by p e r m i s s i o n oí the C o n t r o l l e r of H . M . S t a t i o n e r y O f f i c e .


12

N O T E S ON M E T E O R O L O G Y

C a m b r i c is w r a p p e d r o u n d t h e b u l b of t h e l a t t e r and it is k e p t m o i s t b y m e a n s of a piece of c o t t o n wick leading t o a c o n t a i n e r of distilled w a t e r . T h e e v a p o r a t i o n of water requires h e a t and this is t a k e n from r o u n d t h e w e t b u l b w h i c h , unless t h e air is saturated, shows a lower reading t h a n t h e dry b u l b . T h e screen and t h e r m o m e t e r s should be h u n g u p t o w i n d w a r d away from local draughts or w a r m air c u r r e n t s . More a c c u r a t e readings can be o b t a i n e d b y using a whirling p s y c h r o m e t e r . This l o o k s rather like a football rattle. T h e whirling ensures a steady flow of air over t h e t w o bulbs. By entering tables with t h e dry b u l b t e m p e r a t u r e and t h e difference b e t w e e n t h e w e t and dry bulbs as a r g u m e n t s t h e d e w p o i n t and relative h u m i d i t y can b e f o u n d . WIND M E A S U R I N G I N S T R U M E N T S . These are rarely f o u n d a b o a r d ship although every shore station has o n e . T h e R o b i n s o n Cup A n e m o m e t e r consists of four hemispherical c u p s fixed t o t h e ends of rods set 9 0 ^ from each o t h e r in a h o r i z o n t a l p l a n e . T h e spindle, t o w h i c h t h e rods are a t t a c h e d , is c o n n e c t e d t o a u c h o m e t c r a n d from t h e n u m b e r of r e v o l u t i o n s m a d e in a given t i m e t h e "run** of t h e wind can be calculated.

t^lan view of cups

Wind

T h e Dines a n e m o m e t e r w o r k s o n t h e U-tube principle w h e r e b y t h e w i n d blowing in o n e side forces liquid r o u n d t h e b e n d of t h e U. T h e actual i n s t r u m e n t is very m u c h m o r e refined, a n d usually, t h e r e is a revolving d r u m fitted so t h a t t h e r e c o r d e r arm can m a k e a c o n t i n u o u s record; t h e a n e m o m e t e r t h e n b e c o m e s an a n e m o m o g r a p h . Wind

Showing principle of dines anemometer

Liquid depressed on 'windward' side

Besides t h e t w o principle a n e m o m e t e r s m e n t i o n e d above, t h e r e are several smaller types, s o m e of great a c c u r a c y , o t h e r s n o t so a c c u r a t e .


NOTES ON METEOROLOGY

13

RAIN GAUGE. This is infrequentiy f o u n d a b o a r d ship b u t will b e f o u n d a t all shore s t a t i o n s . It consists of a cylinder 1 3 0 m m (5 ins.) in d i a m e t e r having inside a small j u g . The rain is collected in this j u g by m e a n s of a funnel, which also p r e v e n t s t h e rainfall evaporating. T o m e a s u r e t h e rainfall t h e w a t e r is p o u r e d from t h e j u g i n t o a m e a s u r i n g glass which is g r a d u a t e d in millimetres or h u n d r e d t h s of an inch. Snowfall is m e a s u r e d with a ruler a n d it is r e c k o n e d t h a t 3 0 0 m m (12 ins.) of snow is t h e equivalent of 2 5 m m (1 in.) of rain. I S O H Y E T . A line j o i n i n g places having equal rainfall. SUNSHINE R E C O R D E R . This is f o u n d a t all shore s t a t i o n s a n d a t m o s t coastal resorts b u t n o t a b o a r d ship. In t h e Campbell-Stokes sunshine recorder, t h e sun's rays are focused by m e a n s of a glass sphere o n t o a piece of sensitized p a p e r which discolours w h e n t h e sun shines. The h o u r s of sunshine c a n t h e n be c o u n t e d u p . ISOHEL. A line j o i n i n g places having equal sunshine. HYDROMETER. Although hardly an i n s t r u m e n t for foretelling w e a t h e r in t h e usual sense, t h e h y d r o m e t e r can n o n e t h e less be valuable in helping t o decide which c u r r e n t s m a y b e in­ fluencing t h e vessel. T h e h y d r o m e t e r w o r k s o n A r c h i m e d e s ' principle t h a t a floating b o d y displaces its o w n weight of t h e liquid in which it floats. It consists of a float c h a m b e r t h r o u g h which passes a s t e m , t h e l o w e r e n d of which is weighted so t h a t it floats upright. T h e u p p e r e n a is graduated t o read t h e density of w a t e r . T h e i n s t r u m e n t is frequently m a d e of glass a l t h o u ^ polished steel h y d r o m e t e r s are m o r e r o b u s t . Whichever t y p e is used t h e s u r U c e s h o u l d always b e k e p t clean.

00

Μ

25

Float ΙΪ·>!·*ν>Λ·Λν>'.ν.1 chamber


14

N O T E S ON M E T E O R O L O G Y

SALINOMETER. This is a similar i n s t r u m e n t t o t h e h y d r o m e t e r differing only in t h e g r a d u a t i o n s which are parts of salt per t h o u s a n d ( ^ / o o ) . I S O H A L I N E . A line joining places having equal salinity. THE R A D I O - S O N D E . This is a m i n i a t u r e radio station which is carried i n t o t h e air by a large h y d r o g e n filled b a l o o n . A t t a c h e d t o t h e t r a n s m i t t e r is an aneroid b a r o m e t e r , a t h e r m o m e t e r and a h y g r o m e t e r , and readings of these are radioed back t o t h e p o i n t of release. This is frequently an ocean w e a t h e r ship. T h e u p p e r wind velocity and direction m a y also be found when the radio-sonde is t r a c k e d b y radar. NEPHOSCOPE. This i n s t r u m e n t is used t o find t h e velocity/height ratio of cloud. If t h e height is k n o w n , the velocity is k n o w n , and vice-versa. There are t w o main t y p e s ; o n e , t h e BESSON*S c o m b n e p h o s c o p e , looks r a t h e r like a garden rake and here t h e t i m e of passage of a cloud b e t w e e n t h e spikes is n o t e d . In t h e F I N E M A N reflecting n e p h o s c o p e t h e t i m e of passage of t h e c l o u d ' s re­ flection b e t w e e n rings on a horizontal mirror is n o t e d . I S O N E P H . A line joining places having equal cloud a m o u n t s .


II The Atmosphere Most of t h e w e a t h e r changes t a k e place in t h e lower layer of t h e a t m o s p h e r e which is k n o w n as t h e T R O P O S P H E R E . This layer e x t e n d s a b o u t 11 miles high over the equatorial regions and a b o u t 5 miles high over t h e polar regions. T h e layer above the t r o p o s p h e r e is k n o w n as t h e S T R A T O S P H E R E w h e r e t h e r e is little w a t e r vapour and t h e lower p a r t appears t o be isothermal. T h e r e is a rise in t e m p e r a t u r e t o w a r d s its upper limit. T h e b o u n d a r y b e t w e e n t h e t r o p o s p h e r e and t h e s t r a t o s p h e r e is k n o w n as t h e T R O P O P A U S E . A b o u t 2 0 miles above t h e earth t h e s t r a t o s p h e r e gives way t o t h e O Z O N O S P H E R E , where there is a high c o n c e n t r a t i o n of o z o n e which absorbs ultra violet radiation. T h e I O N O S P H E R E starts a b o u t 5 0 miles above t h e earth and this is t h e layer t h a t contains t h e various radio wave reflecting areas k n o w n as t h e K E N N E L L Y - H E A V I S I D E (55 miles) and A P P L E T O N ( 1 5 0 miles) layers. 150 4- Appleton layer

Stratosphere ! i

g>

Ď&#x2020;

Troposphere

Surface

^ Kennelly Heaviside layer Ozonosphere

I 20

temperature falls with height 90° lat

55 50

10

_ Stratosphere Troposphere

0° lat

T h e density of t h e a t m o s p h e r e decreases with height and high-flying aircraft must be pressurised t o avoid severe discomfort t o passengers. O x y g e n is also rarer at t h e higher levels and oxygen masks m u s t be worn in non-pressurised craft. Approximate percentage volumes of t h e various gases forming dry air in t h e T r o p o s p h e r e m a y be of interest t o the reader. T h e y are N I T R O G E N 7 8 % , O X Y G E N 2 1 % , A R G O N 0.9%, C A R B O N D I O X I D E 0 . 0 3 % , with t h e balance being m a d e up of traces of H Y D R O G E N , HELIUM, N E O N , K R Y P T O N , R A D O N , X E N O N and O Z O N E . As m o s t of t h e w e a t h e r changes take place in t h e t r o p o s p h e r e , it is in this region that t h e changes in t e m p e r a t u r e are m o s t i m p o r t a n t . The sun, at a distance of a b o u t 9 3 , 0 0 0 , 0 0 0 miles and at a t e m p e r a t u r e of a b o u t 6 , 0 0 0 ^ C , is t h e principal source of light and h e a t for t h e e a r t h . T h e heat from t h e sun


16

N O T E S ON M E T E O R O L O G Y

travels t o t h e earth in t h e form of short wave radiation, which passes t h r o u g h t h e a t m o s 足 phere w i t h o u t appreciably warming it. On striking t h e earth s o m e of t h e h e a t will be absorbed t o warm t h e earth. T h e heat received at t h e earth from t h e sun is k n o w n as INSOLATION. The a m o u n t of insolation per u n i t area varies with latitude. It can be seen in t h e sketch below t h a t a b a n d of rays has t o h e a t a very m u c h larger area in a high l a t i t u d e than it does in a low latitude.

T h e increase in t h e t e m p e r a t u r e of t h e earth will d e p e n d , amongst o t h e r things which will be discussed later, on t h e a m o u n t of insolation and t h e specific h e a t of t h e earth. Assuming t h a t t h e insolation is c o n s t a n t , a surface with a high specific heat w a r m s and cools less quickly t h a n a surface with a low specific h e a t . F o r instance, t h e sea t e m p 足 erature in non-tidal waters hardly changes in t h e 24 h o u r s . A c o n t r i b u t i n g factor t o this small change is its d e p t h . It will be found t h a t , in general, sea t e m p e r a t u r e s are less t h a n the t e m p e r a t u r e s of adjacent land by day and greater by night. T h e r e is a similar dif足 ference in s u m m e r and winter. T h e a m o u n t of insolation will d e p e n d o n t h e sun's altitude and t h e degree of cloud cover. It m a y be n o t e d t h a t t h e S O L A R C O N S T A N T , which is t h e intensity of solar radiation at t h e o u t e r b o u n d a r y of t h e a t m o s p h e r e , is 1.39kw/m2. T h e height of the station u n d e r consideration above sea level will be a factor influencing t h e heating, as will t h e prevailing wind. Air which has flowed over warm surfaces will have a smaller cooling effect t h a n air which has flowed over cold surfaces. As the earth is a warm b o d y it radiates h e a t . This radiation will cool t h e sur足 face. If the earth is radiating heat at a greater rate than it is receiving it, t h e n e t result will be a cooling of t h e surface. T h e converse is also t r u e . In the diagram below the curves of insolation and radiation are s h o w n t o increase

0000

0000


N O T E S ON M E T E O R O L O G Y

17

Quite clearly t h e t e m p e r a t u r e falls until p o i n t A is reached w h e n it begins t o rise. T h e rise c o n t m u e s t o p o i n t Β and t h e n , o n c e again, t h e t e m p e r a t u r e falls. It will be n o t e d t h a t t h e lowest t e m p e r a t u r e occurs shortly after sunrise whereas t h e max­ i m u m t e m p e r a t u r e occurs a b o u t 1 4 0 0 h o u r s local t i m e , and n o t w h e n insolation is m i n i m u m a n d m a x i m u m . A similar lag takes place on an a n n u a l basis. In n o r t h e r n latitudes t h e lowest t e m p e r a t u r e s d o n o t usually o c c u r until F e b r u a r y and the m a x i m u m t e m p e r a t u r e s occur in July a n d early August. T h e sea t e m p e r a t u r e lags even further b e h m d t h e sun. T h e m i n i m u m occuring in March a n d t h e m a x i m u m in S e p t e m b e r . It m u s t be u n d e r s t o o d t h a t an t h e above general s t a t e m e n t s can be considerably modified by local w e a t h e r c o n d i t i o n s , particularly c l o u d a m o u n t s . T h e foregoing has described t h e process by which t h e earth is h e a t e d , b u t w h a t a b o u t the air s u r r o u n d i n g it? Heat is transferred t o it by o n e or m o r e of t h e following means: —

1.

1.

Radiation

2.

Conduction

3.

Convection

4.

Turbulence.

RADIATION.

As previously m e n t i o n e d t h e heat rays from t h e sun are in short wave form which pass through t h e a t m o s p h e r e causing very little heating. S o m e heating will o c c u r as water vapour in t n e a t m o s p h e r e absorbs h e a t . T h e h e a t rays from t h e earth are in long wave f o r m , which t e n d t o w a r m t h e lower layers of air. If t h e sky is cloudless m o s t of t h e e a r t h ' s radiated h e a t will go t o o u t e r space. However, if t h e r e is a cloud cover, m o s t of t h e h e a t will be reflected oack to earth. T h e c l o u d acts as an insulator for t h e e a r t h . T h e h e a t radiated from t h e earth on cloudless nights is considerable and a great deal of surface cooling takes place. 2.

CONDUCTION.

The with a w a r m is in c o n t a c t (see page 54) 3.

h e a t is transferred from particle t o particle. T h u s , air which is in c o n t a c t surface is w a r m e d ; this occurs by day w h e n t h e sun is shining. Air which with a cold surface is c o o l e d ; this occurs on clear nights. Air masses acquire their characteristics by t h e same process.

CONVECTION.

Air which is w a r m e d e x p a n d s a n d c o n s e q u e n t l y its density decreases. This m a k e s it lighter t h a n t h e u n w a r m e d air s u r r o u n d i n g it and t h e w a r m air rises. Convectional processes t a k e large a m o u n t s of w a r m air and water vapour from t h e surface to t h e u p p e r levels. When t n e water v a p o u r c o n d e n s e s i n t o water d r o p s and p r e c i p i t a t i o n occurs, t h e latent h e a t remains. Most o t t h e a t m o s p h e r i c heating takes place in this w a y . Conversely, air which is colder t h a n t h e s u r r o u n d i n g air has a greater density, and as it is heavier, it sinks. An e x a m p l e of this is t h e Katabatic wind q.v. 4.

TURBULENCE.

Air which flows over a rough surface t e n d s t o b e deflected u p w a r d s . T h e rising air will be replaced by s o m e air from levels u p t o 6 0 0 m ( 2 , 0 0 0 feet) giving an interchange of air b e t w e e n t h e surface a n d 6 0 0 m ( 2 , 0 0 0 feet). T h e rising air carries its w a r m t h (acquired by c o n d u c t i o n ) with it, a n d t h e falling air brings its coolness.


18

N O T E S ON M E T E O R O L O G Y

LAPSE R A T E . Increase of height above sea level generally m e a n s a decrease of t e m p e r a t u r e . The rate at which t h e t e m p e r a t u r e changes with height is k n o w n as t h e lapse rate, an average value being a b o u t 0 . 7 ^ C / 1 0 0 m (3 " F / 1 , 0 0 0 feet). E N V I R O N M E N T LAPSE R A T E . T h e lapse rate can vary a n d t h e E n v i r o n m e n t Lapse R a t e will be referred t o w h e n a particular air mass is u n d e r consideration. This rate will b e d e p e n d e n t on m a n y factors and will vary with t h e a l t i t u d e , although it is usually b e t w e e n t h e S.A.L.R. and D.A.L.R. Plotting t e m p e r a t u r e s of t h e air at various heights will give e n v i r o n m e n t curves. ADIABATIC TEMPERATURE CHANGE. If a v o l u m e of air rises t o a region of lower pressure, t h e v o l u m e will increase and, following t h e gas laws, t h e t e m p e r a t u r e of t h e v o l u m e will fall. T h e converse will occur if t h e air falls t o an area of greater pressure. T h e change of t e m p e r a t u r e is solely due t o expansion or c o n t r a c t i o n and n o h e a t has been given t o or received from adjacent air. This change of t e m p e r a t u r e is k n o w n as an Adiabatic T e m p e r a t u r e C h a n g e . D R Y A D I A B A T I C LAPSE R A T E ( D . A . L . R . ) . When dry air is forced t o rise it has been found t h a t it decreases its t e m p ­ erature by l ^ C / l O O m ( 5 . 4 ^ F / 1 , 0 0 0 feet) and this is k n o w n as t h e Dry A d i a b a t i c Lapse R a t e . With dry air which is forced d o w n , an increase at a similar rate is f o u n d . Dry air is any air which is n o t saturated. SATURATED ADIABATIC

LAPSE

RATE

(S.A.L.R.)

