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Nº1 THE METEOROLOGIST

THE WIND ISSUE

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UK £ 6.50

H E N R I E T TA H A R R I S • S T E V E N L E V I T T VO R T I C I T Y • M O N S O O N S E A S O N • T H E W I N DY I S L A N D S 1


THE METEOROLOGIST

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Ever since the beginning of mankind, man has wondered about the weather. It has been one of the most mysterious parts of our every day reality. Today, even though we sometimes predict the winds quite impressively, we cannot control it, and to a very limited extent protect ourselves from it. We are very small relative to the atmosphere, and yet, we live and die by it. The Meteorologist is for the weather connosieur, the knowledge hungry or the mindful soul who wishes to be present in the meteorolical moment - beyond deciding whether to put on the wool or the cotton sweater. We wish to expand the understanding of the mechanisms behind meteorological phenomena, and the inner workings of the crafts that depend on or meta-analyze on them. We hope you enjoy The Wind Issue. - The Editors

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CONTENTS

p. 8 Seven Deadly Winds p. 10 Tornado Warning p. 14 Henrietta Harris p. 20 The Windy Islands p. 26 Monsoon p. 34 Vorticity, explained p. 38 Oceanic Diffractions p. 42 Steven Levitt of Freakonomics p. 48 2013 Weather Photo Awards p. 56 Turbulence p. 60 Strand Beest

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JOURNEY OF THE COPY Text by Aron Lindegård Photography by Some Dudeson

Far far away, behind the word mountains, far from the countries Vokalia and Consonantia, there live the blind texts. Separated they live in Bookmarksgrove right at the coast of the Semantics, a large language ocean. A small river named Duden flows by their place and supplies it with the necessary regelialia. It is a paradisematic country, in which roasted parts of sentences fly into your mouth. Even the all-powerful Pointing has no control about the blind texts it is an almost unorthographic life One day however a small line of blind text by the name of Lorem Ipsum decided to leave for the far World of Grammar. The Big Oxmox advised her not to do so, because there were thousands of bad Commas, wild Question Marks and devious Semikoli, but the Little Blind Text didnít listen. She packed her seven versalia, put her initial into the belt and made herself on the way. Pityful a rethoric question ran over her

salia, put her initial into the belt and made herself on the way. On her way she met a copy. The copy warned the Little Blind Text, that where it came from it would have been rewritten a thousand times and everything that was left from its origin would be the word ìandî and the Little Blind Text should turn around and return to its own, safe country. The copy warned the Little Blind Text, that where it came from it would have been rewritten a thousand times and everything that was left from its origin would be the word ìandî and the Little Blind Text should turn around and return to its own, safe country. The Big Oxmox advised her not to do so, because there were thousands of bad Commas, wild Question Marks and devious Semikoli, but the Little Blind Text didnít listen. She packed her seven versalia, put her initial into the belt and made herself on the way. But nothing the copy said could convince her and so it didnít take long until a few insidious Copy Writers ambushed

The Big Oxmox advised her not to do so, because there were thousands of bad Commas, wild Question Marks and devious Semikoli. cheek, then she continued her way. On her way she met a copy. The copy warned the Little Blind Text, that where it came from it would have been rewritten a thousand times and everything that was left from its origin would be the word ìandî and the Little Blind Text should turn around and return to its own, safe country. The Big Oxmox advised her not to do so, because there were thousands of bad Commas, wild Question Marks and devious Semikoli, but the Little Blind Text didnít listen. She packed her seven ver-

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her, made her drunk with Longe and Parole and dragged her into their agency, where they abused her for their projects again and again. And if she hasnít been rewritten, then they are still using her. When she reached the first hills of the Italic Mountains, she had a last view back on the skyline of her hometown Bookmarksgrove, the headline of Alphabet Village and the subline of her own road, the Line Lane. Pityful a rethoric question ran over her cheek, then she continued her way. On her way she met a copy.


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SEVEN DEADLY WINDS

29°16 ' 52 '' N 94°49 ' 33 '' W

14°14 ' N 61°21 ' W

The Atlantic Ocean has been the scene of thousands of hurricanes throughout history. Swift and lethal, these winds destroys all in their path. These are the seven deadliest winds of the Atlantic.

