Weather

WEATHER > Honza Rejmanek

Good Glass-Off Gone Bad Originally published in Cross Country Magazine

:Carefree soaring in widespread lift in the late afternoon is a great way to end a day. However, sometimes and in some locations, the soaring situation can evolve from carefree to worrisome in less than 20 minutes. This is because a lot of energy has been put into the system throughout the day and, due to inertia, it is possible that local winds can peak as much as six hours after peak heating. In the last issue, the phenomenon, or a combination of phenomena referred to as glass-off, was discussed. It is important to examine situations when widespread late afternoon and evening lift is best left to the birds.

Widespread lift with ascent rates that can match or slightly surpass the sink rate of soaring aircraft requires significant low-level convergence and similar divergence at the top of lift. If there are cumulus clouds present, then their tops mark the top of the lift. It is important to be vigilant of the height of the cloud tops. In the late afternoon, the scales of convection can change from a sky full of perfect fair-weather cumulus to a sky dominated by a few cumulus congestus (towering cumulus). Given enough humidity in the lower levels and a favorable environmental lapse rate through the depth of the troposphere, some of the cumulus congestus can blow up to full-blown cumulonimbus. Fighting to get down in the vicinity of a large towering cloud at sunset is worrisome at best.

Even if the towering evening cloud is not in your immediate vicinity, it is important to remain observant. The concept to keep in mind is that when even isolated cumulus congestus begin to appear, it is an indication that the scale and depth of convection are starting to transition from shallow to deep. Rather than the desired boundary layer convection, we are beginning to see hints of deep convection that encompass the depth of the troposphere. This is a much more dynamic state that is no longer reliant on surface heating. Deep convection taps most of its energy from latent heat release due to the condensation of countless tiny droplets. In a sense, the atmosphere stops caring about what time of day it is. In the vicinity of these towering clouds, late afternoon and evening might become the strongest part of the day. Mellow evening lift can increase to undesirable strength.

A wide towering cumulus in the distance is evidence of several cubic kilometers of air that has risen or is in the process of rising. This volume has to be replaced from below, and this can lead to the strengthening of valley winds in the landing area at a time of day that they would normally be decreasing. This increase in valley wind might not be evident up on launch.

Conversely, a cumulonimbus that has stopped growing and is dropping out can send a cool, dense outflow, or gust front, many kilometers down a valley. When air quality is poor and the air is humid, the cloud producing the gust front might be obscured by the haze. Whether and to what extent the gust front will affect your landing area depends on many factors, such as the strength and distance of the storm. The surrounding topography can either channel or block the gust front.

If no clouds are present, it is fairly safe to assume that the top of lift will be near the boundary layer top or just below. In desert or dry mountainous regions, this can still be several thousand meters above the surface. In less arid regions, afternoon top of lift will usually be found 1,500-2,500m above the surface. However, the absence of clouds is not an excuse to be less attentive. Watching the sunset from 2,000m above the landing zone while boating around in omnipresent lift can be mesmerizing. Nonetheless, if you want to be on the ground in less than half an hour, you will need to either

fly to the edge of this lift to glide down in time or employ more radical descent techniques.

Finding the edge of widespread evening lift can be a challenge. If you mapped out the edge of the lift in the past few minutes or if you know the flying site quite well, then you might enjoy the sunset from this high perch with confidence. If not, you might want to plan on landing before sunset.

Converging air does not always do so in a symmetrical textbook pattern. It can be sloped, and a giant invisible ramp can form in the air with layers that can sometimes exhibit strong shear turbulence. Sometimes stronger lift can be encountered as you head out into the valley to land. This band of lift can extend for many kilometers following the valley. At other times, the lift brings you into stronger wind, and the only escape path might be into the wind, which might result in gruelingly slow progress over the ground while still going up.

Once over the landing area, there can still be other surprises. For example, in Pemberton, Canada, a local late afternoon wind called the Whistler Express comes roaring in at 40km/h. Not being aware of this local phenomenon can leave a pilot surprised. At other times, synoptic wind can increase with an approaching system. This is another situation where a lack of heating does not mean less wind. The exception can be near the ground in deep valleys where cold pools

are starting to form. These stable surface patches begin to decouple from the wind above. For a pilot, this can mean possible shear down low.

Fortunately, the synoptic situation is usually fairly well resolved for 12 hours out, so a check of the forecast should alert a soaring pilot to the possibility of a tightening pressure gradient in the evening.

Checking the weather, consulting knowledgeable local pilots, and keeping plenty of escape options are vital to safe and enjoyable soaring in the late afternoon or any time of day.

if you want to be on the ground in less than half an hour, you will need to either fly to the edge of this lift to glide down in time or employ more radical descent techniques.

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