L a t e n t h e a t is t h e h e a t necessary t o change 1 kilogram of water t o 1 kilogram of vapour at s a t u r a t i o n t e m p e r a t u r e . If a q u a n t i t y of w a t e r changes t o v a p o u r an a m o u n t of l a t e n t heat will have been required. This sort of t h i n g will h a p p e n with air at a t e m p ­ erature above d e w p o i n t , (it should be n o t e d t h a t t h e higher t h e air t e m p e r a t u r e t h e greater is the a m o u n t of water vapour t h a t it can h o l d ) . N o w t h e l a t e n t h e a t t h a t has been required t o change t h e water t o v a p o u r is n o t lost, as, w h e n t h e air is cooled t o its sat­ uration or dew p o i n t t e m p e r a t u r e , t h e w a t e r vapour c o n d e n s e s i n t o w a t e r d r o p l e t s and the l a t e n t h e a t is released. This is t h e l a t e n t h e a t of c o n d e n s a t i o n . When the cooling of t h e air t o its d e w p o i n t t e m p e r a t u r e is caused by t h e air rising t h e l a t e n t heat released will modify t h e overall cooling r a t e . For rising air D.A D.A.L.R.

- l.O^C/lOOm

Latent heat

+ 0 . 5 ° C / 1 0 0 m (average for near surface in m i d d l e latitudes)

S.A.L.R.

- O.S^C/lOOm ( Z . 7 Q F / 1 , 0 0 0 feet.)

T h e S.A.L.R. is a variable q u a n t i t y d e p e n d i n g on t h e l a t e n t h e a t of t h e c o n d e n s ­ ing water vapour. T h e S.A.L.R. is low near t h e e q u a t o r a n d high in polar regions. It will also increase with height.


N O T E S ON M E T E O R O L O G Y

19

STABLE AIR. If air which has been forced t o rise (or fall) from its initial level t e n d s t o r e t u r n to that level it is said t o be stable. This c o n d i t i o n occurs when t h e e n v i r o n m e n t lapse rate is less t h a n t h e S.A.L.R. F i g u r e 1 shows b y a n u n b r o k e n line t h e e n v i r o n m e n t c u r v e of s o m e tropical air, whose lapse rate is a b o u t 0.3°C/100m (2°F/1,000 feet). S u p p o s e some d r y air at p o i n t A is forced u p w a r d s , it will cool at t h e D . A . L . R . a n d follow t h e d o t t e d line. W h e n t h e rising air reaches t h e level BBi it will b e m u c h colder t h a n t h e e n v i r o n m e n t or s u r r o u n d i n g air. As it is colder it is denser, a n d will w a n t to r e t u r n to A , a t e n d e n c y s h o w n b y t h e arrow. If instead of t h e air rising from A, it fell, t h e n at t h e level CCi t h e falling air w o u l d h a v e w a r m e d adiabatically to a considerably higher t e m p e r a t u r e t h a n t h e envirorunent or s u r r o u n d i n g air. I t would t h e n be less dense a n d t h e t e n d e n c y w o u l d b e for it to r e t u r n to A as indicated. If t h e air forced to rise or fall from A was saturated instead of d r y , t h e p a t h indicated by t h e d o t / d a s h line would b e followed. Again, for t h e same reasons as are given above, t h e tendency is for t h e air to r e t u r n to A.

4 •·. \

Height

D.A.L.R.

S.A.LR.

• Temperature Figure 1.

UNSTABLE AIR. If air which has been forced to rise (or fall) from its initial level tends to c o n t i n u e its u p w a r d (or d o w n w a r d ) m o v e m e n t , it is said to be unstable. This c o n d i t i o n occurs when the e n v i r o n m e n t I ipse rate is greater t h a n t h e D.A.L.R.


20

NOTES ON M E T E O R O L O G Y

F i g u r e 2 shows b y a n u n b r o k e n line t h e e n v i r o n m e n t curve of some polar air whose lapse rate is a b o u t 1.2°C/100m (6.0°F/1,000 feet). S u p p o s e some saturated air at p o i n t A is forced u p w a r d it will cool at t h e S . A . L . R . a n d follow t h e dot/dash line. W h e n this rising air reaches t h e level BBi it will b e w a r m e r t h a n t h e e n v i r o n m e n t or s u r r o u n d i n g air. As it is w a r m e r it will b e less d e n s e a n d t h e tendency is for it to continue u p w a r d s . If, instead of t h e air rising from A , it fell, t h e n at t h e level CCi t h e falling air would b e at a lower t e m p e r a t u r e t h a n t h e e n v i r o n m e n t or s u r r o i m d ing air. I t would t h e n b e denser a n d it would continue t o fall. If t h e air forced to rise o r fall from A was d r y instead of saturated, t h e p a t h i n d i ­ cated by t h e dotted line would b e followed. Again, for t h e reasons given a b o v e , t h e rising or falling air continues to rise or fall.

^

Temperature Figure 2.

N E U T R A L AIR. This c o n d i t i o n occurs when the e n v i r o n m e n t lapse rate is the same as t h e D.A.L.R. if dry air is u n d e r consideration, or the S.A.L.R. in the case of saturated air. The air which has been forced t o rise or fall will have no t e n d e n c y to c o n t i n u e its up­ ward or d o w n w a r d m o v e m e n t nor will it have any t e n d e n c y t o return to its original position. CONDITIONAL INSTABILITY. T h i s occurs w h e n stable conditions exist for dry air a n d unstable conditions exist w h e n t h e air is saturated. F i g u r e 3 illustrates this. T h e e n v i r o n m e n t curve has a lapse rate between t h e S.A.L.R. a n d t h e D . A . L . R . T h e d o t t e d line representing d r y air forced u p or d o w n follows a similar p a t t e r n to figure 1, whilst t h e dot/dash line representing sat­ u r a t e d air forced u p or d o w n follows a similar p a t t e r n to figure 2. T h i s conditionally u n ­ stable condition is quite c o m m o n , t h e air below t h e condensation level being stable a n d t h a t above being unstable.


N O T E S ON M E T E O R O L O G Y

21

Environment curve

Height

Temperature Figure 3.

TEMPERATURE

INVERSION

The\isual tendency for t h e t e m p e r a t u r e to fall with height is s o m e t i m e s reversed. When the t e m p e r a t u r e increases with height an inversion is said to exist. An inversion also exists if a layer of air is isothermal. Inversions may occur at any level at all. They frequently occur at ground level on clear nights. They also occur just above cloud layers. As an inversion gives rise t o very stable c o n d i t i o n s , convection and t u r b u l e n c e are d a m p e d o u t and there will be no u p w a r d m o v e m e n t of air through t h e inversion. Inversions may be caused in anticyclonic c o n d i t i o n s by subsiding air warming adiabatically to a t e m p e r a t u r e above that of t h e lower layers of air.


22

N O T E S ON M E T E O R O L O G Y

Figure 4 shows a curve for the environment where there is an inversion above the surface. Dry and saturated air forced up from A will clearly return showing stable conditions, whereas if forced up from Î&#x2019; (or d o w n from A) unstable c o n d i t i o n s exist.

Environment curve

\ D.A.LR\^

S.A.L.R.

Height

Temperature Figure 4.

TEPHIGRAM. For accurate forecasting a knowledge of u p p e r air c o n d i t i o n s is essential. If upper air t e m p e r a t u r e s are k n o w n (from Radio-sonde m e a s u r e m e n t s ) the o t h e r c o n d i t i o n s can be d e d u c e d from a tephigram. T h e tephigram is a graticule formed by plotting a series of straight lines rep­ resenting t e m p e r a t u r e , water vapour c o n t e n t , and dry adiabatics (also called lines of c o n s t a n t potential t e m p e r a t u r e ) against curves (almost horizontal straight lines) which indicate height-pressure for a standard c o n d i t i o n of t h e a t m o s p h e r e ( k n o w n as the ICAN a t m o s p h e r e ) . Also included on t h e graticule are curves to show saturated adiabatics for the same standard a t m o s p h e r e . Such curves are shown on Metform 2 8 1 0 b u t they are unlikely t o be found a b o a r d ship.


Ill C l o u d and P r e c i p i t a t i o n

When air is cooled b e l o w its d e w p o i n t t h e w a t e r v a p o u r therein starts t o con­ dense o u t i n t o water droplets. T h e water d r o p l e t s form either fog or cloud d e p e n d i n g o n the process by which t h e air is c o o l e d . It is generally u n d e r s t o o d t h a t t h e fog is f o r m e d when t h e cooling of t h e air takes place at t h e surface by c o n d u c t i v e processes, whereas cloud generally forms above t h e surface d u e t o t h e adiabatic cooling of t h e rising air. T h e cooling of t h e air occurs as t h e air is forced t o rise as follows : — (a) (b) (c) (d) (e)

at a warm f r o n t at a cold front by convectional processes by high g r o u n d by t u r b u l e n c e

C o n t r i b u t o r y causes in cloud f o r m a t i o n m a y be : (i) radiation of h e a t by water v a p o u r in t h e a t m o s p h e r e (ii) m i x m g of t w o masses of nearly s a t u r a t e d air at different atures.

temper­

Clouds are classed as being either high a b o v e 6 0 0 0 m e t r e s ( 2 0 , 0 0 0 feet), m e d i u m 2 5 0 0 - 6 0 0 0 m e t r e s ( 8 , 0 0 0 - 2 0 , 0 0 0 feet) or low below 2 5 0 0 m e t r e s ( 8 , 0 0 0 f e e t ) . There are several t y p e s in each class, t h e main ones being s h o w n in t h e following table:-

CLASS

STRATIFORM (Stable air conditions)

FREE CONVECTIVE (Unstable air conditions)

LIMITED CONVECTIVE (Air c o n d i t i o n s t e n d i n g t o be u n s t a b l e )

LOW

Stratus (St)

C u m u l u s (Cu)

Stratocumulus

MEDIUM

N i m b o s t r a t u s (Ns) A l t o s t r a t u s (As)

Cumulonimbus

HIGH

Cirrostratus (Cs)

Cirrus (Ci)

(Cb)

(Sc)

A l t o c u m u l u s (Ac) C i r r o c u m u l u s (Cc)

T h e r e are o t h e r forms of cloud such as : — F r a c t o s t r a t u s or Scud. F r a c t o m e a n s b r o k e n and m a y be applied t o o t h e r t y p e s of clouds besides stratus. Lcnticularis. Indicates c l o u d s of lens or airship s h a p e .


24

N O T E S ON M E T E O R O L O G Y Env. Stable Path'

Condensation level Path Unstable Env>

Condensation "^^vel Path Conditionally unstable c) Cu

r

d) St

Env. i\ ^ Path I e) St

Stable


N O T E S ON M E T E O R O L O G Y

25

Castellatus. Here t h e t o p s of t h e cloud m a y e x t e n d vertically t o form 'castles'. often occurs with a l t o c u m u l u s .

This

M a m m a t u s . The lower side of t h e c l o u d projects to form festoons. N o t very c o m m o n , b u t may be found with unstable c o n d i t i o n s , where t h e r e are s t r o n g d o w n as well as up c u r r e n t s . T h e water vapour m e n t i o n e d earlier c a n n o t c o n d e n s e into large size water d r o p s unless c o n d e n s a t i o n nuclei are present. These nuclei, which m a y be salt or dust particles, are always present although the n u m b e r s will vary from a b o u t 1 0 0 0 per c m 3 over t h e sea to a b o u t 1 5 0 , 0 0 0 per c m ^ over industrial areas. Water drops of less t h a n 0 . 1 m m d iam eter c a n n o t reach the g r o u n d as they evap­ orate on t h e way. These drops are cloud d r o p s and their average d i a m e t e r is 0 . 0 1 m m . Drops over 0 . 1 m m are drizzle d r o p s and over 0 . 5 m m are rain d r o p s . T h e m a x i m u m size to which a drop can grow is 5 . 5 m m . T h e process of g r o w t h of these w a t e r d r o p s has brought forth m a n y theories o n e of which is t h a t in t h e original stages of c o n d e n s a t i o n taking place, some d r o p s form, larger t h a n the rest. Their g r o w t h , due t o t h e a t t r a c t i o n of the smaller drops, is then assured. A n o t h e r t h e o r v is t h a t for growth to t a k e place, part of the c l o u d m u s t be above the freezing level, in which case t h e ice crystal will grow at t h e e x p e n s e of w a t e r d r o p s . When these get sufficiently large they will fall t h r o u g h t h e c l o u d , w a r m i n g and m e l t m g as they fall, until they arrive at t h e lower level as rain. This is k n o w n as t h e B E R G E R O N P R O C E S S . It should be n o t e d t h a t t h e freezing level over t h e British Isles is a b o u t 1 0 0 0 m ( 3 , 0 0 0 ft.) in winter and a b o u t 2 5 0 0 m ( 8 , 0 0 0 ft.) in s u m m e r . This latter t h e o r y w o u l d account for the t e n d e n c y t o greater rainfall in w i n t e r t h a n in s u m m e r .

Ice crystals and water drops

^^^^y^^^

Freezing level Water droplets Condensation level Water vapour and condensation nuclei Surface

Rain is usually described as being either F r o n t a l , Convectional or O r o g r a p h i c . This is in a c c o r d a n c e with t h e process of c l o u d f o r m a t i o n (see page 23). Appreciable rainfall is unlikely with t u r b u l e n t cloud. F R O N T A L R A I N o c c u r s at a w a r m front. T h e precipitation c o m m e n c e s from A l t o s t r a t u s cloud and increases in intensity as t h e lower n i m b o s t r a t u s cloud c o m e s in. Where t h e front is n o t so m a r k e d O C C A S I O N A L P R E C I P I T A T I O N may occur. This has t h e same characteristics as frontal rain and lasts less t h a n 30 m i n u t e s . C O N V E C T I O N A L R A I N falls w h e n t h e s t r o n g u p c u r r e n t s , within t h e c u m u l u s t y p e cloud, cease or are so r e d u c e d t h a t t h e y can n o longer keep t h e rain d r o p s within t n e cloud. It m a y be n o t e d t h a t an up c u r r e n t of 8 m / s (25 feet per s e c o n d ) will k e e p t h e m a x i m u m sized raindrops from falling. If this velocity is slightly r e d u c e d , only t h e largest sized d r o p s will fall and t h e smaller d r o p s will remain in t h e c l o u d . A feature of con­ vectional t y p e rain is t h a t r2.infall always starts heavily a n d often with little warning. A n o t h e r n a m e for convectional rain is showers. These last less t h a n 3 0 m i n u t e s althougn showers frequently merge t o give a period of p r o l o n g e d rainfall.


26

N O T E S ON M E T E O R O L O G Y

;

'J\'''

· ·, . 1 ' · · · · Rapid growth in drop size • · · • / · * · ' · ' * " · c a u s e d by collision of drops

Very strong vertical currents

O R O G R A P H I C R A I N . This falls on t h e w e a t h e r side of m o u n t a i n ranges and gives s o m e of the heaviest rain k n o w n . SNOW. When water vapour c o n d e n s e s at a t e m p e r a t u r e below freezing p o i n t ice crystals form. These j o i n t o g e t h e r t o form snowflakes. If t h e air t e m p e r a t u r e is m u c h above freezing p o i n t t h e flakes will melt. In general t h e t e m p e r a t u r e near t h e surface m u s t n o t exceed 3^C. ( 3 7 ^ F . ) for snow t o fall. It should be u n d e r s t o o d t h a t at t e m p e r a t u r e s less than 3^C rain is still very likely a n d is n o t necessarily replaced by s n o w . SLEET. This is partially m e l t e d s n o w . THE C U M U L O N I M B U S C L O U D . This is a cloud of great vertical d e v e l o p m e n t f o r m e d in u n s t a b l e c o n d i t i o n s a n d may e x t e n d up t o 13.000 m ( 4 0 , 0 0 0 ft.) from a base which m a y be as low as 5 0 0 m ( 1 , 5 0 0 feet). T h e Cb cloud m a y be f o r m e d (i) at a cold f r o n t , (ii) at a m o u n t a i n range, (iii) when t h e r e is considerable heating of t h e g r o u n d , or (iv) w h e n cold air flows over a warm surface. However, by whichever m e t h o d it is f o r m e d it will still have its character­ istic a p p e a r a n c e . Air c u r r e n t s at high levels m a y , and often d o , p r o d u c e an 'anvil'. Within t h e c l o u d will be water d r o p s and ice crystals and in b e t w e e n t h e t w o there m a y be supercooled water drops. These latter are w a t e r d r o p s in their liquid state at a t e m p e r a t u r e below freezing. As long as t h e y d o n o t c o n t a c t a n y t h i n g t h e y can remain liquid at t e m p e r a t u r e s d o w n t o - 4 0 ^ . If, however, t h e y t o u c h a n y t h i n g , t h e y i m m e d i a t e l y freeze into a globule of clear ice. HAIL. Due t o t h e t u r b u l e n c e within t h e Cb cloud ice crystals frequently c o n t a c t t h e supercooled water d r o p s . When this occurs t h e s u p e r c o o l e d d r o p freezes t o give a layer of clear ice r o u n d t h e ice crystal. This is t h e start of t h e hailstone, which will t h e n g r o w as c o n v e c t i o n c u r r e n t s carry it t h r o u g h t h e cloud. During this process t h e s t o n e acquires a l t e r n a t e coats, o n e of clear ice (from the supercooled drops) and o n e o p a q u e ice (from t h e ice crystals). If a h a i l s t o n e is c u t open t h e various layers can be seen, a n d t h e n u m b e r of t i m e s it has b e e n carried u p a n d d o w n can be e s t i m a t e d . When t h e hailstone can n o longer be s u p p o r t e d within t h e cloud it will fall t o t h e ground. T h e hailstone varies considerably in size being a n y t h i n g from 5 m m t o 5 0 m m in diameter.


