Windshear and Microburst — Page 181, Lesson 276

I want to walk you through the external meteorological clues that can warn you of low-level windshear near an airport. Recognizing these clues early lets you make a proactive decision — you can go around, delay your approach, or hold off on take-off until conditions improve, rather than flying into a hazard you could have avoided. Let's start with the first clue: strong, gusty surface winds. This is especially significant when the aerodrome is located near hills or where there are comparatively large buildings near the runway. Those obstacles can create turbulence and shear as the wind flows around them. Next, look for lenticular cloud. That's a smooth, lens-shaped altocumulus cloud. Its presence tells you there are standing waves in the atmosphere, usually downwind from a mountain. Those waves can produce strong vertical currents and shear at low level. Then there's virga. Virga is precipitation falling from the base of a cloud that evaporates before it reaches the ground. You'll often see it under convective cloud. The key point here is that even though the rain itself has evaporated, the downdraughts that carried it may still exist and can reach the ground. So virga doesn't mean the danger is gone — the downward rush of air can still be there. Another clue is a roll cloud girding the base of a thunderstorm and advancing ahead of the rain belt. That roll cloud indicates the presence of a gust front — the leading edge of cold, dense air rushing out from the thunderstorm. You might also see areas of dust raised by wind, particularly when that dust forms a ring below convective clouds. That ring pattern is a direct visual sign of a downburst — a strong downward burst of air that hits the ground and spreads out. Check the wind socks around the airfield. If different wind socks are responding to different winds, that tells you the wind direction or speed is varying significantly across the field — a classic sign of shear. Look at smoke plumes if there are any nearby. If the upper and lower sections of a smoke plume are moving in different directions, the plume is being sheared apart by changing wind with height. Finally, and this is a hard rule: thunderstorms should always be assumed to have the capability of producing hazardous windshear. Even if you don't see any of the other specific clues, a thunderstorm in the vicinity means you treat the area as potentially dangerous. Now, let me explain what happens to the aeroplane when it enters a downdraught from a horizontal airflow. The aeroplane's momentum will at first keep it on its original path relative to the new direction of flow. So the aircraft doesn't instantly change direction — it continues along its old trajectory for a moment. That change in relative airflow affects two things: first, you get a loss of airspeed. Second, the change in relative airflow also affects the angle of attack of the wing. The resulting decrease in angle of attack causes a loss of lift. So you're losing both speed and lift at the same time, which is a serious situation, especially close to the ground.