The Great Hurricane of 1780 15°55 ' N 6°00 ' W Hurricane Mitch

The Great Hurricane of 1780, also known as Huracán San Calixto, the Great Hurricane of the Antilles, and the 1780 Disaster, is probably the deadliest Atlantic hurricane on record. Between 20,000 and 22,000 people died when the storm passed through the Lesser Antilles in the Caribbean starting on October 10 and ending on October 16. Specifics on the hurricane's track and strength are unknown since the official Atlantic hurricane database only goes back to 1851.

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Hurricane Mitch was the most powerful hurricane and the most destructive of the 1998 Atlantic hurricane season, with maximum sustained winds of 180 mph (285 km/h). The storm was the thirteenth tropical storm, ninth hurricane, and third major hurricane of the season. Deaths due to catastrophic flooding made it the second deadliest Atlantic hurricane in history; nearly 11,000 people were killed with over 11,000 left missing by the end of 1998. Additionally, roughly 2.7 million were left homeless as a result of the hurricane.

Galveston Hurricane

The Hurricane of 1900 made landfall on September 8, 1900, in the city of Galveston, Texas, in the United States. It had estimated winds of 145 miles per hour (233 km/h) at landfall, making it a Category 4 storm. The hurricane caused great loss of life with the estimated death toll between 6,000 and 12,000 individuals; the number most cited in official reports is 8,000, making it the deadliest hurricane in US history, and the second costliest hurricane in U.S. history.


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14°14 ' N 61°21 ' W

14°14 ' N 61°21 ' W

Hurricane Fifi

Pointe-à-Pitre Hurricane

15°55 ' N 6°00 ' W

Hurricane Fifi (later Hurricane Orlene) was a catastrophic tropical cyclone that killed between 3,000 and 10,000 people in Honduras in September 1974, ranking it as the fourth deadliest Atlantic hurricane on record. In a single town, between 2,000 and 5,000 people were killed overnight after a massive flood inundated the area. Fifi brought continuous rainfall to the area for three days, hampering relief efforts in what was the worst disaster in Honduras' history at the time.

Dominican Republic Hurricane 29°16 ' 52 '' N 94°49 ' 33 '' W Hurricane Flora

The 1930 Dominican Republic Hurricane, also known as Hurricane San Zenon, is the fifth deadliest Atlantic hurricane on record. The second of two known tropical cyclones in the 1930 Atlantic hurricane season, the hurricane was first observed on August 29 to the east of the Lesser Antilles. The cyclone was a small but intense Category 4 hurricane, killing as many as 8,000 people when it crossed the Dominican Republic.

Hurricane Flora is among the deadliest Atlantic hurricanes in recorded history, with a death total of over 7,000. The seventh tropical storm and sixth hurricane of the 1963 Atlantic hurricane season, Flora developed from a disturbance in the Intertropical Convergence Zone on September 26 while located about 755 miles (1,215 km) southwest of the Cape Verde islands.

The 1776 Pointe-à-Pitre hurricane was at one point the deadliest North Atlantic hurricane on record. Although its intensity and complete track is unknown, it is known that the storm struck Guadeloupe on September 6, 1776 near Pointe-à-Pitre, which is currently the largest city on the island. At least 6,000 fatalities occurred on Guadeloupe, which was a higher death toll than any known hurricane before it. The storm struck a large convoy of French and Dutch merchant ships, sinking or running aground 60% of the vessels.

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TORNADO WARNI NG Text by Vicky Pediah Photography by Some Dudeson

A tornado is a violently rotating column of air that is in contact with both the surface of the earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud. They are often referred to as twisters or cyclones, although the word cyclone is used in meteorology, in a wider sense, to name any closed low pressure circulation. Tornadoes come in many shapes and sizes, but they are typically in the form of a visible condensation funnel, whose narrow end touches the earth and is often encircled by a cloud of debris and dust. Most tornadoes have wind speeds less than 110 miles per hour (177 km/h), are about 250 feet (76 m) across, and travel a few miles (several kilometers) before dissipating. The most extreme tornadoes can attain wind speeds of more than 300 miles per hour (483 km/h), stretch more than two miles (3.2 km) across, and stay on the ground for dozens of miles (more than 100 km). Various types of tornadoes include the landspout, multiple vortex tornado, and

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waterspout. Waterspouts are characterized by a spiraling funnel-shaped wind current, connecting to a large cumulus or cumulonimbus cloud. They are generally classified as non-supercellular tornadoes that develop over bodies of water, but there is disagreement over whether to classify them as true tornadoes. These spiraling columns of air frequently develop in tropical areas close to the equator, and are less common at high latitudes. Other tornado-like phenomena that exist in nature include the gustnado, dust devil, fire whirls, and steam devil. Tornadoes have been observed on every continent except Antarctica. However, the vast majority of tornadoes in the world occur in the Tornado Alley region of the United States, although they can occur nearly anywhere in North America. They also occasionally occur in south-central and eastern Asia, northern and east-central South America, Southern Africa, northwestern and southeast Europe.