N O T E S ON M E T E O R O L O G Y 27 T H U N D E R S T O R M . When r a i n d r o p s reach their m a x i m u m size of 5 . 5 m m t h e y break u p . This spHtting gives t h e air a negative electrical charge whilst t h e r a i n d r o p retains a positive electrical charge. This action t a k e s place within t h e C b c l o u d and it is f o u n d t h a t t h e r e is a large positive charge near the t o p of t h e cloud. At t h e b o t t o m t h e r e is a large negative charge with a small positive charge d u e t o t h e r a i n d r o p s . When t h e r e is sufficient difference of p o t e n t i a l b e t w e e n t h e t o p a n d b o t t o m of the cloud, or b e t w e e n t w o clouds, or b e t w e e n a cloud and t h e e a r t h , there is a visible electrical discharge which is called L I G H T N I N G . Thunderhead often forms anvil

Ice crystals

Water drops

Cumulonimbus cloud

:Water vapour and condensation nuclei


28

NOTES ON M E T E O R O L O G Y

When t h e P.D. is b e t w e e n t h e cloud and t h e e a r t h , t h e negative charge on t h e underside of t h e cloud induces a positive charge on t h e e a r t h . A leader s t r o k e k n o w n as a stepped leader finds a p a t h from t h e cloud t o a p o i n t near the earth w h e n an up­ ward stroke from t h e earth c o m e s u p t o m e e t it. This up s t r o k e c o n t i n u e s along t h e ionised path m a d e by t h e s t e p p e d leader. T h e r e will p r o b a b l y be m a n y up a n d d o w n strokes in t h e space of a m i c r o s e c o n d or so, each striking t h e same place. T h e hghtning c o n d u c t o r c o n c e n t r a t e s t h e charge a n d ensures t h a t t h e lightning will be discharged w i t h o u t damage t o t h e building. In any place t h e u p w a r d stroke is likely t o originate from t h e highest p o i n t in t h e vicinity of t h e d o w n w a r d leader. Ships will n o t suffer any damage when struck by lightning as t h e charge will go directly t o earth, if vessels have w o o d e n t o p m a s t s , lightning c o n d u c t o r s properly b o n d e d t o earth m u s t b e fitted t o them. Negative charge

Note: Lightning conductor gives protection over an angle of about 45° each side of vertical

Positive charge induced on surface

T h e intense h e a t generated b y t h e lightning spark causes t h e air t o e x p a n d rapidly with t h e r e s u l t a n t noise called T H U N D E R . T h e noise lasts a considerable time c o m p a r e d with t h e d u r a t i o n of t h e lightning mainly d u e t o t h e difference in speed of light, 3 0 0 X l O ^ m / s ( 1 8 6 , 0 0 0 miles p e r s e c o n d ) a n d s o u n d 3 3 5 m / s ( 1 , 1 0 0 feet p e r second). T h e s o u n d m a y also e c h o from c l o u d or m o u n t a i n s . It m a y be n o t e d t h a t the sound of t h u n d e r is unlikely t o travel m o r e t h a n a b o u t four miles, c o n s e q u e n t l y m u c h lightning m a y be seen w i t h o u t t h u n d e r being h e a r d . This is particularly so a t sea w h e n lightning may be seen over 1 0 0 miles. S t o r m s occuring d u r i n g t h e a f t e r n o o n are nearly always d u e t o h e a t i n g of t h e surface. S t o r m s occuring a t o t h e r t i m e s are usually of t h e frontal t y p e . S t o r m s fre­ quently occur off t h e east coast of N . America and Siberia during t h e w i n t e r d u e t o t h e cold c o n t i n e n t a l air flowing over t h e relatively warm sea surface. During t h u n d e r s t o r m s a brush-like discharge m a y be seen e m a n a t i n g from masts, yards, aerials or stays. This discharge is d u e t o electrical charges o n these p a r t s and is k n o w n as ST. E L M O ' S F I R E .


29

N O T E S ON M E T E O R O L O G Y

So far t h e p h e n o m e n a caused by air cooHng t h r o u g h rising have been considered. If air is cooled below its d e w p o i n t by c o n d u c t i o n (i.e. c o n t a c t with a cold surface) t h e n t h e water v a p o u r will c o n d e n s e i n t o d r o p l e t s on or near t h a t surface. As has been seen on page 1 7 , t h e radiation from t h e earth on cloudless nights will cause considerable surface cooling, with t h e possible deposit of water droplets if t h e air near t h e earth is cooled until s a t u r a t e d . DEW. T h e water droplets deposited on t h e surface by t h e above process. Clear skies and practically n o wind are necessary c o n d i t i o n s for its f o r m a t i o n . H O A R F R O S T . F o r m a t i o n is t h e same as for d e w e x c e p t t h a t t h e d e w p o i n t t e m p e r a t u r e is b e l o w freezing. Hoar frost m a y also be formed by t h e freezing of w a t e r d r o p l e t s which were d e p o s i t e d as dew. F O G occurs when visibility is r e d u c e d t o less t h a n 1 0 0 0 m e t r e s . m e a n s by which fog is f o r m e d and these are detailed b e l o w .

T h e r e are different

MIST is similar to fog b u t here t h e visibility is 1 0 0 0 m t o 2 0 0 0 m . It should be n o t e d t h a t H A Z E giving t h e same visibility as mist is f o r m e d of dust particles n o t w a t e r d r o p l e t s . R A D I A T I O N F O G occurs over land at night w h e n radiation from the earth has cooled its surface t o b e l o w t h e d e w p o i n t of t h e air adjacent t o it. C o n d u c t i o n cools this air until c o n d e n s a t i o n takes place. Slight t u r b u l e n c e (wind 3 - 7 k n o t s ) will carry this cold sat­ urated air t o higher levels a n d will bring t o t h e surface drier air, which will in t u r n be cooled and s a t u r a t e d . As t h e process c o n t i n u e s t h e w h o l e lower layer of air will b e c o m e saturated. A t e m p e r a t u r e inversion is often associated with radiation fog, w h i c h , acting as a "lidV keeps t h e fog low d o w n and causes it t o t h i c k e n . Stronger winds will disperse t h e fog as will t h e h e a t from t h e sun, p r o v i d e d t h e fog layer is n o t t o o thick. In winter there is rarely sufficient h e a t t o disperse fog and persistent fogs occur. If over industrial areas, " S M O G " frequently forms as t h e s m o k e from c h i m n e y s is t r a p p e d u n d e r t h e t e m p e r a t u r e inversion. This t y p e of fog m a y occur overnight on t h e b a n k s of rivers b u t usually disperses fairly quickly in t h e m o r n i n g . Inversion level

Environment curve

C/ o G ο· o

CrOG Ο· Ο·

G G G. SURFACE COOLED BY

HAOIATION

Very »light wind

Inversion

Ht

Temp.


30

N O T E S ON M E T E O R O L O G Y

A D V E C T I O N F O G . This occurs over t h e sea w h e n w a r m tropical m a r i t i m e air is cooled below its d e w p o i n t by c o n d u c t i o n from a cold sea surface. T h e likelihood of fog occuring may be j u d g e d if graphs of t h e air d e w p o i n t t e m p e r a t u r e s and t h e sea t e m p e r a t u r e s are drawn and p l o t t e d against t i m e . In t h e graph below fog is likely t o o c c u r a b o u t 2 0 0 0 h o u r s , if t h e present t r e n d s are m a i n t a i n e d . A change of course t o t h e s o u t h w a r d would alter c o n d i t i o n s considerably. Northern hemisphere

0000

0400

0800

1200

1600

2000

^ Local mean time

In general t e r m s fog is likely t o occur if tropical m a r i t i m e air crosses seven sea isotherms from its source Sea fog occurs mainly in t h e s u m m e r m o n t h s in t h e following areas : - British Isles, Banks of N e w f o u n d l a n d , West Coast of N o r t h America, J a p a n , West Coasts of South Africa and S o u t h America. If reference is m a d e t o t h e c u r r e n t chart on pages 4 4 and 4 5 , it will be seen t h a t the areas m e n t i o n e d have cold sea c u r r e n t s flowing t h r o u g h t h e m . SEA SMOKE OR A R C T I C S M O K E . This occurs over t h e sea w h e n t h e sea is warm c o m p a r e d with the air which flows over it. T h e air n e x t t o t h e water is w a r m e d by c o n d u c t i o n . This w a r m e d air takes u p water vapour before rising through t h e colder air above, where cooling c o n d e n s e s o u t t h e water vapour. T h e overall effect is t h a t of visible steaming. This frequently occurs in winter over rivers and also off t h e East Coasts of continents.

Very cold air

Sea surface relatively warm

OROGRAPHIC FOG. orographic cloud.

F o u n d on w i n d w a r d coasts.

Temp.

Its f o r m a t i o n is t h e same as t h a t of


N O T E S ON M E T E O R O L O G Y

31

O t h e r p h e n o m e n a associated with w a t e r d r o p s are ; RIME. This o c c u r s when supercooled water d r o p s (often in fog) freeze i n t o o p a q u e ice on c o m i n g i n t o c o n t a c t with rigging o n ships or trees or telegraph wires on land. T h e ice formed grows o u t t o w i n d w a r d .

Wind

Rime grows to windward

t G L A Z E D F R O S T O R BLACK F R O S T . This o c c u r s w h e n w a t e r d r o p s fall o n t o a surface which is below freezing. A layer of clear ice is f o r m e d and this creates c h a o s on r o a d s as t h e ice c o n d i t i o n s c a n n o t b e seen. However, t h e effect at sea is m u c h m o r e serious as, if water falls o n t o d e c k s , rigging and s u p e r s t r u c t u r e s which are b e l o w freezing, t h e weight of t h e ice which f o r m s can affect t h e stability so t h a t t h e safety of t h e vessel is endangered. A l t h o u g h ice axes and s t e a m hoses are partially effective in r e m o v i n g t h e u n w a n t e d ice, t h e o n l y sure way of c o m b a t i n g black rrost is t o s t e a m i n t o w a r m e r con­ ditions. O P T I C A L P H E N O M E N A of various sorts are associated with w a t e r v a p o u r or ice crystals in t h e a t m o s p h e r e , o t h e r s being associated with dust particles. S o m e of t h e m o r e c o m m o n ones are described briefly below. A U R E O L E . A b r o w n i s h red ring s u r r o u n d i n g a bluish-white glow 2^—3^ in d i a m e t e r a r o u n d t h e sun or m o o n . T h e light is diffracted b y w a t e r d r o p s . BISHOP'S R I N G . A reddish b r o w n ring 1 0 ^ - 2 0 ^ r a d i u s , occasionally seen r o u n d the sun with a clear sky. T h e light is defracted by dust particles. C O R O N A . A n u m b e r of coloured rings r o u n d t h e sun or m o o n which a p p e a r o u t s i d e t h e aureole. T h e radius is generally less t h a n 1 0 ^ , b u t this differs with t h e size of t h e w a t e r d r o p s from which t h e light is diffracted. T h e smaller t h e d r o p s t h e larger is t h e radius of t h e c o r o n a . If c o l o u r s are visible, violet will b e nearest t h e sun and red a w a y from it. C o l o u r is rarely visible with l u n a r c o r o n a e . HALO.

Occurs when light from t h e sun or m o o n is refracted t h r o u g h ice crystals. In its m o s t c o m m o n form it is a ring of 2 2 ^ radius. When t h e c o l o u r s can be distinguished red is nearest t o t h e sun and violet o n t h e side of t h e h a l o f u r t h e s t from t h e sun. A halo of 4 6 ^ radius is s o m e t i m e s seen. T h e difference in radii is due t o t h e different angles which t h e faces of t h e ice crystal m a k e with o n e another.

MOCK SUN OR P A R H E L I O N . This m a y f o r m at t h e same a l t i t u d e as t h e sun a b o u t 220—360 from t h e real sun. When c o l o u r e d , red (as with t h e h a l o ) is nearest t h e sun. Mock m o o n s or paraselenae m a y also o c c u r . T h e f o r m a t i o n is b y refrac­ tion of light t h r o u g h ice crystals.


32

N O T E S ON M E T E O R O L O G Y

P A R H E L I C CIRCLE OR MOCK SUN R I N G . This is a n o t h e r of the halo p h e n o m e n a and is a white circle parallel t o t h e h o r i z o n at t h e same altitude as t h e sun. Its width is a b o u t 1^ a n d , u n d e r ideal c o n d i t i o n s , can be seen all r o u n d t h e hor­ izon. 22° Halo CORNONA

Brownish red ring

HALO

Aureole

Green (blue and violet rare)

RAINBOW. This forms when light from t h e sun (or m o o n ) enters a r a i n d r o p and is reflected from t h e far side. An observer looking t o w a r d s raindrops with his back t o t h e sun will see a r a i n b o w radius a b o u t 4 2 ^ provided t h a t t h e sun's altitude does n o t exceed 4 2 ^ . T h e colours of a primary b o w are red, orange, yellow, green, blue, indigo and violet, t h e red being on t h e outside. T h e second­ ary bow m a y form a b o u t 9 ° outside t h e p r i m a r y . T h e colours of t h e secondary bow are in t h e o p p o s i t e order t o those of the p r i m a r y . A secondary b o w c a n n o t form if t h e sun's altitude is m o r e than 5 4 ^ . T h e colours seen d e p e n d on the intensity of light and if the rainbow is f o r m e d by m o o n l i g h t it will nearly Secondary bow Violet RAINBOW

A well k n o w n p h e n o m e n a which is u n c o n n e c t e d with either w a t e r d r o p s or dust particles is the M I R A G E . This occurs when there is abnormal refraction. This may occur with a t e m p e r a t u r e inversion, particularly in t h e polar regions. With these con­ ditions a distant ship may be seen in triplicate with o n e of t h e images inverted. Mirages formed when the air n e x t t o t h e g r o u n d is less dense t h a n t h a t above appear t o c u t off the b o t t o m of objects so t h a t land a n d ships appear t o float in t h e air.


IV Wind Consider the t w o c o l u m n s of air AB and CD in t h e figure b e l o w . T h e s e are of equal cross sectional area, height and t e m p e r a t u r e , so t h a t t h e pressures at A a n d C are equal, as are those at Î&#x2019; a n d D. S u p p o s e t h e c o l u m n AB is w a r m e d it will e x p a n d t o AE, if t h e area remains c o n s t a n t , and although t h e pressure at A will remain u n a l t e r e d , the pressure at Î&#x2019; will have increased relative t o t h a t at D, and t o equalise these air will flow from Î&#x2019; t o D. This transfer of air from c o l u m n AB t o CD will cause a d r o p in pressure at A and a rise at C, and air will n o w flow from C t o A. Upper air

Surface air

Low (convergence)

High (divergence)

This illustrates t h e vertical circulation of air in t h a t t h e outflowing or diverging air from C is replaced by subsidising air from above, whilst t h e inflowing or converging air at A escapes t o the u p p e r levels and t h e r e c o m p l e t e s t h e circulation back t o D. T h e rate of flow of air from C t o A will d e p e n d on t h e pressure gradient (see page 3) b e t w e e n C and A. The m o v e m e n t of air straight from a high pressure t o a low pressure only occurs either very close t o the e q u a t o r or in places w h e r e t h e high and low pressure areas are small and their centres are near t o each o t h e r , (see land and sea breezes). Elsewhere, d u e to the r o t a t i o n of t h e e a r t h , there is a deflective force which prevents t h e air flowing directly from high pressure t o low. This force is k n o w n as t h e G E O S T R O P H I C or C O R I O L I S force.


34

NOTES ON M E T E O R O L O G Y

Figure (i) below is a view of t h e earth from a p o i n t above t h e N o r t h Pole (Pn) whilst figure (ii) is t h e earth viewed from a p o i n t above t h e South Pole (Ps). In each case a parcel of air is sent from p o i n t A t o w a r d s a p o i n t C in space. At t h e outset p o i n t Β o n t h e earth is in transit with A a n d C, b u t by t h e time t h e parcel of air reaches a p o i n t above B, Β will have m o v e d t o B j d u e t o t h e earth*s r o t a t i o n . T o an observer at Β in t h e N o r t h e r n Hemisphere t h e air appears to have been deflected t o t h e right, whereas in t h e S o u t h e r n Hemisphere it appears t o have been deflected t o t h e left. Rotation

(i)

Rotation

The geostrophic force for any wind velocity is zero at t h e e q u a t o r a n d m a x i m u m at t h e poles. Its force in any l a t i t u d e increases with t h e wind velocity. It always acts 9 0 ^ from the direction in which t h e wind is blowing. In the diagrams below a situation is s h o w n where t h e isobars are r u n n i n g in parallel straight lines. T h e force Ρ d u e t o t h e pressure gradient starts t h e particle of air A moving t o w a r d s the low pressure. T h e geostrophic force G acting at right angles t o t h e existing direction of m o t i o n of t h e particle causes t h e particle t o follow t h e curved p a t h A, B, C. When t h e particle reaches C t h e forces G a n d Ρ are equal and o p p o s i t e and t h e wind is blowing parallel t o t h e isobars. This is k n o w n as t h e G E O S T R O P H I C WIND. Low pressure

Low pressure P. Ρ Β

ρ ^

c J

ρ

Geostrophic wind

ρ

C

ρ 1

^ Ρ·

Á

/

High Northern hemisphere

\

kP

^^'9^ Southem hemisphere

It can be seen from t h e diagram t h a t if an observer faces t h e w i n d in t h e N o r t h e r n H e m i s p h e r e , t h e low pressure lies t o his right, whilst in t h e S o u t h e r n h e m i ­ sphere t h e l o w pressure lies t o his left. These facts were first p r o p o u n d e d by Buys Ballot a n d are k n o w n as BUYS B A L L O T ' S LAW.


