Volume 40 No 4 Aug-Sept 1989.pdf - Lakes Gliding Club

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TOM BRADBURY One cannot always be sure what the low level wind is like. This can make landing one of the hazardous periods, 01 a flight. Gliders with their relatively slow approach speed are sensitive to sudden changes 01 wind. However, the problems are not confined to gliders. Large and heavy airliner!! have also experienced landing accidents due to sudden changes in the lOw level wind flow. This is a briel description of what may be encountered on the 'final approach. Wind shear Wind shear refers to the change of wind v.elocity over a specified distance. From the pilot's point of view the important feature is the vertical wind shear, the change 01 wind velocity as you alter height. This is often called the "wind gradient". (Do not confuse this with a similar sounding term "gradient wind" which is concerned with geostrophic winds which blow round curved isobars.) The most familiar ,type of shear is that between the "geostrophic wind" and the surface wind. The geostrophic wind is measured from the direction and spacing of isobars on a weather map and is assumed to represent the wind at about 2000ft. Tne surface wind is almost always less than the 2000ft wind. Affects low _ level turbulence ----. The relationship depends on the stability of the air and the roughness 01 the terrain. The roughness of the ground' influences the surface drag. This reduces the wind speed near the ground and also aHects low level ttJrbulence. When itis unstable the up and down currents tend tQ stir up the air so that the wind aloft is not very diHerent from the wind near the surface. However, as the air becomes more stable the flow aloft becomes decoupled from the flow near the ground. Then the lowest layers are no longer speeded up by eddies of last moving air coming down from above and' the surface wind falls very light. Here is an example of the kind of change observed in diHer~nl circumstances. Wind at 2000fl Wind at 30ft: Over a smooth sea Over land on a sunny day Over land around sunset Over land on a clear night 270°/25kt 260 0 /22kt 250 0 /15kt 2<strong>40</strong> 0 /1Okt 230 0 /06kt r----:-------.......---~ 2000 ~~~----+---J, 5 10 15 20 knots 500 1000 500 Fig 1. Vertical wind shear between 2000ft and the surface on a sunny day. The wind shear is not constant from top to tottom of the layer; most of the shear occurs near the " WIND SHEAR, DOWNDRAFTS ANDVORTCES Part 1 Tom Bradbury continues his Met series with a two :part article on the I'ow level wind flow and its influence on landing ground. Fig 1 shows the variation in wind speed between 2000ft and the surface during a sunny day. The solid line shows the mean wind speed. the shaded section either side shows how the actual speed might fluctuate due to gustiness. The gusts are most noticeable near the surface where tur.bulence is greatest. How the, shear changes between day and night - When the lowest levels of the air cool down during, the evening a temperature inversion starts to form near the surface. Tllis inversion stops the mixing 01 air between high and low ,levels and the surface wind then decreases. The wind shear becomes greater but the turbulence dies away because there is no longer a strong flow over surface obstacles. 250 o 10 20 knols Ag '2. Wind shear between Sooft and the surface showing the change as a nocturllal inversion develops. Fig. 2 shows the kind of change expected between 50011 and the surface. There are three curves. Tile right hand one represents conditions just after thermals cease. At this time the wind speed might only decrease from 20 to 15kt in the lowest 500ft. The left hand curve represents the effect of a big inversion when the wind speed becomes nearly calm at the surface, It must be pointed out that these are slliloothed curves which are only theoretically represent
WIND SHEAR, DQWNDRAFTS AND VORTICES I I ....... f "pi n Separation of flow Most pilots are familiar with the separation of flow ovef a. stalled wing. Separation of flow can also occur when Ihe wind blows
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Page 10 and 11: Are you covered 168 Even in the bes
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Volume 40 No 4 Aug-Sept 1989.pdf - Lakes Gliding Club

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