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Dejah Thoris caught her breath at my knew that as my arm rested there across take him home and nurse him back to last words, and gazed upon me with her shoulders longer than the act of health,” she laughed. “That is precisely dilated eyes and quickening breath, and adjusting the silk required she did not what we do on Earth,” I answered. then, with an odd little laugh, which draw away, nor did she speak. The storm has been brewing for brought roguish dimples to the corners And so, in silence, we walked the quite a while now, do you feel you’re of her mouth, she shook her head and surface of a dying world, but in the being perceived the right way in the cried: “What a child! A great warrior and breast of one of us at least had been born public eye? yet a stumbling little child.” “What have that which is ever oldest, yet ever new. I “At least among civilized men.” This I done now?” I asked, in sore perplexity. loved Dejah Thoris. The touch of my arm made her laugh again. She could not “Some day you shall know, John Carter, if upon her naked shoulder had spoken to understand it, for, with all her tenderness we live; but I may not tell you. And I, the me in words I would not mistake, and I and womanly sweetness, she was still a daughter of Mors Kajak, son of Tardos knew that I had loved her since the first Martian, and to a Martian the only good Mors, have listened without anger,” she moment that my eyes had met hers that enemy is a dead enemy; for every dead soliloquized in conclusion. first time in the plaza of the dead city of foeman means so much more to divide Then she broke out again into one of Korad. My first impulse was to tell her of between those who live. her gay, happy, laughing moods; joking my love, and then I thought of the helpI was very curious to know what I had with me on my prowess as a Thark lessness of her position wherein I alone said or done to cause her so much perwarrior as contrasted with my soft heart could lighten the burdens of her captivity, turbation a moment before and so I conand natural kindliness. “I presume that and protect her in my poor way against tinued to importune her to enlighten me. should you accidentally wound an enemy the thousands of hereditary enemies she “No,” she exclaimed, “it is enough that you you would take him home and nurse him must face upon our arrival at Thark. I have said it and that I have listened. And back to health,” she laughed. “That is pre- could not chance causing her additional when you learn, John Carter, and if I be cisely what we do on Earth,” I answered. pain or sorrow by declaring a love which, dead, as likely I shall be ere the further “At least among civilized men.” This in all probability she did not return. moon has circled Barsoom another made her laugh again. She could not Should I be so indiscreet, her posi- twelve times, remember that I listened understand it, for, with all her tenderness tion would be even more unbearable and that I smiled.” It was all Greek to me, and womanly sweetness, she was still a than now, and the thought that she but the more I begged her to explain the Martian, and to a Martian the only good might feel that I was taking advantage more positive became her denials of my enemy is a dead enemy; for every dead of her helplessness, to influence her request, and, so, in very hopelessness, I foeman means so much more to divide decision was the final argument which desisted. between those who live. sealed my lips? Day had now given away to night I was very curious to know what I “Why are you so quiet, Dejah Thoris?” I and as we wandered along the great had said or done to cause her so much asked. “Possibly you would rather return avenue lighted by the two moons of perturbation a moment before and to Sola and your quarters.” “No,” she mur- Barsoom, and with Earth looking so I continued to importune her to mured, “I am happy here. I do not know down upon us out of her luminous enlighten me? why it is that I should always be happy green eye, it seemed that we were “No,” she exclaimed, “it is enough that and contented when you, John Carter, a alone in the universe, and I, at least, you have said it and that I have listened. stranger, are with me; yet at such times. was content that it should be so? And when you learn, John Carter, and if I Dejah Thoris caught her breath at my The chill of the Martian night was be dead, as likely I shall be ere the further last words, and gazed upon me with upon us, and removing my silks I threw moon has circled Barsoom another dilated eyes and quickening breath, and them across the shoulders of Dejah twelve times, remember that I listened then, with an odd little laugh, which Thoris. As my arm rested for an instant and that I smiled.” It was all Greek to me, brought roguish dimples to the corners upon her I felt a thrill pass through every but the more I begged her to explain the of her mouth, she shook her head and fiber of my being such as contact with no more positive became her denials of my cried: “What a child! A great warrior and other mortal had even produced; and it request, and, so, in very hopelessness, I yet a stumbling little child.” “What have seemed to me that she had leaned slightly desisted. I done now?” I asked, in sore perplexity. toward me, but of that I was not sure. Day had now given away to night and “Some day you shall know, John Carter, if Only I knew that as my arm rested there as we wandered along the great avenue across her shoulders longer than the act lighted by the two moons of Barsoom, of adjusting the silk required she did not “A violently rotating column of draw away, nor did she speak. and with Earth looking down upon us air, in contact with the ground.” out of her luminous green eye, it seemed And so, in silence, we walked the that we were alone in the universe, and I, surface of a dying world, but in the at least, was content that it should be so. we live; but I may not tell you. And I, the breast of one of us at least had been born The chill of the Martian night was upon daughter of Mors Kajak, son of Tardos that which is ever oldest, yet ever new. I us, and removing my silks I threw them Mors, have listened without anger,” she loved Dejah Thoris. The touch of my arm across the shoulders of Dejah Thoris. As soliloquized in conclusion. upon her naked shoulder had spoken to my arm rested for an instant upon her I Then she broke out again into one of her me in words I would not mistake, and I felt a thrill pass through every fiber of gay, happy, laughing moods; joking with knew that I had loved her since the first my being such as contact with no other me on my prowess as a Thark warrior as moment that my eyes had met hers that mortal had even produced; and it seemed contrasted with my soft heart and natural first time in the plaza of the dead city of to me that she had leaned slightly toward kindliness. “I presume that should you Korad. My first impulse was to tell her of me, but of that I was not sure. Only I accidentally wound an enemy you would my love, and then I thought. •