N O T E S ON M E T E O R O L O G Y

35

If t h e pressure gradient is k n o w n t h e gcostrophic w i n d speed can b e calculated from t h e formula : -

g Where V Ρ

t h e geostrophic wind speed

g

t h e pressure gradient t h e density of air

Ρ cosing

2p a ; s i n 0

=

rate of t u r n t a b l e m o t i o n in l a t i t u d e (

If a scale distance is t o b e used for isobar spacing a diagram s h o w i n g wind speeds for different pressure gradients can be d r a w n . Such a diagram is s h o w n on Mctform 1 2 5 8 - t h e N o r t h Atlantic Plotting C h a r t - a n d is r e p r o d u c e d b e l o w with t h e permission of t h e controller of H.M. S t a t i o n e r y Office and t h e Director of t h e M e t ­ eorological Office.

Geostrophic wind scale for isobars at 4 mb. intervals The wind speed is in knots and the scale correct for 1000 mb., 50°F

Scale 1: 20,000.000 at latitudes 30° and 60° Lambert conformal conical projection

T o use t h e scale t a k e a pair of dividers a n d m e a s u r e t h e p e r p e n d i c u l a r distance b e t w e e n isobars spaced at 4 millibar intervals at t h e r e q u i r e d p o s i t i o n on t h e p l o t t i n g c h a r t (i.e. after drawing t h e s y n o p t i c c h a r t as detailed in C h a p t e r V I I I ) . T h e distance is n o w transferred t o t h e geostrophic wind scale so t h a t o n e leg of t h e dividers is placed on t h e vertical scale o n t h e l a t i t u d e of t h e p o s i t i o n , whilst t h e o t h e r will be at a p o i n t on or b e t w e e n t h e wind speed curves h o r i z o n t a l l y t o t h e right of t h e vertical. F o l l o w t h e curve d o w n t o t h e base line w h e r e t h e wind speed can b e read off either directly or b y inteφolation. F o r e x a m p l e if t h e distance b e t w e e n isobars 4 m b a p a r t in l a t i t u d e 4 8 ° N is represented by A B , t h e geostrophic wind is 13 k n o t s . It m u s t b e emphasised t h a t this is t h e geostrophic wind which b l o w s at and above 6 0 0 m ( 2 0 0 0 ft.), t h e surface wind speed is a b o u t 2/3 t h a t of t h e g e o s t r o p h i c .


36

N O T E S ON M E T E O R O L O G Y

It is useful to k n o w t h a t for equal pressure gradients t h e wind velocity in t h e tropics ( l a t . 1 5 ^ ) is three times as great as t h a t in t e m p e r a t e latitudes ( l a t . 5 2 ^ ) . If t h e isobars are curved, as a r o u n d a high or low pressure area, a further force will c o m e i n t o being. This force, an o u t w a r d force from t h e c e n t r e of high or low pressure, is called t h e C Y C L O S T R O P H I C F O R C E . In t h e diagrams below it can b e seen t h a t t h e c y c l o s t r o p h i c force C acts in t h e same direction as Ρ with t h e high pressure circulation, so t h a t G = Ρ + C, a n d in t h e o p p o s i t e direction to Ρ with t h e low pressure system such t h a t G = Ρ - C. T h e resulting wind is t h e G R A D I E N T WIND. It m a y be n o t e d t h a t t h e wind will be stronger a r o u n d the high pressure system t h a n a r o u n d t h e low pressure system always provided t h a t the pressure gradient is t h e same in each case.

G=P+C

The direction of wind circulation should be noted.

Geostrophic wind

G=P

C Gradient G wind

• G = P-C

Northern hemisphere


N O T E S ON M E T E O R O L O G Y

37

T h e winds, so far referred t o , have been unaffected by friction; such w i n d s will blow above 6 0 0 m e t r e s ( 2 , 0 0 0 ft.). T h e effect of friction is t o r e d u c e t h e w i n d velocity. T h e r e d u c t i o n gets progressively greater b e t w e e n 6 0 0 m e t r e s ( 2 , 0 0 0 ft.) and sea level. A t sea level the w m d velocity is only half t o t w o t h i r d s t h a t at 6 0 0 m ( 2 , 0 0 0 ft.) d e p e n d i n g on the t y p e of surface, t h e r e d u c t i o n being greater over t h e land t h a n over t h e sea. T h e sketch shows t h a t although t h e wind velocity is r e d u c e d by friction t h e pressure gradient is u n c h a n g e d . In order t h a t t h e equilibrium of p o i n t A shall be main­ tained, t h e direction of t h e wind m u s t be m o v e d t o w a r d s t h e force P. T h u s t h e surface wind is inclined t o w a r d s the low pressure a n d away from t h e high pressure. T h e angle which t h e wind m a k e s with the isobars is k n o w n as t h e A N G L E O F I N D R A U G H T . Resultant ot 'G' and 'F'

Geostrophic wind

Friction

Angle ot \^ indraught

During t h e d a y t i m e t u r b u l e n c e d u e t o surface heating causes t h e higher velocity winds at 6 0 0 m e t r e s ( 2 , 0 0 0 ft.) t o be b r o u g h t t o t h e surface and it will be n o t e d t h a t during this t i m e t h e surface wind veers and increases in velocity. By night t u r b u l e n c e is d a m p e d o u t and it will be n o t e d t h a t t h e surface wind backs and decreases in velocity. The effects m e n t i o n e d above are hardly noticeable over t h e sea as t u r b u l e n c e d u e t o surface heating is negligible. The direction of t h e t r u e wind should be n o t e d in t h e logbook and in w e a t h e r messages. It m a y be e s t i m a t e d by n o t i n g t h e direction of t h e waves. T h e a p p r o x i m a t e speed of t h e wind m a y be estimated from t h e a p p e a r a n c e of t h e sea (see Beaufort scale). The a p p a r e n t wind direction and speed is t h e r e s u l t a n t of t h e ship's course a n d speed and t h e true wind direction a n d speed. If any t w o of t h e foregoing are k n o w n t h e third can be f o u n d by p l o t t i n g a triangle of velocities. E.g. An officer on a vessel steaming N N E at 1 k n o t s observes t h e a p p a r e n t wind t o be 6 p o i n t s o n t h e p o r t b o w at 15 k n o t s . What w o u l d be t h e t r u e w i n d direction a n d speed? AB represents t h e ship's course and speed. This is t h e wind caused by t h e s h i p ; h e n c e the direction of t h e a r r o w h e a d is o p p o s i t e t o t h e c o u r s e . AC represents t h e a p p a r e n t wind direction and speed. A d o u b l e a r r o w h e a d d e n o t e s this as it is t h e resultant of t h e wind caused by t h e vessel a n d t h e t r u e w i n d . J o i n i n g Î&#x2019; t o C c o m p l e t e s t h e triangle of velocities. BC will be t h e direction and speed of t h e t r u e wind. True Wind West 15 k n o t s .


38

N O T E S ON M E T E O R O L O G Y B E A U F O R T WIND SCALE

Beaufort Scale Number

Description and limit of wind speed in k n o t s .

Sea Criterion

0

Calm Less than 1

Sea like a mirror.

1

Light air 1-3 Light breeze 4-6

Ripples with the appearance of scales are formed b u t w i t h o u t foam crests. Small wavelets, still short b u t m o r e p r o n o u n c e d , crests have a glassy appearance and d o n o t break.

3

Gentle breeze 7-10

Large wavelets. Crests begin to break. F o a m of glassy a p p e a r a n c e . Perhaps scattered w h i t e horses.

4

Moderate breeze 11-16

5

Fresh breeze 17-21

Small waves, b e c o m i n g longer; fairly frequent white horses. Moderate waves, taking a m o r e p r o n o u n c e d long form; m a n y white horses are formed. (Chance or some spray).

6

Strong breeze 22-27

Large waves begin t o form; the white foam crests are m o r e extensive everywhere (probably some spray).

7

Near gale 28-33

Sea heaps up and white foam from breaking waves begins t o be blown in streaks along t h e direction of t h e wind, (spindrift begins to be seen).

8

Gale 34^40

Moderately high waves of greater length; edges of crests break into spindrift. T h e foam is b l o w n in well m a r k e d streaks along the direction of t h e wind.

9

Strong gale 41-47

High waves. Dense streaks of foam along t h e direction of the wind. Sea begins to roll. Spray may affect visibility.

10

Storm 48-55

Very high waves with long overhanging crests. T h e resulting foam in great p a t c h e s is blown in dense white streaks along t h e direction of t h e wind. On t h e whole t h e surface of the sea takes a white a p p e a r a n c e . T h e rolling of t h e sea b e c o m e s heavy and shocklike. Visibility affected.

11

Violent storm 56-63

12

Hurricane 64-71

Exceptionally high waves. (Small and m e d i u m size ships might be for a t i m e lost t o view b e h i n d the waves). T h e sea is completely covered with long white p a t c h e s of foam lying along t h e dir­ ection of t h e wind. Everywhere the edges of t h e wave crests are blown into froth. Visibility affected. T h e air is filled with foam and spray. Sea c o m ­ pletely white with driving spray; visibility very seriously affected.

2


39

N O T E S ON M E T E O R O L O G Y There is an e x t e n d e d scale for exceptional wind as follows : Knots

Force 13

72-

80

14

81 -

89

15

90-

99

16

100 -

109

17

110 -

118

STORM W A R N I N G S are hoisted at stations on t h e coasts of t h e British Isles w h e n winds of force 8 or m o r e are e x p e c t e d shortly within 50 — 100 miles of t h e station. T h e day signal consists of a c o n e 1 m e t r e (3 feet) wide and 1 m e t r e (3 feet) high. A night t i m e signal is hoisted at a few stations and consists of a triangle of lights 1.3 m e t r e s (4 feet) wide at t h e base. If t h e gale is e x p e c t e d t o c o m m e n c e from a n o r t h e r l y p o i n t t h e c o n e or triangle of lights is hoisted p o i n t u p w a r d s . If t h e gale is e x p e c t e d from a s o u t h e r l y p o i n t t h e signal is s h o w n with its p o i n t d o w n w a r d s . Gales starting from east or west and likely to change t o a n o r t h e r l y direction will be indicated by a n o r t h c o n e whilst t h o s e likely t o change to a southerly direction will be indicated by a s o u t h c o n e . The warnings will be t a k e n d o w n only if a period of at least 12 h o u r s is e x p e c t e d t o be free of gales. Southerly gales

Northerly gales

Daytime signal

/

Night signal /

\

7¿í

WIND R O S E S are found on climatological c h a r t s and they depict t h e frequency and strength of t h e winds blowing from t h e various d i r e c t i o n s . T h e r e are m a n y forms of wind rose, each having its o w n features.


40

N O T E S ON M E T E O R O L O G Y

The Baillie rose shown below is comprehensive y e t simple. F r o m the circle t o the inner end of the wind arrow represents a scale of 5%. A thin line represents wind force 1 - 3, a double line represents forces 4 - 7 , and a broad line forces 8 upwards. T h e figures in t h e centre show t h e n u m b e r of observations, the percentage frequency of variables and t h e percentage frequency of calms in t h a t order.

Thus t h e figure shows N o r t h winds 4% light, 10% m o d e r a t e and 2% gales; NE winds 10% light, 9% m o d e r a t e ; Î&#x2022; winds 5% light, 4% gales; S winds 5% light, 5% mod­ erate; SW w m d s 6% m o d e r a t e . 7% gales; W w m d s 2% hght, 4% m o d e r a t e , 9% gales; NW winds 3% m o d e r a t e , 12% gales. T h e total n u m b e r of observations is 3 7 6 , t h e frequency of variables is 1% and there is a 2% frequency of calms. T h e figure which follows shows trie planetary wind circulation for t h e "ideal" earth which would be o n e covered all over with water.

Note: to preserve clarity only a lew wind arrows are drawn

The dotted lines show upper air circulation

T h e circulation will be modified wherever t h e r e are land masses. T h e general circulation o n t h e earth is s h o w n on t h e following pages for F e b r u a r y and A u g u s t .


NOTES ON M E T E O R O L O G Y


42

υ o

C 0) I S

CO

i! o CL Ζ

NOTES ON M E T E O R O L O G Y


43

NOTES ON M E T E O R O L O G Y As can be seen the trades move slightly north and south with the sun; a p p r o x i m a t e limits are as follows :-August

February

qo .

Atlantic D o l d r u m s N.E. Trade S.E. Trade

their

5^N - lO^N

- 25^N

lO^^N

- 30ON

5^N-25^S

Pacific D o l d r u m s N.E. Trade S.E. Trade

- δ^Ν 8^N - 25^N 40N - 3 0 ^ S

S^N - 12^N

n^N -

30^N S^N - 25^'S

15^S-30«S

Indian S.E. Trade

-25^S

The A N T I - T R A D E S arc winds which blow above a b o u t 2 5 0 0 m e t r e s ( 8 , 0 0 0 feet) in the o p p o s i t e direction to the trades on the surface. L A N D A N D SEA B R E E Z E S . As the land is heated during the d a y t i m e the air over it will be h e a t e d by con­ d u c t i o n . This heating causes a decrease in t h e density of t h e air and t h e pressure falls. The sea t e m p e r a t u r e remains m o r e or less t h e same and t h e jpressure over it is high c o m p a r e d with t h a t over t h e land. T h e pressure gradient is sufficient for air to flow from over t h e :ea t o the land; this is t h e SEA B R E E Z E . The sea breeze sets in during t h e m o r n i n g , reaches its m a x i m u m strength a b o u t 1400 h o u r s and then dies awav t o w a r d s sunset.

Pressure relatively high over sea

Air warned by conduction causes low pressure . overland

Wind blows across isobars

/

/

/

Land heated by sun

Sea temperature constant

After sunset t h e land cools rapidly and t h e air above it also cools and its density increases giving rise to an increase in pressure. T h e pressure over t h e sea is n o w low c o m p a r e d with t h a t over the land. T h e pressure gradient causes air t o flow from the land to t h e sea; this is t h e L A N D B R E E Z E . The land breeze sets in shortly after sunset and c o n t i n u e s until d a w n . Conductive cooling of air over land causes high pressure Pressure relatively low over sea Wind blows across isobars

Sea temperature constant

The sea breeze is the m o r e n o t i c e a b l e ; it reaches force 3 - 4 and gives a pleasantly cool breeze on the coast during t h e heat of t h e day. T h e land breeze is generally lighter t h a n t h e sea breeze.


44

N O T E S ON M E T E O R O L O G Y WORLD 40°

80'

m m

m m

/Jill k

m a m —

·'·^

'5

North Atlantic^ Λ PORTUGALíx^í

:::::3·7<^^;:.

¡•^^-

Ν. equatorial . Equatorial»

.Ccounter •φ y S -

GUINEA J

equatorial Equatoriar

^ b a r f

MADAGASCAR

L —

^^ar-^—

The outline of t h e world is r e p r o d u c e d from British Admiralty C h a r t N o . 5 0 1 0 with t h e permission of H.M. Stationery Office and of t h e H y d r o g r a p h e r of t h e Navy.


N O T E S ON M E T E O R O L O G Y CURRENTS 5—-

General currents ',^:^:/ί!2!2^ S.W. monsoon currents N.E. monsoon currents - - • - • • Q Warm currents Cold currents Ice limits Fog areas

z^^^-^n-^-^

lOYASIWO

45


46

NOTES ON M E T E O R O L O G Y

N.E. MONSOON O F THE CHINA SEAS AND INDIAN O C E A N . During t h e n o r t h e r n winter t h e Asian c o n t i n e n t is cooled and an intense high pressure area forms over Eastern Siberia. The winds circulating r o u n d this form tne N.E. M o n s o o n . In t h e n o r t h e r n part of the China Sea t h e pressure gradient is large and winds are likely t o be North Westerly force 6 - 7 , further south where t h e pressure gradient is smaller t h e winds will be Northerly force 5 - 6. In the Bay of Bengal and Arabian Sea winds are N.E.*ly force 3 - 4 . Rainfall of the showery t y p e is likely on windward coasts especially in t h e China Sea. As t h e air is of Polar origin t h e relative h u m i d i t y will be low. T h e N.E. m o n s o o n b e c o m e s established in early December and c o n t i n u e s until April.