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“Remember that I listened and that I smiled.”

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HENRIETTA HARRIS Text by Aron Lindegård Illustrations by Henrietta Harris Far far away, behind the word mountains, far from the countries Vokalia and Consonantia, there live the blind texts. Separated they live in Bookmarksgrove right at the coast of the Semantics, a large language ocean. A small river named Duden flows by their place and supplies it with the necessary regelialia. It is a paradisematic country, in which roasted parts of sentences fly into your mouth. Even the all-powerful Pointing has no control about the blind texts it is an almost unorthographic life One day however a small line of blind text by the name of Lorem Ipsum decided to leave for the far World of Grammar. The Big Oxmox advised her not to do so, because there were thousands of bad Commas, wild Question Marks and devious Semikoli, but the Little Blind Text didnít listen. She packed her seven versalia, put her initial into the belt and made herself on the way. Pityful a rethoric question ran over her cheek, then she continued her way. On her

herself on the way. On her way she met a copy. The copy warned the Little Blind Text, that where it came from it would have been rewritten a thousand times and everything that was left from its origin would be the word ìandî and the Little Blind Text should turn around and return to its own, safe country. The copy warned the Little Blind Text, that where it came from it would have been rewritten a thousand times and everything that was left from its origin would be the word ìandî and the Little Blind Text should turn around and return to its own, safe country. The Big Oxmox advised her not to do so, because there were thousands of bad Commas, wild Question Marks and devious Semikoli, but the Little Blind Text didnít listen. She packed her seven versalia, put her initial into the belt and made herself on the way. But nothing the copy said could convince her and so it didnít take long until a few insidious Copy Writers ambushed her, made her drunk with Longe and Parole and dragged her into their agency, where

The Big Oxmox advised her not to do so, because there were thousands of bad Commas, wild Question Marks and devious Semikoli. way she met a copy. The copy warned the Little Blind Text, that where it came from it would have been rewritten a thousand times and everything that was left from its origin would be the word ìandî and the Little Blind Text should turn around and return to its own, safe country. The Big Oxmox advised her not to do so, because there were thousands of bad Commas, wild Question Marks and devious Semikoli, but the Little Blind Text didnít listen. She packed her seven versalia, put her initial into the belt and made

they abused her for their projects again and again. And if she hasnít been rewritten, then they are still using her. When she reached the first hills of the Italic Mountains, she had a last view back on the skyline of her hometown Bookmarksgrove, the headline of Alphabet Village and the subline of her own road, the Line Lane. Pityful a rethoric question ran over her cheek, then she continued her way. On her way she met a copy. Pityful a rethoric question ran over her cheek, then she continued her way.

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She packed her seven versalia, put her initial into the belt and made herself on the way.

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Far far away, behind the word mountains, far from the countries Vokalia and Consonantia, there live the blind texts. Separated they live in Bookmarksgrove right at the coast of the Semantics, a large language ocean. A small river named Duden flows by their place and supplies it with the necessary regelialia. It is a paradisematic country, in which roasted parts of sentences fly into your mouth. Even the all-powerful Pointing has no control about the blind texts it is an almost unorthographic life One day however a small line of blind text by the name of Lorem Ipsum decided to leave for the far World of Grammar. The Big Oxmox advised her not to do so, because there were thousands of bad Commas, wild Question Marks and devious Semikoli, but the Little Blind Text didnít listen. She packed her seven versalia, put her initial into the belt and made herself on the way. When she reached the first hills of the Italic Mountains, she had a last view back on the skyline of her hometown Bookmarksgrove, the headline of Alphabet Village and the subline of her own road, the Line Lane. Pityful a rethoric question ran over her cheek, then she continued her way. On her way she met a copy. The copy warned the Little Blind Text, that where it came from it would have been rewritten a thousand times and everything that was left from its origin would be the word ìandî and the Little Blind Text should turn around and return to its own, safe country. But nothing the copy said could convince her and so it didnít take long until a few insidious Copy Writers ambushed her, made her drunk with Longe and Parole and dragged her into their agency, where they abused her for their projects again and again. And if she hasnít been rewritten, then they are still using her. Far far away, behind the word mountains, far from the countries Vokalia and Consonantia, there live the blind texts. Separated they live in Bookmarksgrove right at the coast of the Semantics, a large language ocean. A small river named Duden flows by their place and supplies it with the neces-

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sary regelialia. It is a paradisematic country, in which roasted parts of sentences fly into your mouth. Even the all-powerful Pointing has no control about the blind texts it is an almost unorthographic life One day however a small line of blind text by the name of Lorem Ipsum decided to leave for the far World of Grammar. The Big Oxmox advised her not to do so, because there were thousands of bad Commas, wild Question Marks and devious Semikoli, but the Little Blind Text didnít listen. She packed her seven versalia, put her initial into the belt and made herself on the way. When she reached the first hills of the Italic Mountains, she had a last view back on the skyline of her hometown Bookmarksgrove, the headline of Alphabet Village and the subline of her own road, the Line Lane. Pityful a rethoric question ran over her cheek, then she continued her way. On her way she met a copy. The copy warned the Little Blind Text, that where it came from it would have been rewritten a thousand times and everything that was left from its origin would be the word ìandî and the Little Blind Text should turn around and return to its own, safe country. But nothing the copy said could convince her and so it didnít take long until a few insidious Copy Writers ambushed her, made her drunk with Longe and Parole and dragged her into their agency, where they abused her for their projects again and again. And if she hasnít been rewritten, then they are still using her. Far far away, behind the word mountains, far from the countries Vokalia and Consonantia, there live the blind texts. Separated they live in Bookmarksgrove right at the coast of the Semantics, a large language ocean. A small river named Duden flows by their place and supplies it with the necessary regelialia. It is a paradisematic country, in which roasted parts of sentences fly into your mouth. Even the all-powerful Pointing has no control about the blind texts it is an almost unorthographic life One day however a small line of blind text by the name of Lorem Ipsum. •


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THE WINDY ISLANDS

Text by Aron LindegĂĽrd Photography by Willy Vanderperre

The Faroe Islands (from Greek (photos), meaning "light", and (graphos), meaning "written") is a person who takes photographs. Amateur photo and graphers take photo for pleasure and to record an event, in the emotion, place, or person. A professional photographer may be an employee, for example of a newspaper, or may contract to cover a particular event such as a wedding or graduation, or to illustrate an advertisement. Others, including paparazzi and fine art photographers, are freelancers, first making a picture and then offering it for sale or display. Some workers, such as policemen, estate agents, journalists and scientists, make photographs as part of other work.