C2i

N.W. MONSOON O F A U S T R A L I A . During the S o u t h e r n s u m m e r a low pressure forms over t h e deserts of Australia. The pressure gradient from t h e high over Siberia is c o n t i n u e d to the a f o r e m e n t i o n e d low. The N.E. m o n s o o n winds blow d o w n to and past the e q u a t o r . On crossing the e q u a t o r these winds are deflected to the left giving N.W. winds on the coasts of Australia. Having blown over t h o u s a n d s of miles of ocean the air has a high relative h u m i d i t y and periods of rain may be e x p e c t e d . The wind force is unlikely to exceed


N O T E S ON M E T E O R O L O G Y

47

force 6 - 7 and is frequently less. Intensification or otherwise of t h e Siberian high will affect this m o n s o o n as well as t h e N.E. m o n s o o n . The d u r a t i o n of this m o n s o o n is similar t o t h a t of t h e N . E . m o n s o o n . S.W. MONSOON O F T H E INDIAN O C E A N A N D C H I N A SEA. During t h e n o r t h e r n s u m m e r t h e Asian c o n t i n e n t is w a r m e d ; t h e high pressure over Siberia declines and is replaced by a low pressure over N.W. India. There is a pressure gradient b e t w e e n t h e high pressure over the S o u t h Indian Ocean and the lows over Africa and India. This causes t h e S.E. T r a d e t o blow up t o the e q u a t o r , and on crossing it, t h e wind near t h e African coast circulates r o u n d t h e low over Africa and brings rainfall t o East Africa. Away from t h e coast t h e wind is deflected to the right and blows from a south westerly direction. In t h e Arabian sea t h e wind is S.W. force 7 â&#x20AC;&#x201D; 8, in t h e Bav of Bengal S.W. winds of force 6 - 7 can be e x p e c t e d , whilst in t h e s o u t h e r n part of t h e China Sea t h e wind is S'ly force 4 - 5 . T h e n o r t h e r n p a r t of t h e China Sea has light S.E.'ly winds. T h e air forming t h e S.W. m o n s o o n has travelled over t h o u s a n d s of miles of ocean and is saturated. This results in heavy rain, especially near t h e coasts, and p o o r visibility. The S.W. m o n s o o n sets in during J u n e and lasts until O c t o b e r , t h e early m o n t h s being the worst.

1004 1008 1012 Equator 1016

1020


48

NOTES ON METEOROLOGY

S.W. WINDS O F WEST A F R I C A . During t h e n o r t h e r n s u m m e r t h e l o w pressure over Africa m o v e s n o r t h of t h e e q u a t o r . T h e pressure gradient b e t w e e n t h e high over t h e S o u t h A t l a n t i c and t h e low over Africa causes t h e S.E. Trade t o be deflected a n d it arrives on t h e West Coast of Africa as a S.W. wind often called a m o n s o o n , although its intensity is n o t h i n g like t h a t of t h e S.W. m o n s o o n of India.

1020

N.E. WINDS O F B R A Z I L . The S o u t h American c o n t i n e n t is h e a t e d during t h e s o u t h e r n s u m m e r and a low pressure area forms over N . E . Brazil. T h e S.E. T r a d e is deflected t o circulate a r o u n d this low giving East t o N . E . winds on t h e coasts. Having travelled over t h o u s a n d s of miles of sea, the relative h u m i d i t y of t h e air is high, giving heavy rainfall on t h e coast.

Equator


2 eg

S5

NOTES ON M E T E O R O L O G Y 49


50

N O T E S ON M E T E O R O L O G Y Further local winds

Low

35°S


N O T E S ON M E T E O R O L O G Y

51

KATABATIC A N D A N A B A T I C E F F E C T S . When the earth's surface cools by night, the air n e x t t o t h e surface is also cooled and thus its density is increased. If this cold air is on high g r o u n d there is a t e n d e n c y for it to sink d o w n to lower ground. If t h e high ground is a cliff t o p or a high coastal plateau, t h e downflowing cold air will move h o r i z o n t a l l y when it reaches sea level to form a K A T A B A T I C WIND, which may reach force 6 - 7 . Air cooled by conduction

Sea

The Katabatic wind is c o m m o n off t h e coasts of Greenland where cooling is helped by the snow covered land surface. Also experienced in the Adriatic and m a n y other areas having high land adjacent to the coast. The A N A B A T I C WIND is less noticeable as it blows u p t h e sides of valleys with considerably less force than the K A T A B A T I C wind. T h e air at the b o t t o m of the valley is warmed by c o n d u c t i o n from the h e a t e d land during t h e day, and this air, being less dense than the air above it, takes the easiest path to the top of the valley by following the warm sides. Anabatic wind


52

N O T E S ON M E T E O R O L O G Y

F O H N WIND E F F E C T . This is felt on the lee side of high ground and results in t e m p e r a t u r e s o n t h e lee side being higher than those on t h e weather side if t h e air passing over t h e high ground has been cooled to below its d e w p o i n t . The sketch below shows m a r i t i m e tropical air rising over a m o u n t a i n range. As it rises t h e air is first cooled t o its d e w p o i n t and, as precipitation occurs, t h e absolute h u m i d i t y decreases. Due t o the decrease in absolute h u m i d i t y t h e d e w p o i n t t e m p e r a t u r e on t h e lee side will be considerably lower than t h a t on t h e w e a t h e r side, which m e a n s that t h e c o n d e n s a t i o n level will be higher on the lee side. Below t h e c o n d e n s a t i o n level the air will change its t e m p e r a t u r e at t h e D.A.L.R. whilst above it, it will change its t e m p e r a t u r e at t h e S.A.L.R. As t h e c o n d e n s a t i o n level on the lee side is higher t h a n on the weather side it follows t h a t t h e descending air will warm at a greater rate than t h e ascending air cools; this results in higher t e m p e r a t u r e s on the lee side. The difference in t e m p e r a t u r e s can be considerable d e p e n d i n g on t h e height of the land and the h u m i d i t y of t h e rising air.

Lee side Air sinks and warms adiabatically

Weather side Rising air cools adiabatically

Evaporation level OX (32°F)'

Condensation level^ "lO°C ( 5 0 ° F ) ^ ^ \ D.A.L.R. 18°C (64°F)

27°C (80 Note- The S.A.L.R. has been assumed constant at 0.5X/100 m (2.7°F/1000 ft) " whereas it increases with height. The difference between weather and lee side temperatures is thus slightly over-emphasised in the above diagram.

LOCAL WINDS. !n m a n y parts of t h e world seasonal a n d occasional winds are given local names; some of t h e better k n o w n of these are illustrated on pages 4 9 and 50.


Isobaric Systems A N T I C Y C L O N E is t h e n a m e given t o a system of high pressure. T h e wind circulation a r o u n d this system will be clockwise in t h e -northern h e m i s p h e r e and anti­ clockwise in t h e s o u t h e r n h e m i s p h e r e . A n t i c y c l o n e s are associated with small pressure gradients a n d c o n s e q u e n t l y light winds. As t h e y are areas of subsidence ramfall is unlikely. They m a y be classed as either cold or w a r m and each of these m a y be either p e r m a n e n t (long lasting) or t e m p o r a r y .

\

{

Air subsides at about 300 m (1000 ft.) per day

Divergence at surface

When t h e high pressure is b r o u g h t a b o u t by t h e air over an area being denser t h a n t h a t nearby a cold a n t i c y c l o n e is f o r m e d , cold air having a greater density t h a n w a r m air. On t h e o t h e r h a n d a high pressure can be f o r m e d by larger t h a n n o r m a l a m o u n t s of warm air over an area, in whicn case a w a r m a n t i c y c l o n e is f o r m e d . T h e cold a n t i c y c l o n e gives brilliant frosty w e a t h e r in t h e c e n t r e b u t near t h e o u t e r regions dull foggy w e a t h e r is frequently e x p e r i e n c e d , as t h e r e is usually a t e m p ­ erature mversion close t o t h e surface. T h e inversion is f o r m e d b y t h e subsiding air warming adiabatically. T h e best k n o w n of t h e cold a n t i c y c l o n e s is t h a t over Siberia in winter which is a p e r m a n e n t o n e . Cold a n t i c y c l o n e s are unlikely over oceans or over t h e land in s u m m e r ; if t h e y d o form they rapidly b e c o m e w a r m ones or disperse. T e m p o r a r y cold anticyclones occur b e t w e e n t h e m i d d l e l a t i t u d e depressions a n d bring a w e l c o m e t h o u g h brief spell of very bright w e a t h e r . Occasionally t n e ridge with which t h e t e m p o r a r y a n t i c y c l o n e is associated will intensify t o build u p an a n t i c y c l o n e c o m p o s e d entirely of cold air. If this occurs over t h e land in w i n t e r it m a y merge with the c o n t i n e n t a l high and be k e p t as a cold a n t i c y c l o n e by c o n t i n u o u s cooling of t h e land. If it occurs over t h e land in s u m m e r or over t h e sea at any t i m e it is h a r d l y likely t o persist as a cold a n t i c y c l o n e ; it m a y collapse or b e c o m e a w a r m a n t i c y c l o n e .


54

N O T E S ON M E T E O R O L O G Y

Warm anticyclones occur w h e n t h e air in t h e t r o p o s p h e r e is w a r m e r t h a n t h e surrounding air and t h e high pressure is caused by an excessive d e p t h of this w a r m air. P e r m a n e n t warm anticyclones are those which are f o u n d over t h e o c e a n s and are generally referred t o as t h e sub-tropical highs. T h e y are c o m p o s e d of w a r m dry air and the visibility is excellent. T e m p o r a r y warm anticyclones m a y occur when there is an e x t e n s i o n from t h e sub-tropical high or w h e n a t e m p o r a r y cold a n t i c y c l o n e warms adiabatically. Over t h e land t h e air is very dry and by day fine h o t w e a t h e r will be experienced during t h e summer. By night there m a y be sufficient radiation cooling t o give fog, this being particularly likely during t h e a u t u m n . Over t h e sea advection fog may occur at any time b u t this is m o s t p r o b a b l e during spring a n d early s u m m e r . T h e pressure tendencies of t h e anticyclone are frequently helpful in forecasting the m o v e m e n t of depressions, t h e general rules for these being given on page 6 2 A R I D G E O F HIGH P R E S S U R E is an extension of an a n t i c y c l o n e and m a y b e in any direction from its p a r e n t . During t h e winter t i m e ridges from t h e sub-tropical high and from a high over Greenland frequently join to give a c o n t i n u o u s period of N o r t h e r l y winds over t h e British Isles(see page 6 7 ) . T h e ridge may also be referred t o as a wedge.

Low

AIR MASSES arc large areas of air which have t h e same characteristic t h r o u g h o u t . F o r such c o n d i t i o n s t o occur t h e air m u s t remain over an area for several d a y s . T h e ideal conditions will be f o u n d where t h e r e is a p e r m a n e n t a n t i c y c l o n e a n d t h e s e arc t h e principal S O U R C E R E G I O N S for t h e air masses. Air masses may be cither w a r m or cold d e p e n d i n g o n w h e t h e r t h e i r source is tropical or polar. They arc further described as being cither m a r i t i m e or c o n t i n e n t a l . The following table and t h e diagram descriDc and show t h e a p p r o x i m a t e cracks of air masses which affect t h e British Isles.


AIR MASS AND ABBREVUTION

NOTES ON M E T E O R O L O G Y

55

SOURCE TEMPERATURE RELAΉVε HUMIDITY

AIR CONDITION

SKY

Maritime tropical Azores mT

High

High

Covered with stratus

Stable

Continental tropical cT

Sahara

Very high

Low

Clear

Stable

Maritime polar mP

Greenland

Low-high depending on time over sea

Medium

Cumulus cloud

Unstable

Continental polar

cP

Siberia

Very low

Low

Possible stratus ttuOveh turbulence

SUble

Arctic

A

Arctic Ocean

Very low

low -

Cumulus

Unstable

Biedium

It can be seen t h a t each air mass has a different characteristic t o t h e o t h e r air masses a n d c o n s e q u e n t l y t h e y will n o t m i x with o n e a n o t h e r . T h e b o u n d a r y of each air mass is generally well definea and on t h e g r o u n d it is k n o w n as a F R O N T . T h e b o u n d a r y above t h e g r o u n d is k n o w n as a F R O N T A L S U R F A C E . Most fronts are in m o t i o n , b u t if a front is stationary or behaving as if it were s t a t i o n a r y , it is referred t o as being QUASI-STATIONARY.


56

N O T E S ON M E T E O R O L O G Y

When t h e a p p r o a c h i n g air is colder t h a n t h e present air t h e b o u n d a r y will be a C O L D F R O N T whereas if t h e a p p r o a c h i n g air is w a r m e r t h a n t h a t at present t h e b o u n d a r y

Warm front symbol (red line on working chart)

Cold front symbol (blue line on working chart)

T h e general line of d e m a r c a t i o n b e t w e e n polar and tropical air masses is k n o w n as the P O L A R F R O N T . T h e density of t h e polar air is greater t h a n t h a t of t h e tropical and t h e frontal surface c a n n o t be vertical, b u t is inclined p o l a r w a r d s so t h a t t h e tropical air overrides the polar air. T h e slope of t h e frontal surface is a b o u t 1 in 150.

Polar air Cold and dense

Tropical air \Narm with high absolute humidity

The warm air is moving relatively faster t h a n t h e cold air and every n o w and again it forces its way i n t o t h e cold air m a k i n g a bulge in t h e polar front. This is t h e c o m ­ m e n c e m e n t of a F R O N T A L D E P R E S S I O N , for as t h e warm less dense air p u s h e s i n t o t h e cold denser air t h e pressure falls. T h e sketches show t h e successive stages in t h e f o r m a t i o n and degeneration of t h e depression. It will be seen t h a t w h e n t h e depression is fully developed t h e air b e h i n d t h e cold front is moving faster t h a n t h e air in t h e w a r m sector. This causes t h e w a r m sector t o be u n d e r c u t and lifted off t h e g r o u n d ; t h e front is t h e n said t o be O C C L U D E D . T h e air ahead of and b e h i n d t h e occlusion was originally of t h e same t e m p e r a t u r e b u t circumstances ( t h e w a r m sector intervening and t h e passage of t i m e ) cause a change in their t e m p e r a t u r e s . If t h e air b e h i n d t h e occluded front is colder t h a n t h a t in front the occlusion is said t o be a cold o n e , b u t if t h e air b e h i n d t h e occlusion is w a r m e r t h a n that ahead of it, t h e occlusion is a w a r m o n e .

Blue line L. behind Îś. purple on working charts

Red line behind purple on working Chans

Warm occlusion

Cold occlusion


57

N O T E S ON M E T E O R O L O G Y Sketches illustrating the life of a frontal depression High

Cold air

-^Warm

Warm

Warm air pushes across polar front

Fronts start to occlude

Warm

/ / ' I

Warm

Cold air lifts warm y off ground ' \

Warm

Warm air pushed out

High

T h e depression t e n d s t o move in a N.E.My direction in t h e N o r t h e r n H e m i s p h e r e and S.E.'ly in t n c S o u t h e r n H e m i s p h e r e . However, if t r a c k s of depressions are studied, they will be seen t o be in every direction, a l t h o u g h t h e above directions p r e d o m i n a t e . T e m p e r a t e l a t i t u d e depressions moving w e s t w a r d s are rare and even if t h e y d o m o v e in this direction, they will n o t d o so for any length of t i m e .


58

N O T E S ON M E T E O R O L O G Y

If t h e way in which the depression is formed is considered carefully t h e changes of weather which occur as t h e depression passes any p o i n t will be a p p a r e n t . T h e reader should check his d e d u c t i o n s for the w e a t h e r changes as t h e depression m o v e s along t h e line AB with those shown in t h e table o p p o s i t e . Plan Cc

Ci Ac,,

Sc J Heaviest ^'precipitation ^/Ayy^

Cu

As

/Ns

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1

— — -

1

Warm sector

-

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Ί

1

1

fwarm front

I.'.

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ti

NOTES ON M E T E O R O L O G Y

o »4

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ts "i l g C

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59


60

N O T E S ON M E T E O R O L O G Y

A s o u t h e r n heniisphere frontal depression is formed in exactly t h e same way as one in t h e n o r t h e r n h e m i s p h e r e â&#x20AC;&#x201D; namely oy a w a r m m a r i t i m e air mass e n c r o a c h i n g on a cold polar air mass. T h e reader should a t t e m p t t h e drawing of a f o r m a t i o n of such a depression a n d t h e n c o m p a r e his fully developed depression with t h e plan view s h o w n be o w . The section along AB is exactly t h e same as t h a t s h o w n on page 58 and t h e w e a t h e r changes on page 59 are also applicable e x c e p t t h a t for t h e wind. T h e wind will back from N.W.Uy t o S.W.'ly as t h e depression a p p r o a c h e s a n d passes. It s h o u l d be n o t e d t h a t t h e feathers on t h e wind arrows m t h e s o u t n e r n h e m i s p h e r e should always be placed on t h e left h a n d side of t h e wind a r r o w w h e n facing t h e w i n d .

Sub-tropical high

Plan of southern hemisphere frontal depression

PLAN

OF SOUTHERN HEMISPHERE FRONTAL DEPRESSION


61

N O T E S ON M E T E O R O L O G Y

F R O N T O L Y S I S is t h e t e r m applied t o "spreading" of the i s o t h e r m s at a front which means t h a t t h e front is less active and t h e depression will b e filling. This occurs when t h e isobars are cyclonic. F R O N T O G E N E S I S is t h e t e r m applied t o t h e " c l o s i n g " of t h e isotherms at a front." This m e a n s t h a t t h e activity at t h e front is increasing with a deepening of t h e depression or m o r e likely the f o r m a t i o n of a secondary depression. It occurs when t h e isobars are anticyclonic. T h e ideal c o n d i t i o n s for t h e f o r m a t i o n of a S E C O N D A R Y D E P R E S S I O N are usually f o u n d on the cold front b e t w e e n t h e p o i n t w h e r e t h e warm and cold fronts are occluded and t h e p o i n t where the front runs parallel t o t h e isobars. T h e s e c o n d a r y , which may only show as a bulge in the isobars, moves a r o u n d t h e primary ( t h e original depres­ sion) in an anticlockwise direction and t e n d s to d e e p e n as the p r i m a r y fills. T h e p h e n o m e n a associated with a secondary are often m o r e violent t h a n those a r o u n d t h e p r i m a r y .

Secondary likely here /

Back bent occlusion often deepens depression

Secondary depressions may form where there is a kink or wave on t h e cold they m a y ' t h e n be referred to as WAVE D E P R E S S I O N S . As the cold front of a frontal depression 'trails' t o join up with t h e polar front it is quite likely t h a t when t h e n e x t depression is already forming on t h e p o l a r front, t h e trailing cold front will join the warm front of t h e new depression. This m a y be repeated with several depressions, and is k n o w n as a F A M I L Y O F D E P R E S S I O N S which m a v be

front;

Low "A"

Most of t h e sketches show t h e frontal depression in t h e N o r t h e r n H e m i s p h e r e . The m a n n e r in which it is formed in t h e S o u t h e r n H e m i s p h e r e is similar, as s h o w n opposite. The reader should have no difficulty in drawing t h e o t h e r sketches for t h e S o u t h e r n Hemisphere.


62

N O T E S ON M E T E O R O L O G Y

Depressions may also be of non-frontal origin, these being f o r m e d w h e n there is either a large a m o u n t of surface h e a t i n g or in t h e lee of m o u n t a i n ranges or w h e r e t h e r e is great vertical instability. T h e latter t y p e o c c u r particularly in polar air s t r e a m s ; t h e depression thus formed may remain small or it m a y grow t o q u i t e a large size. A k n o w l e d g e of the m o v e m e n t of depressions is essential if any a t t e m p t is t o be made t o forecast t h e weather. Although each depression m u s t be t r e a t e d on the merits of the general s y n o p t i c situation t h e following general rules can usually be applied. 1.

A frontal depression t e n d s t o m o v e in a direction parallel t o t h e isobars in t h e w a r m sector. It moves at a speed a p p r o x i m a t e l y equal t o t h a t of t h e geostrophic wind in t h e w a r m sector. T h e warm front will move s o m e w h a t slower t h a n this whereas t h e cold front will move s o m e w h a t faster. T h e greater t h e angle which t h e front makes with t h e warm sector isobars t h e faster it moves.

2.

An occluded depression moves slowly or is often s t a t i o n a r y .

3.

A depression which has t h e same pressure gradient on all sides is s t a t i o n a r y .

4.

A depression t e n d s t o m o v e in t h e direction of t h e strongest winds circulating r o u n d it.

5.

A depression t e n d s t o move t o w a r d s t h e isallobaric low, i.e. w h e r e t h e pressure is falling fastest.

6.

Small depressions t e n d t o move quickly in t h e general direction of t h e air stream in which they form.


63

N O T E S ON M E T E O R O L O G Y 7.

Secondary depressions move cyclonicall)^ a r o u n d their p r i m a r y and if t w o depressions are m o r e or less t h e same size f o r m i n g a dumb-bell shape these t e n d t o revolve cyclonically a b o u t a c o m m o n c e n t r e s o m e w h e r e b e t w e e n t h e

8.

Elongated or elliptical shaped depressions t e n d t o m o v e in a direction b e t w e e n t h e cnrection of t h e major axis and t h e direction of t h e isallobaric l o w .

9.

Depressions having wind less t h a n n o r m a l t e n d t o d e e p e n a n d t h o s e with winds greater t h a n n o r m a l t e n d t o fill. A T R O U G H O F LOW P R E S S U R E , s o m e t i m e s called a V-shaped depression, is an ex­ tension of a depression i n t o a high pressure area. It nearly always p o i n t s t o w a r d s t h e e q u a t o r . T h e trough m a y b e frontal, in which case t h e r e is a marlced change in t h e direction of t h e isooars on t h e trough line, or non-frontal w h e r e t h e isobars are well rounded. F r o n t a l t r o u g h s m a y be associated with w a r m , cold, secondary cold or o c c l u d e d fronts and t h e w e a t h e r changes will be similar t o t h o s e associated with tíiese fronts.

\%

Frontal

1000 1004

Non-trontal


64

N O T E S ON M E T E O R O L O G Y

A LINE S Q U A L L occurs when t h e cold front is well m a r k e d . In o t h e r w o r d s , there is a large t e m p e r a t u r e difference b e t w e e n t h e w a r m and cold air. T h e line squall is c o m m o n off t h e East Coast of South America during t h e spring where it is k n o w n as a P A M P E R O . In S o u t h Eastern Australia a similar squall is k n o w n as a S O U T H E R L Y B U S T E R .

Cold polar air

Warm tropical air

The following changes in the w e a t h e r m a y be e x p e c t e d at the passage of a line squall. (a)

S u d d e n rise in b a r o m e t r i c pressure.

(b)

Sudden drop in t e m p e r a t u r e

(c)

Change in wind direction and increase in velocity.

(d)

Heavy black cloud often appearing to roll along j u s t above t h e surface.

(e)

Heavy rain.

(f)

T h u n d e r and lightning.

THE T O R N A D O of North America also forms on a cold front w h e r e t h e r e is strong convergence. T h e winds of hurricane force circulate cyclonically r o u n d t h e c e n t r e where there is a very low pressure and which is small in diameter, seldom m o r e t h a n a c o u p l e of h u n d r e d m e t r e s (yards). The path rarely exceeds 2 0 miles, b u t everything on t h e track is likely to have suffered great damage:â&#x20AC;&#x201D; roofs ripped off, trees u p r o o t e d and w i n d o w s blown o u t . Similar t o r n a d o e s occur infrequently in England. A similar b u t smaller p h e n o m e n o n over the sea forms a W A T E R S P O U T . T h e low pressure and t h e convergence cause t h e sea t o be sucked u p t o w a r d s t h e lower p o r t i o n of t h e c u m u l o n i m b u s cloud from which a funnel shaped cloud goes d o w n t o w a r d s t h e sea. T h e t o r n a d o e s of West Africa are very different from t h o s e of N o r t h America, in that they are violent squalls of wind, w i t h o u t any cyclonic circulation, b l o w i n g o u t w a r d from an a p p r o a c h i n g t h u n d e r s t o r m . They occur mainly during t h e rainy season.


N O T E S ON M E T E O R O L O G Y

65

When t w o anticyclonic systems and t w o cyclonic systems are diametrically opposed there is an area in t h e centre of t h e four systems which c a n n o t be considered as a high pressure or a low pressure area. This area is k n o w n as a COL and t h e pressure is lower than t h a t r o u n d the high pressures and higher t h a n t h a t r o u n d the low pressures. In a Col the pressure gradients are small giving light variable winds. In general the relative h u m i d i t y is high and there may be fog, or there m a y be t h u n d e r s t o r m s .

1020

1016

1012

When the distance from a centre of high or low pressure is large t h e isobars may run in parallel straight lines over a large area, a situation which is k n o w n as STRAIGHT ISOBARS. The weather associated with straight isobars is usually of t h e changeable t y p e , although in certain situations a spell of fine w e a t h e r can be e x p e c t e d . The sketch on page 66 shows pictorially t h e following principal isobaric forms. (a)

Cyclonic

(b)

Secondary

(c)

Trough

(d)

Anticyclonic

(e)

Ridge

(f)

Col

(g)

Straight isobars.


66 NOTES ON METEOROLOGY


N O T E S ON M E T E O R O L O G Y

67

T h e disposition of anticyclones a r o u n d t h e British Isles gives rise t o N o r t h e r l y , Easterly, S o u t h e r l y , and Westerly t y p e s of w e a t h e r . It is emphasised t h a t a n o r t h e r l y wind does n o t of itself indicate a Northerly t y p e of w e a t h e r ; t h e r e m u s t also be t h e right disposition of high and low pressure areas. N O R T H E R L Y TYPE occurs when a high pressure over t h e G r e e n l a n d - I c e l a n d area has extensions t o w a r d s t h e Azores high, with relatively low pressure over Scandinavia. Most likely in Spring and early S u m m e r . T h e w e a t h e r is cold and bright with s h o w e r s of rain, sleet or snow on high g r o u n d and e x p o s e d eastern coasts.

1028

1024

1020 1020

1016

1000


68

N O T E S ON M E T E O R O L O G Y

E A S T E R L Y TYPE occurs When there is a high pressure over Scandinavia which e x t e n d s towards Iceland. Most likely in winter and spring when very cold easterly winds flow over t h e c o n t i n e n t and t o the British Isles. There is heavy frost by day and night. By night skies are usually clear whilst by day a layer of t u r b u l e n t cloud (low stratus) frequendy covers the sky. Due t o the A d a n t i c depressions being prevented from following their usual track up to the Norwegian Sea, they f r e q u e n d y move south to pass over F r a n c e where t h e pressure is s o m e w h a t lower. As these depressions pass t o t h e south of t h e British Isles, easterly gales may be experienced together with prolonged snowfall over S o u t h e r n England. If t h e Scandinavian high is joined t o t h e Siberian high t h e Easterly t y p e weather will be very persistent. When this t y p e occurs in s u m m e r very warm w e a t h e r will be e x p e r i e n c e d , b ' t is unlikely t o last long. 1024

1028

1032


N O T E S ON M E T E O R O L O G Y

69

S O U T H E R L Y TYPE occurs mainly during t h e a u t u m n and early winter w h e n there is a high pressure over t h e c o n t i n e n t . T h e British Isles experiences a w a r m , h u m i d southerly to south westerly airstream, which is associated with the w a r m sectors of depressions o n

1000

1004

1008

1012


70

N O T E S ON M E T E O R O L O G Y

W E S T E R L Y TYPE m a y occur at any t i m e . In winter changeable w e a t h e r may be expected as t h e high pressure is situated well t o t h e south over the Azores, and t h e weather is influenced by passing depressions. In s u m m e r when t h e subtropical high pressure area is often situated well north of the Azores, fine sunny weather with light westerly winds can be e x p e c t e d .

1016

1028

1024

>.··.·.

1020


VI Tropical Storms It will be r e m e m b e r e d t h a t t h e air circulations of t h e N o r t h e r n a n d S o u t h e r n hemispheres are in o p p o s i t e directions, so t h a t disturbances which form in o n e hemi­ sphere c a n n o t cross i n t o t h e o t h e r h e m i s p h e r e . T h e area near t h e e q u a t o r is o n e of convergence as t h e N o r t h e r n H e m i s p h e r e N.E. T r a d e wind blows t o w a r d s it, as does t h e S.E. T r a d e of t h e S o u t h e r n H e m i s p h e r e . The general n a m e for this area is t h e I N T E R T R O P I C A L C O N V E R G E N C E Z O N E ( I T C Z ) , also k n o w n as t h e D O L D R U M S . When t h e ITCZ is well t o t h e n o r t h o r south of t h e e q u a t o r t h e change in direction of t h e trades after crossing t h e e q u a t o r will cause very s t r o n g convergence currents and it is possible t h a t a cyclonic d i s t u r b a n c e will form in this area. T h e possioility is increased w h e n t h e ITCZ is in t h e vicinity of islands w h e n local surface h e a t i n g of air of high h u m i d i t y gives rise t o very u n s t a b l e c o n d i t i o n s . L o w pressure areas frequently o c c u r in t h e ITCZ b u t cyclonic circulations c a n only result if t h e geostrophic force is sufficiently large ( t h e r e is n o g c o s t r o p h i c force o n t h e e q u a t o r ) a n d this is unlikely in l a t i t u d e s less t n a n 5 ^ . T h e cyclonic d i s t u r b a n c e , o n c e f o r m e d , is k n o w n as a T R O P I C A L R E V O L V I N G STORM (T.R.S.) whose d i a m e t e r varies b e t w e e n 5 0 a n d 8 0 0 miles, 5 0 0 miles being an average. Wind speeds of over 1 3 0 k n o t s m a y be e x p e r i e n c e d in t h e s t o r m field wnich should be avoided if at all possible. After f o r m a t i o n Dctwccn 5 ^ a n d 1 0 ^ of l a t i t u d e t h e storm moves westwards a t 1 0 - 1 2 k n o t s until reaching t h e tropic w h e r e it slows d o w n before recurving eastwards a n d p r o c e e d i n g a t 1 5 - 2 0 k n o t s t o t h e higher l a t i t u d e s . Tropical revolving s t o r m s d o n o t usually cross t h e land, b u t w h e n t h e y d o , t h e supply of warm m o i s t air necessaiv t o their e x i s t e n c e is c u t off a n d t h e y t e n d t o fill. If tney r e t u r n t o t h e sea t h e y usually d e e p e n again. Wind velocities vary greatly d e p e n d i n g o n t h e pressure g r a d i e n t . Winds of 1 3 0 k n o t s or over m a y be e x p e r i e n c e d . T h e strongest w i n d s are usually f o u n d j u s t abaft t h e trough line in t h e dangerous semi-circle. T h e velocity is greatest h e r e b e c a u s e t h e s t o r m tends t o force itself m t o t h e sub-tropical high pressure a n d t h e r e f o r e t h e isobars arc p a c k e d m o r e closely t o g e t h e r , giving a greater pressure gradient and w i n d t h a n in the navigable semi-circle w h e r e t h e isobars are n o t so p a c k e d . A similar p a c k i n g of t h e isobars takes place o n t h e equatorial side of m a n y t e m p e r a t e l a t i t u d e depressions. In t h e central p a r t of t h e s t o r m ( t h e e y e o r v o r t e x ) t h e winds are q u i t e light, the sky is usually fairly clear as well, a l t h o u g h c o n d i t i o n s arc far from p l e a s a n t as ^crc is usually a high confused swell. It m u s t b e emphasised t h a t n o t all s t o r m s follow such an ideal path as t h a t out­ lined above a n d s h o w n in t h e diagram o n page 7 3 . R e f e r e n c e t o c h a r t s s h o w i n g s t o r m tracks shows w h a t varied p a t h s a T . R . S . c a n t a k e , b u t if t h e rules for avoiding storms are followed t h e s t o r m c e n t r e c a n a l m o s t always b e avoided.


72

N O T E S ON M E T E O R O L O G Y

The following table shows t h e areas in which tropical s t o r m s are likely t o be e n c o u n t e r e d , t o g e t h e r with t h e m o s t likely season and t h e local n a m e . It will b e n o t e d that t h e T.R.S. is n o t f o u n d in t h e South Atlantic or t h e eastern side of t h e S o u t h Pacific. Reference t o t h e charts on pages 4 1 and 4 2 will show t h a t t h e ITCZ is at no time south of 5^S in these areas.

AREA

SEASON

NAME

N o r t h Atlantic Ocean Western side

Hurricane

June

to

November

North

Hurricane or Cordonazo

June

to

November

Typhoon Baguios

All t h e year b u t greatest frequency and intensity J u n e to November.

Pacific

Ocean Eastern side

Western side

or

South

Pacific

Ocean Western side

Hurricane

December

to

April

South

Indian

Ocean Eastern side

Willy-willy

December

to

April

Western side

Cyclone

December

to

April

and Arabian Sea

Cyclone

June and but they during monsoon

Bay

of

Bengal

November m a y occur the S.W. season.

T h e following t e r m s are in c o m m o n usage w h e n reference is m a d e t o a T . R . S . , some are s h o w n in t h e drawings o n t h e following page : â&#x20AC;&#x201D; PATH. T h e d i r e c d o n in which t h e s t o r m is moving. T R A C K . T h e area which t h e s t o r m c e n t r e has traversed. STORM F I E L D . T h e h o r i z o n t a l area covered by t h e cyclonic c o n d i t i o n s of t h e s t o r m . S O U R C E R E G I O N . T h e region w h e r e t h e s t o r m first forms. V E R T E X O R C O D . T h e furthest westerly p o i n t reached by t h e s t o r m c e n t r e . EYE O F T H E S T O R M . T h e s t o r m c e n t r e . BAR O F T H E S T O R M . T h e advancing edge of t h e s t o r m field. A N G L E O F I N D R A U G H T . T h e angle which t h e w i n d m a k e s with t h e isobars. V O R T E X . T h e central calm of t h e s t o r m . D A N G E R O U S SEMI-CIRCLE.

T h e half of t h e s t o r m which lies t o t h e right of t h e p a t h

in t h e N o r t h e r n H e m i s p h e r e and t o t h e left of t h e p a t h in t h e S o u t h e r n H e m i s p h e r e . DANGEROUS QUADRANT.

T h e leading p o r d o n of t h e d a n g e r o u s s e m i - c i r d e w h e r e

t h e winds b l o w t o w a r d s t h e p a t h .


73

NOTES ON M E T E O R O L O G Y

N A V I G A B L E SEMI-CIRCLE. T h e half of t h e s t o r m which lies t o t h e left of t h e p a t h in t h e N o r t h e r n Hemisphere and t o t h e right of t h e p a t h in t h e S o u t h e r n Hemi­ sphere. T R O U G H L I N E . A line t h r o u g h t h e c e n t r e of t h e s t o r m at right angles t o t h e p a t h . T h e dividing line b e t w e e n falling and rising pressure. Path

Vertex

Equator Track

Vertex

Navigable semi-circle


74

NOTES ON M E T E O R O L O G Y Path

Northern hemisphere

Track

Southern hemisphere

Navigable


N O T E S ON M E T E O R O L O G Y

75

Some or all of t h e following signs will be evident as a T R S a p p r o a c h e s . L 2.