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Photographers who produce moving rather than still pictures are often called cinematographers, videographers or camera operators, depending on the commercial context. Photographers are also categorized based on the subjects they photograph. Some photographers explore subjects typical of paintings such as landscape, still life, and portraiture. Other photographers specialize in subjects unique to photography, including street photography, documentary photography, fashion photography, wedding photography, war photography, photojournalism, and commercial photography. The exclusive right of photographers to copy and use their products is protected by copy-

right. Countless industries purchase photographs for use in publications and on products. The photographs seen on magazine covers, in television advertising, on greeting cards or calendars, on websites, or on products and packages, have generally been purchased for this use, either directly from the photographer or through an agency that represents the photographer. A photographer uses a contract to sell the "license" or use of his or her photograph with exact controls regarding how often the photograph will be used, in what territory it will be used (for example U.S. or U.K. or other), and exactly for which products. This is usually referred to as usage fee and is used to distinguish from

production fees (payment for the actual creation of a photograph or photographs). An additional contract and royalty would apply for each additional use of the photograph. The time duration of the contract may be for one year or other duration. The photographer usually charges a royalty as well as a one-time fee, depending on the terms of the contract. The contract may be for non-exclusive use of the photograph (meaning the photographer can sell the same photograph for more than one use during the same year) or for exclusive use of the photograph (i.e. only that company may use the photograph during the term). The contract can also stipulate that. •


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She packed her seven versalia, put her initial into the belt and made herself on the way.

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MONSOON by Mustafah Abdulaziz

Each summer the monsoon winds of South-West Asia transform the Indian landscape, bringing rainfilled clouds to the dry farmlands. However, with the lifegiving clouds come the deadly floods.

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A young boy runs through the ocean in joy as the monsoon approaches the Indian shores.

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People fighting the floods waiting to be rescued by a Red Cross helicopter.

Opposite page: Two men try to cross a flooded river in an attempt to reach safety.

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Four boys play in the flooded river.

Oppostie page: A man stands on a flooded landfill as he scavenges for supplies in the rubble.

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With the roads rendered useless by the floods farmers must use alternative transports to cross the lands.

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When the water recedes the ground is left moist and fertile and within days life returns to the previosly dead farmlands. •

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VORTICITY, EXPLAINED Text by Vicky Pediah Illustrations by Peter Jellitsch

In fluid dynamics, the vorticity is a pseudovector field that describes the local spinning motion of a fluid near some point, as would be seen by an observer located at that point and traveling along with the fluid.

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Conceptually, the vorticity could be determined by marking the particles of the fluid in a small neighborhood of the point in question, and watching their relative displacements as they move along the flow. The vorticity vector would be twice the mean angular velocity vector of those particles relative to their center of mass, oriented according to the right-hand rule. This quantity must not be confused with the angular velocity of the particles relative to some other point. Many phenomena, such as the blowing out of a candle by a puff of air, are more readily explained in terms of vorticty rather than the basic concepts of pressure and velocity. This applies, in particular, to the formation and motion of vortex rings. EXAMPLES In a mass of fluid that is rotating like a rigid body, the vorticity is twice the angular velocity vector of that rotation. This is the case, for example, of water in a tank that has been spinning for a while around its vertical axis, at a constant rate. The vorticity may be nonzero even when all particles are flowing along straight and parallel pathlines, if there is shear (that is, if the flow speed varies across streamlines). For example, in the laminar flow within a pipe with constant cross section, all particles travel parallel to the axis of the pipe; but faster near that axis, and practically stationary next to the walls. The vorticity will be zero on the axis, and maximum near the walls, where the shear is largest. Conversely, a flow may have zero vorticity even though its particles travel along curved trajectories. An example is the ideal irrotational vortex, where most particles rotate about some straight axis, with speed inversely proportional to their distances to that axis. A small parcel of fluid that does not straddle the axis will be rotated in one sense but sheared in the opposite sense, in such a way that their mean angular velocity about their center of mass is zero. EVOLUTION The evolution of the vorticity field in time is described by the