A swell for n o a p p a r e n t reason. This may be felt up t o 1 0 0 0 miles from t h e storm centre. Irregularity in t h e diurnal range of t h e b a r o m e t e r . In t h e tropics a baro­ m e t e r reading appreciably ( 3 m b ) differently from t h e c o r r e c t e d reading for t h a t t i m e as shown on t h e w e a t h e r c h a r t , should be regarded with suspicion. F r e q u e n t l y , a slow fall occurs b e t w e e n 5 0 0 and 120 miles from t h e c e n t r e ; the diurnal range is still n o t i c e a b l e on t h e barograph trace. Between 120 and 6 0 miles from' t h e c e n t r e , t h e diurnal range is masked and t h e fall is distinct. F r o m 6 0 miles t o t h e c e n t r e t h e fall is very rapid, and after t h e c e n t r e has passed t h e rise will be j u s t as rapid. Back to normal

Storm centre Barograph trace in vicinity of a T.R.S.

3.

A change in the appearance of t h e sky. Cirriform cloud first appears with cirrus m b a n d s converging t o w a r d s t h e c e n t r e . This is f o n o w e d bv cirrostratus, c i r r o c u m u l u s a n d t h e n a l t o c u m u l u s with b a n k s of black clouds on the h o r i z o n . T h e black c l o u d ( n i m b o s t r a t u s ) is called t h e bar of t h e s t o r m . Up to this time p r e c i p i t a t i o n has been sporadic b u t it n o w b e c o m e s c o n t i n u o u s and is t o r r e n t i a l .

4.

An increase in velocity or change in direction of t h e t r a d e w i n d , particularly with an u n s t e a d y b a r o m e t e r , usually indicates t h a t a T R S is n o t very far away.

5.

An oppressive feeling in t h e air.

A V O I D I N G THE STORM C E N T R E . If a vessel gets within t h e s t o r m field she should find her p o s i t i o n relative t o the centre as soon as possible. This can be d o n e in t h e following m a n n e r ; after observing t h e w i n d direction, the storm c e n t r e can be e s t i m a t e d as being 12 p o i n t s t o t h e right of this direction in the N o r t h e r n Hemisphere w h e n t h e b a r o m e t e r starts t o fall. If t h e b a r o m e t e r has fallen 10 m b t h e s t o r m c e n t r e is a p p r o x i m a t e l y 10 p o i n t s t o t h e right of t h e wind and, w h e n the b a r o m e t e r has fallen a further 10 m b , t h e c e n t r e is a b o u t 8 p o i n t s t o t h e right of t h e wind. If t h e vessel is in t h e S o u t h e r n H e m i s p h e r e , left should b e s u b s t i t u t e d for right in t h e foregoing p a r a g r a p h . T h e s t o r m s on t h e western side of t h e S o u t h Indian Ocean have a very large angle of indraught, a n d in s o m e p a r t s t h e wind blows straight across t h e isobars t o w a r d s the s t o r m c e n t r e . In these cases t h e m e t h o d of finding t h e a p p r o x i m a t e bearing of t h e centre fails. A freshening S.E. t r a d e with a falling b a r o m e t e r indicates t h a t t h e vessel is on the p a t h of or in t h e dangerous semi-circle of a T . R . S .


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The vessel's position in relation t o t h e p a t h of t h e s t o r m m a y be f o u n d by n o t i n g the wind direction and t h e n after an interval of t i m e n o t i n g it o n c e m o r e . If t h e wind shifts t o t h e right (veers) t h e vessel is in t h e right h a n d semi-circle, and if it shifts t o t h e left (backs) the vessel is in the left h a n d semi-circle. This applies t o b o t h h e m i s p h e r e s . If there is n o shift of wind t h e vessel is o n t h e direct p a t h if t h e b a r o m e t e r is falling, or is proceeding at t h e same speed and in t h e same direction as t h e s t o r m if t h e b a r o m e t e r is u n c h a n g e d . If t h e r e is d o u b t as t o t h e relative m o v e m e n t of t h e ship a n d s t o r m t h e ship should be s t o p p e d until this m o v e m e n t is f o u n d . A c o n t i n u a l check on t h e s t o r m ' s m o v e m e n t m u s t be m a d e e i t h e r by radio weather r e p o r t s or, if these are n o t available, by e s t i m a t i o n as o u t l i n e d a b o v e . When radio warnings of a s t o r m ' s position and track are received an allowance for error in t h e forecast position and track may be m a d e as follows : â&#x20AC;&#x201D; Plot t h e given position of t h e s t o r m c e n t r e . With this as c e n t r e d r a w a circle with t h e s t o r m ' s radius (half t h e forecast d i a m e t e r ) . Draw t h e s t o r m ' s forecast p a t h . Draw lines ahead of t h e s t o r m tangential t o t h e o u t s i d e d i a m e t e r of t h e s t o r m and m a k i n g an angle of a b o u t 4 0 ^ with t h e p a t h . With t h e s t o r m ' s c e n t r e as c e n t r e , a n d a forecast day's m o v e m e n t as radius, d r a w an arc b e t w e e n t h e t w o 4 0 ^ lines. D o likewise with a radius equal t o t w o d a y ' s forecast m o v e m e n t . T h e area enclosed by t h e t w o day arc and the 4 0 ^ lines is a danger area and t h e ship's course and speed should be adjusted t o try and avoid this area. T h e danger area should be r e p l o t t e d each t i m e a n e w radio r e p o r t is received.

Forecast path

Forecast diameter


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T h e following rules apply t o vessels navigating in t h e vicinity of a N o r t h e r n Hemisphere T R S . If t h e vessel is in t h e dangerous semi-circle she should m a k e as m u c h speed as possible k e e p i n g t h e wind on t h e s t a r b o a r d b o w . If it is impossible t o m a k e h e a d w a y because of b a d weather or t h e p r o x i m i t y of land, t h e vessel should heave t o with t h e wind and sea in as c o m f o r t a b l e a position as possible. T o have t h e sea on o n e b o w and t h e wind on t h e o t h e r b o w is often a good plan, as there is q u i t e an angle b e t w e e n t h e wind and swell. If t h e vessel is in t h e navigable semi-circle she should run with t h e wind on t h e starboard q u a r t e r , or if there is n o r o o m t o run t h e n she should heave t o as above. If t h e vessel is in t h e direct p a t h she should run with t h e wind j u s t abaft t h e starboard beam i n t o t h e navigable semi-circle. If t h e vessel is in t h e S o u t h e r n H e m i s p h e r e t h e m a n o e u v r e s o u t l i n e d above should be carried o u t reading Port for S t a r b o a r d . If t h e vessel is in a h-.irricane or t y p h o o n anchorage she should have b o t h a n c h o r s d o w n with a good scope of cable; she s h o u l d also have t h e engines in readiness t o ease t h e strain on t h e cables. If t h e vessel is in an open r o a d s t e a d , she will p r o b a b l y have a b e t t e r c h a n c e of weathering t h e s t o r m if o u t at sea, b u t t h e decision t o p u t t o sea m u s t be t a k e n early, otherwise t h e vessel m a y well be c a u g h t on a lee shore. If t h e vessel is alongside a wharf, all h a t c h e s should b e securely b a t t e n e d d o w n and derricks lowered and secured. E x t r a m o o r i n g s should be p u t o u t fore and aft. A d e q u a t e fenders should be placed b e t w e e n the ship and t h e q u a y . It m a y be advisable t o lay o u t t h e anchors. T h e engines should be ready for instant use. In m a n y p o r t s it is usual for t h e vessel t o leave t h e wharf and p r o c e d t o a t y p h o o n a n c h o r a g e . In all cases a vessel should be m a d e as stable as possible, ballast tanks filled and free surface in t a n k s reduced as m u c h as possible, as during a T R S t h e force of wind pressing on a ship's side will cause a considerable heeling m o m e n t which could have disastrous c o n s e q u e n c e s . Cargo and o t h e r moveable objects should b e well secured or t o m m e d off.


VII Currents and Ice Most places have a daily or twice daily rise and fall of water level k n o w n as a tide. The change in level at a p o r t is caused by water flowing i n t o or o u t of t h a t p o r t . This water flow is a T I D A L S T R E A M which is caused b y t h e gravitational effect of t h e sun and m o o n . OCEAN C U R R E N T S are n o t d e p e n d e n t u p o n gravitational effect b u t are caused by either a difference in level or a difference in density, in which case they are called STREAM C U R R E N T S , or by the wind blowing c o n t i n u o u s l y over t h e surface in the same direction, in which case they are called D R I F T C U R R E N T S . Stream c u r r e n t s may attain a rate of 3 t o 4 k n o t s . T h e best k n o w n e x a m p l e s of these are t h e Gulf Stream, Kuro Siwo and t h e surface c u r r e n t through t h e Strait of Gibraltar into the Mediterranean (whose level has been lowered by e v a p o r a t i o n ) . An example of the latter t y p e is t h e c u r r e n t which flows o u t of t h e M e d i t e r r a n e a n b e l o w the surface ( t h e salinity of the Mediterranean is increased by the evaporation of w a t e r ) . Drift c u r r e n t s are usually of t h e order of 1 t o 2 k n o t s , t h e principal ones being the S o u t h e r n Ocean or Great West Wind Drift and t h e T r a d e Drifts. It will be realised t h a t t h e m e a s u r e m e n t of c u r r e n t sets and drifts is difficult, as accurate D.R. and observed positions m u s t be w o r k e d o u t by m a n y ships on a world wide basis before being collected and finally p l o t t e d . Drift b o t t l e s or plastic envelopes are extensively used, b u t even when these are recovered, o n e c a n n o t tell w h e t h e r or n o t t h e bottle or envelope has travelled direct from the position it was t h r o w n overboard or h o w long it has taken on its j o u r n e y , as it m a y have lain on t h e beach for d a y s or weeks before being found. Since t h e advent of gyro compasses and wireless time signals, m a n y of t h e stronger c u r r e n t s and certain local c u r r e n t s have disappeared! Many local c u r r e n t s still remain b u t these are mainly c o u n t e r c u r r e n t s which, because of land or sea bed con­ figurations, flow in the o p p o s i t e direction t o t h e main c u r r e n t . T h e chart on pages 4 4 and 45 shows t h e principal c u r r e n t s . T h e w a r m c u r r e n t s are shown in red and t h e cold in blue. These t e r m s are relative as the n o r t h flowing p o r t i o n of t h e North Atlantic Drift is shown as being warm whereas the s o u t h e r n branch is shown as being cold. The t e m p e r a t u r e of t h e sea will vary from almost 3 2^C ( 9 0 ^ F ) in t h e equatorial regions to -2^C (ISVi^F) at the edge of t h e ice in t h e polar regions.


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If t h e t e m p e r a t u r e of t h e water is - 2 ° C for a considerable d e p t h ice will start t o form at t h e surface. Navigation will n o t be impaired by thin ice, b u t unless specially strengthened for navigation in ice, n o a t t e m p t should be m a d e t o force a ship t h r o u g h the thicker ice. However, in m a n y areas where this occurs, icebreaker assistance is available. Due a t t e n t i o n should be paid t o b r o a d c a s t warnings of ice c o n d i t i o n s in t h e area t h e vessel is t o transit. Broadcast messages frequently refer t o t h e following ice terms, which are given in t h e order in which they generally f o r m . SLUDGE O R S L U S H . T h e initial stages in t h e freezing of sea water, w h e n its con­ sistency is gluey or soapy. BRASH. Small fragments and r o u n d e d n o d u l e s ; t h e wreckage of o t h e r forms of ice. P A N C A K E ICE. Small pieces of new ice, a p p r o x i m a t e l y circular and with raised rims. Y O U N G ICE. Newly f o r m e d ice. BAY ICE. The y o u n g ice which first forms o n t h e sea in a u t u m n , and is of sufficient thickness t o i m p e d e or p r e v e n t navigation. PACK ICE. T e r m used in i wide sense t o include any area of sea ice, o t h e r than fast ice, n o m a t t e r w h a t form it takes or h o w disposed. FLOE.

An area of ice, o t h e r t h a n fast ice, w h o s e limits are within sight.

F I E L D ICE. Area of pack ice of such e x t e n t t h a t its limits c a n n o t be seen from t h e masthead. L E V E L ICE.

All u n h u m m o c k e d ice, n o m a t t e r of w h a t age or thickness.

HUMMOCK.

A ridge or elevation on a floe due t o pressure.

P R E S S U R E R I D G E . H u m m o c k e d ice where floes have been pressed t o g e t h e r and b r o k e n against each other. BERGY-BITS. Medium sized pieces of glacier ice, or of h u m m o c k y pack, washed clear of snow. Typical bergy-bits have been described as a b o u t t h e size of a c o t t a g e . G R O W L E R S . Smaller pieces of ice t h a n bergy-bits, appearing greenish in c o l o u r , because barely showing above water. R O T T E N ICE.

Floes which have b e c o m e m u c h h o n e y c o m b e d in the process of melting.

F A S T I C E . Sea ice which remains fast in t h e position of growth t h r o u g h o u t the winter, and s o m e t i m e s even during t h e ensuing s u m m e r . L A N D ICE. Ice a t t a c h e d t o t h e shore, within which there is n o channel.


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Large masses of land ice which b e c o m e d e t a c h e d are k n o w n as I C E B E R G S . In the N o r t h e r n Hemisphere these bergs are " c a l v e d " from t h e glaciers o n t h e East a n d West Coasts of Greenland. Many of these bergs get stuck fast in t h e p a c k ice o n t h e East Greenland Coasts, b u t t h o s e t h a t d o n o t are t a k e n s o u t h by t h e East Greenland c u r r e n t t o Cape Farewell w h e n c e t h e y are carried i n t o t h e Davis Strait w h e r e t h e y melt. O t h e r large bergs calved on t h e West Greenland coast are carried n o r t h w a r d t o Baffin Bay before r e t u r n i n g south in t h e L a b r a d o r c u r r e n t t o reach t h e shipping lanes off t h e Banks of N e w f o u n d l a n d . Due t o t h e s h o r t s u m m e r t h e bergs usually reach t h e shipping lanes t h e year after t h e y have calved, having been frozen in t h e p a c k ice for their first winter. T h e m a x i m u m size of a n o r t h e r n berg is unlikely t o e x c e e d mile in length. Its height may b e - u p t o 9 0 m m ( 2 7 0 feet). It should be n o t e d t h a t a b o u t V9ths of t h e iceberg is submerged and t h e u n d e r w a t e r p o r t i o n m a y e x t e n d laterally quite a way from the p o r t i o n above water. T h e eastern limit of t h e area in which icebergs m a y be f o u n d is a b o u t 4 0 ^ W and t h e s o u t h e r n limit a b o u t 4 0 ^ N .

In t h e S o u t h e r n Hemisphere t h e bergs are calved from t h e ice f o o t a n d t e n d t o be flat t o p p e d or tabular u p t o 6 0 m ( 1 8 0 feet) high. Their length is considerably greater t h a n t h e N o r t h e r n Hemisphere bergs. Bergs up t o 7 0 miles long and 10 miles wide have been r e p o r t e d . T h e bergs are rarely e n c o u n t e r e d t o t h e n o r t h w a r d of 4 0 ^ S a n d , as t h e r e is n o t m u c h traffic in t h e higher S o u t h e r n latitudes, less i m p o r t a n c e is a t t a c h e d t o S o u t h e r n bergs than t o their N o r t h e r n c o u n t e Ď&#x2020; a r t s . I N D I C A T I O N S O F ICE. 1. 2. 3. 4. 5. 6. 7.

8.

Ice blink by day and night. A yellowish w h i t e light reflected from t h e ice o n t o t h e sky near t h e h o r i z o n . There is often a wall of thick fog at t h e ice edge. A rapid fall in t h e sea t e m p e r a t u r e t o below freezing. Herds of seals or flocks of birds far from land. Absence of sea or swell in a fresh breeze is an indication of ice or land to windward. Noise of t h e ice cracking or falling i n t o t h e sea m a y b e h e a r d . Radar. T h e echoes will d e p e n d on t h e configuration of t h e ice. Also in t h e case of a berg, on t h e a m o u n t of Moraine (glacial material) in it. It should be n o t e d t h a t radar is n o t infallible in picking u p ice. On calm days e c h o e s m a y be h e a r d from higher ice.