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vorticity equation, which can be derived from the Navier-Stokes equations. In many real flows where the viscosity can be neglected (more precisely, in flows with high Reynolds number), the vorticity field can be modeled well by a collection of discrete vortices, the vorticity being negligible everywhere except in small regions of space surrounding the axes of the vortices. This is clearly true in the case of 2-D potential flow (i.e. 2-D zero viscosity flow), in which case the flowfield can be modeled as a complex-valued field on the complex plane. Vorticity is a useful tool to understand how the ideal potential flow solutions can be perturbed to model real flows. In general, the presence of viscosity causes a diffusion of vorticity away from the vortex cores into the general flow field. This flow is accounted for by the diffusion term in the vorticity transport equation. Thus, in cases of very viscous flows (e.g. Couette Flow), the vorticity will be diffused throughout the flow field and it is probably simpler to look at the velocity field than at the vorticity. VORTEX LINES AND VORTEX TUBES A vortex line or vorticity line is a line which is everywhere tangent to the local vorticity vector. A vortex tube is the surface in the fluid formed by all vortex-lines passing through a given (reducible) closed curve in the fluid. The 'strength' of a vortex-tube (also called vortex flux) is the integral of the vorticity across a cross-section of the tube, and is the same at every-

volume integral of its squared magnitude) can be intensified when a vortex-line is extended — a phenomenon known as vortex stretching. This phenomenon occurs in the formation of a bathtub vortex in out-flowing water, and the build-up of a tornado by rising air-currents. Helicity, which is vorticity in motion along a third axis in a corkscrew fashion. AERONAUTICS In aerodynamics, the lift distribution over a finite wing may be approximated by assuming that each segment of the wing has a semi-infinite trailing vortex behind it. It is then possible to solve for the strength of the vortices using the criterion that there be no flow induced through the surface of the wing. This procedure is called the vortex panel method of computational fluid dynamics. The strengths of the vortices are then summed to find the total approximate circulation about the wing. According to the Kutta–Joukowski theorem, lift is the product of circulation, airspeed, and air density. ATMOSPHERIC SCIENCES In the atmospheric sciences, the relative vorticity is the vorticity of the air velocity field relative to the Earth. This is often modeled as a two-dimensional flow parallel to the ground, so that the relative vorticity vector is generally perpendicular to the ground, and can then be viewed as a scalar quantity, positive when the vector points upward, negative when it points downwards. Therefore, vorticity is positive then the wind turns

In words, the vorticity tells how the velocity vector changes when one moves by an infinitesimal distance in a direction perpendicular to it. where along the tube (because vorticity has zero divergence). It is a consequence of Helmholtz's theorems (or equivalently, of Kelvin's circulation theorem) that in an inviscid fluid the 'strength' of the vortex tube is also constant with time. Viscous effects introduce frictional losses and time dependence. In a three dimensional flow, vorticity (as measured by the

counter-clockwise (looking down onto the Earth's surface). In the Northern Hemisphere, positive vorticity is called cyclonic rotation, and negative vorticity is anticyclonic rotation; the nomenclature is reversed in the Southern Hemisphere. The absolute vorticity is computed from the air velocity relative to an inertial frame, and therefore includes a term due to

the Earth's rotation, the Coriolis parameter. The potential vorticity is absolute vorticity divided by the vertical spacing between levels of constant entropy (or potential temperature). The absolute vorticity of an air mass will change if the air mass is stretched (or compressed) in the z direction, but the potential vorticity is conserved in an adiabatic flow, which predominates in the atmosphere. The potential vorticity is therefore useful as an approximate tracer of air masses over the timescale of a few days, particularly when viewed on levels of constant entropy. The barotropic vorticity equation is the simplest way for forecasting the movement of Rossby waves (that is, the troughs and ridges of 500 hPa geopotential height) over a limited amount of time (a few days). In the 1950s, the first successful programs for numerical weather forecasting utilized that equation. In modern numerical weather forecasting models and general circulation models (GCM's), vorticity may be one of the predicted variables, in which case the corresponding time-dependent equation is a prognostic equation. Helicity of the air motion is important in forecasting supercells and the potential for tornadic activity. Potential vorticity (PV) is a quantity which is proportional to the dot product of vorticity and stratification that, following a parcel of air or water, can only be changed by diabatic or frictional processes. It is a useful concept for understanding the generation of vorticity in cyclogenesis (the birth and development of a cyclone), especially along the polar front, and in analyzing flow in the ocean. It is also useful in tracing intrusions of stratospheric air deep into the troposphere in the vicinity of jet streaks, a concentrated region within a jet stream where the wind speeds are the strongest. It acts as a flow tracer in the ocean as well. It can also be used to explain how a range of mountains like the Andes can make the upper westerly winds swerve towards the equator and back. Baroclinic instability requires the presence of a potential vorticity gradient along which waves amplify during cyclogenesis. •


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