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N A V I G A T I N G IN ICE. Vessels navigating in areas w h e r e icebergs m a y be e n c o u n t e r e d should go at a m o d e r a t e speed k e e p i n g a good l o o k o u t . If in fog, as frequently occurs on t h e Banks of N e w f o u n d l a n d , e x t r a special care m u s t be t a k e n . If navigating through pack ice t h e lanes a n d leads should be followed. T h e leads may be seen showing u p as dark lines or p a t c h e s on t h e ice blink. T h e greatest care m u s t be exercised t o p r e v e n t t h e vessel b e c o m i n g "nipped*' in t h e ice. This m a y occur if the ice is h u m m o c k i n g , and n o a t t e m p t should be m a d e t o e n t e r or cross ice which is forming these pressure ridges. If collision with a berg c a n n o t be avoided t h e n every e n d e a v o u r should be m a d e t o collide with it h e a d o n . I N T E R N A T I O N A L ICE P A T R O L S E R V I C E . This is financed by t h e c o u n t r i e s whose shipping uses t h e N o r t h e r n A t l a n t i c , b u t is m a i n t a i n e d by United States Coast G u a r d vessels whose call sign whilst o n ice p a t r o l is N I D K . T h e object of this patrol is t o locate by air and surface scouting, and radio information from all sources, t h e icebergs and field ice nearest t o and m e n a c i n g t h e N o r t h Atlantic Lane R o u t e s . T h e S o u t h e r l y limits of the ice will be d e t e r m i n e d and c o n t a c t m a i n t a i n e d with t h e ice as it moves s o u t h by a c o n t i n u o u s surface p a t r o l . The p a t r o l c o n t i n u e s t h r o u g h o u t t h e ice season from a p p r o x i m a t e l y 1st March t o 1st J u l y or later or earlier if c o n d i t i o n s w a r r a n t it. Ships sighting ice should r e p o r t its position t o t h e ice p a t r o l vessel, or t o t h e h e a d q u a r t e r s of t h e service at Argentia N e w f o u n d l a n d , call sign N I K . NORTH ATLANTIC LANE ROUTES. Vessels trading accross t h e N o r t h Atlantic are e x p e c t e d t o keep t o t h e track specified for t h e t i m e of year. T h e r e are seven of these t r a c k s , each track having Westbound and E a s t b o u n d r o u t e s . T h e Transatlantic Track Association comprises t h e principal N o r t h Atlantic S h i p o w n e r s a n d decisions m a d e by t h e m are c o m m u n i c a t e d b y m e a n s of Notices t o Mariners a n d t h e shipping press.


VIII Weather Charts and Routeing SYNOPTIC C H A R T S . In addition t o the Ocean Weather Ships, which are stationed p e r m a n e n t l y at positions in t h e N o r t h Atlantic, there are Merchant Ships observing a n d radioing information every six h o u r s . F r o m this information and from information by land stations all over t h e N o r t h e r n Hemisphere, t h e Meteorological Office weather charts. Selected and S u p p l e m e n t a r y Ships radio their position, t h e wind direction and speed, the pressure and t e m p e r a t u r e , t h e a m o u n t of cloud and cloud t y p e s , t h e visibility, and t h e past and present weather. Selected ships also radio information a b o u t t h e sea t e m p e r a t u r e , t h e d e w p o i n t t e m p e r a t u r e , waves and t h e ship's course a n d speed. T h e information t o be radioed is coded in t h e form given in t h e '*Decode for Use of Shipping'* (Met.O.509) to which reference should be m a d e for full details of w e a t h e r messages. Weather forecasts for shipping are broadcast by t h e BBC on t h e 1 5 0 0 m e t r e wavelength p r o g r a m m e . T h e forecast areas lie to t h e east of 4 0 ^ W and b e t w e e n 3 5 ^ N and 6 0 ^ N , t h e areas a r o u n d t h e coasts of t h e British Isles being smaller t h a n t h o s e in t h e Adantic. An A d a n t i c Weather Bulletin for Shipping from "Bracknell W e a t h e r " t o "All S h i p s " is broadcast from Portishead radio. This broadcast is in six parts, p a r t s I, II, and III consisting of storm warnings, a synopsis of w e a t h e r c o n d i t i o n s and w e a t h e r forecasts for areas from 35^N t o 6 5 ^ N b e t w e e n 15^W and 4 0 ^ W . These are all given in plain language and are broadcast at 0 9 3 0 and 2 1 3 0 G.M.T. Parts V and VI are also broadcast at 0 9 3 0 and 2 1 3 0 G.M.T., and consist of a selection of ship and shore r e p o r t s . These are broadcast in c o d e and besides t h e ships' positions and station identification n u m b e r s t h e following information is included: â&#x20AC;&#x201D; the wind direction and speed in k n o t s , t h e b a r o m e t r i c pressure, t h e air t e m p e r a t u r e , cloud a m o u n t , visibility, and past a n d present weather. This i n f o r m a t i o n can be p l o t t e d in t h e form of a station m o d e l , as shown below, at t h e a p p r o x i m a t e position on a chart. T h e ideal chart for p l o t t i n g weather observations is Metform 1 2 5 8 . selected weather supplied prepares

TT Www

PPP


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PPP is t h e pressure; t h e initial 9 or 10 is usually o m i t t e d T T is t h e air t e m p e r a t u r e VV is the c o d e figure for the visibility Î? is t h e Beaufort symbol for t h e cloud a m o u n t WW is the Beaufort symbol for the present w e a t h e r W is the Beaufort symbol for t h e past w e a t h e r The line with the feathers represents t h e wind direction and speed. T h e arrow flies with t h e wind and a speed of 5 k n o t s is represented by a short feather, 10 k n o t s by a long feather, 15 k n o t s by a short and a long feather, a n d so forth. T h e feathers always p o i n t t o w a r d s t h e low, so t h a t in t h e n o r t h e r n h e m i s p h e r e t h e feathers are always on the observer's right h a n d when he faces t h e wind. In t h e e x a m p l e a wind of 0 7 5 ^ T 25 k n o t s has been p l o t t e d . Part IV of the bulletin is broadcast at 1 1 3 0 GMT and consists of a w e a t h e r analysis. C o n t a i n e d herein is sufficient information to c o n s t r u c t a w e a t h e r chart. Metform 1258 should be used for the p l o t t i n g of t h e following given d a t a : The positions of high, low or o t h e r pressure systems. The t y p e of front and its positions T h e positions of selected isobars. When the information given in parts IV, V and VI of t h e Atlantic Weather Bulletin has been p l o t t e d on Metform 1 2 5 8 , the chart should be c o m p l e t e d by drawing in isobars at 4 millibar intervals b e t w e e n those given in Part IV. Bearing in mind the rules for t h e m o v e m e n t of depressions given on page 62 and the pressure characteristic (broadcast in Part IV) a w e a t h e r forecast m a y be m a d e for a selected position for the n e x t few h o u r s . If e x a m i n a t i o n candidates have to c o n s t r u c t a w e a t h e r chart from c o d e d infor­ mation similar to that broadcast in the Atlantic Weather Bulletin, t h e D e c o d e Book Met. 0 . 5 0 9 will be supplied but, nevertheless, practice is required in p l o t t i n g t h e d a t a and drawing in the isobars if t h e chart is to be c o m p l e t e d within t h e time available. It is suggested that this practice could be d o n e m o s t profitably at sea, using actual broadcast information. It is also suggested that candidates w h o are pressed for time should only p u t in the wind, t e m p e r a t u r e and pressure on the station m o d e l . It may be found occasionally that some information in Part IV, V or VI does not correspond with the general synoptic situation. This should be disregarded, as there may have been an error in receiving or decoding t h e message.


84

N O T E S ON M E T E O R O L O G Y

FACSIMILE W E A T H E R C H A R T S . Vessels fitted with facsimile p l o t t e r s can receive signals by radio which will p r o d u c e , on a moving roll of p a p e r , a weather c h a r t in t h e same form as t h o s e d r a w n by the Meteorological Office. This removes any possibility of mistakes in reading, d e c o d i n g or p l o t t i n g weather information sent o u t by c o d e as in t h e Atlantic Bulletin. Impressions of black or white signals are m a d e on a roll of p a p e r as it passes an electronic m a r k e r p e n . Different t y p e s of transmissions require different d r u m speeds and in order t h a t t h e c h a r t r e p r o d u c t i o n is perfect t h e receiver m u s t b e k e p t correctly t u n e d and t h e d r u m speed also correct t h r o u g h o u t t h e transmission. A n y s p o t s which are t o o dark or insufficiently dark c a n n o t be overprinted by t h e m a c h i n e as t h e transmission takes several m i n u t e s and t h e chart c a n n o t be fed back i n t o t h e m a c h i n e . If a bad c h a r t is p r o d u c e d a further chart will have t o be p r o d u c e d at t h e n e x t transmission. Full details of the scale and t y p e of the m a p p r o j e c t i o n , t h e d r u m speed, t h e t y p e of c h a r t (surface analysis, surface prognosis i.e. forecast, wave analysis, wave prog­ nosis, sea ice observations etc.), t h e t r a n s m i t t i n g stations with wavelengths a n d times are given in V o l u m e III of t h e List of Radio Signals. WEATHER

ROUTEING.

This is carried o u t by private c o m p a n i e s in the U.S.A. and by t h e British Meteorological Office in t h e U.K. A small charge is m a d e for t h e service. During t h e comparatively short period t h a t it has been in o p e r a t i o n m a n y h u n d r e d s of crossings of t h e N o r t h Atlantic have been m a d e by vessels using this service. Whilst it is impossible t o give the time saved or r e d u c t i o n in heavy w e a t h e r damage for individual passages, t h e average time taken by w e a t h e r - r o u t e d vessels is less t h a n t h a t of similar vessels not so r o u t e d and the damage caused by heavy w e a t h e r d u r i n g a r o u t e d period is significantiy less than t h a t during a similar period when n o t r o u t e d . T h e following description of weather routeing is taken from the Meteorological Office leaflet on weather r o u t e i n g with t h e permission of t h e Controller of H.M. Stationery Office and the Director of the Meteorological Office. To enable ships t o m a k e t h e best of t h e w e a t h e r t h e British Meteorological Office has recently i n t r o d u c e d a weather r o u t e i n g service for vessels trading on t h e N o r t h At­ lantic. Voyages in b o t h east-west and west-east directions are covered and ships of any nation may participate. Each ship is given individual advice on t h e best course t o steer in order t o avoid heavy weather as far as possible. T h e Master still remains responsible for deciding his course, b u t experience has shown t h a t t h e use of weather-routeing advice can offer the following advantages. a.

Higher speeds

b.

Quicker crossings

c.

Better p u n c t u a l i t y

d.

Greater c o m f o r t for passengers and crew

e.

Less damage to ships

f.

Less damage t o cargoes

g.

More o p p o r t u n i t i e s for m a i n t e n a n c e at sea.


N O T E S ON M E T E O R O L O G Y

85

J u s t before sailing t h e Master c o n t a c t s t h e Meteorological Office at Bracknell where the Central Forecast Office gives advice covering at least t h e n e x t 4 8 h o u r s . This can be very i m p o r t a n t . F o r e x a m p l e , should a ship leaving Liverpool go n o r t h a r o u n d Ireland, or will it pay t o go t h e longer way via F a s t n e t ? T h e s h o r t e s t r o u t e is n o t always the quickest. This initial advice is followed by further advisory messages t r a n s m i t t e d t o t h e ship by Portishead R a d i o . These are normally sent every 4 8 h o u r s , b u t messages can be sent m o r e frequently - every 12 h o u r s if necessary. These messages advise the course t o follow a n d , if required, a special w e a t h e r forecast for 4 8 h o u r s and forecasts of wind and sea can also be provided. T h e Central Forecast Office at Bracknell receives a c o n s t a n t flow of m e t e o r o ­ logical d a t a from t h e N o r t h Atlantic and adjacent areas. These d a t a are fed into a high speed electronic c o m p u t e r which p r o d u c e s regular forecasts for t h e N o r t h Atlantic for 2â&#x20AC;&#x201D;3 days ahead. T h e c o m p u t e r converts t h e p r e d i c t e d wind field i n t o wave heights and m a k e s allowance for swell. Comprehensive i n f o r m a t i o n a b o u t c u r r e n t s and ice m o v e m e n t s is also available. In o r d e r t o k n o w as m u c h as possible a b o u t t h e p e r f o r m a n c e of an individual ship in varying wave and w e a t h e r c o n d i t i o n s , the Office asks in advance for all t h e necessary details a b o u t each ship and each voyage, including, for e x a m p l e , t h e size, d r a u g h t and loading of t h e vessel and any special characteristics which might affect her p e r f o r m a n c e on a particular crossing. A r m e d with all this i n f o r m a t i o n , e x p e r t w e a t h e r forecasters, aided by nautical officers with long ocean-going experience, calculate t h e best course for t h e ship t o follow during t h e n e x t 4 8 h o u r s t o enable it t o reach its destination in t h e s h o r t e s t time and with t h e m i n i m u m of bad weather. S o m e t i m e s , of c o u r s e , bad w e a t h e r is t o o widespread t o avoid c o m p l e t e l y , b u t practical experience has proved t h a t over a few voyages w e a t h e r routeing soon pays for itself in t e r m s of time saved and damage avoided.


86

SOLE

PORTLAND COASTAL STATIONS A Β ΒΑ BL BR BS

FINISTERRE

c

CP D J KH KL LE LK LS MD MH

Aberdeen Boulmer Benbecula Blackpool Bell Rock Butt ot Lewis Channel Lt V St Catherine's Pt Dowsing Jersey Kiliough Kilkeei Lands End Lerwick Leuctiars Malm Head Machnhanish

MN MS Ν

0

PB PK RS RY SC SH SP SY Τ V VA VY W

Mansion Mumbles Noord Hinder Orlock Head Portland Bill Prestwick Royal Sovereign Ronaldsway Scilly Sumburgh Spurn Point Stornoway Tiree Varne Valentía Valley Wick

TRAFALGAR

Sea areas and coastal stations used in marine forecasts prepared by the Meteorological Office at Bracknell and broadcast by the BBC and Coast Radio Stations, incorporating changes to areas made in 1984. Weather within an estuary, and particularly visibility, may often be expected to diflfer from that forecast for the coastal sea areas within which the estuary lies, though this will not necessarily be mentioned in the forecast.


Index ABSOLUTE HUMIDITY 10 Absolute temperature 9 Adiabatic temperature changes . . . . 18 Advection fog 30 Air Masses 54.55 Anabatic wind 51 Anemometers 12 Aneroid Barometer 6,7 Angle of Indraught 37 Anticyclone 53 Anti-trades 43 Arctic smoke 30 Atmosphere 15 Aureole 31 BAILLIE WIND ROSE Barograph Barometer Barometer errors Beaufort wind scale Bergeron process Bishop's Ring Black frost Buys Ballot's Law

40 8 4 5 38 25 31 31 34

CAPACITY Capillarity Celsius Centigrade aoud Col Conditional instability Conduction Convection Coriolis Corona Cumulonimbus cloud Current chart Currents Cyclostrophic force

5 5 9 9 23,24 65 20 17 17 33 31 26 44,45 78 36

DEPRESSIONS Dew Dew point

56,63 29 10

Diurnal range

3

Dry Adiabatic Lapse Rate

18

EASTERLY TYPE

68

FACSIMILE CHARTS Fahrenheit Fiducial temperature Fog Fohn wind Forecast Areas Friction Frontogenesis Frontolysis Fronts Frost GEOSTROPHIC WIND Geostrophic Wind Scale Glazed frost Gold slide Gradient wind HAIL Halo Haze Hoar frost Humidity Hydrometer Hygrometer ICE Ice Patrol Service Insolation Inversion Isallobar Isobar Isobaric forms Isohaline Isohcl Isohyet Isoncph Isotherm KATABATIC WIND

84 9 5 29 52 86 37 61 61 55,56 29 34 35 31 6 36 26 31 29 29 10 13 11 79,80 81 16 21 3 3 66 14 13 13 14 9 51


Kew barometer

4

LAND AND SEA BREEZES Lapse rates Latent heat Line squall Local winds

43 18 15 64 49,50

MAXIMUM THERMOMETER Mediterranean winds Mercurial barometer Millibar Minimum thermometer Mirage Mist Mock sun Monsoons Movement of depressions ,

10 49 4 3 10 32 29 31 46,47 62

NEPHOSCOPE N.E. Monsoon N.E. winds of Brazil N.W. Monsoon Neutral air Northerly Type

14 46 48 46 20 67

OCCLUSION Optical phenomena Orographic fog

56 31 30

PLANETARY QRCULATION 40 Precipitation 25 Precision Aneroid Barometer . . . . . . 7 Pressure 3 Pressure Gradient 3 Prevailing winds, N. Summer 42 Prevailing winds, N. winter 41 Psychrometcr 11 Pumping 5 QUASI-STATIONARY FRONT RADIATION Radio-sonde Rain Rainbow Raingauge Reamur Relative Humidity

55 17,29 14 25 32 13 9 10

Rime Ridge

31 54

ST. ELMO'S FIRE 28 Salinometer 14 Saturated Adiabatic Lapse Rate . . . . 18 Sea breeze 43 Sea fog 30 Sea smoke 30 Secondary Depression 61 Sixes Thermometer 11 Sleet 26 Snow 26 Solar Constant 16 Source region . . . 54 Southerly type 69 S.W. Monsoon 47 S.W. winds of Africa 48 Specific heat 9 Stable air . 19 Standard Temperature 5 Storm warnings 39 Straight isobars 65 Sunshine recorder 13 Synoptic charts 82 TEMPERATURE Tephigram Thermograph Thermometers Thunderstorm Tornado Trade wind limits Tropical Revolving Storms Troposphere Trough Turbulence UNSTABLE AIR WATERSPOUT Wave depression Weather charts Weather Routeing Westerly type Wind Wind and Pressure - summer Wind and Pressure - winter Wind rose

9,10 22 11 10 27 64 43 . . . . 71-78 15 63 17 19 64 61 82 84 70 33 . . . . 42 41 39,40

Kemp & Young - Notes on Meteorology  

Kemp & Young - Notes on Meteorology

Kemp & Young - Notes on Meteorology  

Kemp & Young - Notes on Meteorology

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