Chapter 3b: Effects of Weather on Small Unmanned Aircraft Performance  Wind shear is dangerous to an aircraft. It can rapidly change the performance of the aircraft and  disrupt the normal flight attitude. For example, a tailwind quickly changing to a headwind causes an  increase in airspeed and performance. Conversely, a headwind changing to a tailwind causes a  decrease in airspeed and performance. In either case, a pilot must be prepared to react immediately to  these changes to maintain control of the aircraft. The most severe type of low-level wind shear, a microburst, is associated with convective precipitation  into dry air at cloud base. Microburst activity may be indicated by an intense rain shaft at the surface  but virga at cloud base and a ring of blowing dust is often the only visible clue. A typical microburst has  a horizontal diameter of 1–2 miles and a nominal depth of 1,000 feet. The lifespan of a microburst is  about 5–15 minutes during which time it can produce downdrafts of up to 6,000 feet per minute (fpm)  and headwind losses of 30–90 knots, seriously degrading performance. It can also produce strong  turbulence and hazardous wind direction changes. During an inadvertent microburst encounter, the  small UA may first experience a performance-increasing headwind, followed by performance- decreasing downdrafts, followed by a rapidly increasing tailwind. This can result in terrain impact or  flight dangerously close to the ground. An encounter during approach involves the same sequence of  wind changes and could force the small UA to the ground short of the intended landing area.  It is important to remember that wind shear can affect any flight at any altitude. While wind shear may  be reported, it often remains undetected and is a silent danger to aviation. Always be alert to the  possibility of wind shear, especially when flying in and around thunderstorms and frontal systems.  The stability of the atmosphere depends on its ability to resist vertical motion. A stable atmosphere  makes vertical movement difficult, and small vertical disturbances dampen out and disappear. In an  unstable atmosphere, small vertical air movements tend to become larger, resulting in turbulent  airflow and convective activity. Instability can lead to significant turbulence, extensive vertical clouds,  and severe weather. The combination of moisture and temperature determine the stability of the air and the resulting  weather. Cool, dry air is very stable and resists vertical movement, which leads to good and generally  clear weather. The greatest instability occurs when the air is moist and warm, as it is in the tropical  regions in the summer. Typically, thunderstorms appear on a daily basis in these regions due to the  instability of the surrounding air. As air rises and expands in the atmosphere, the temperature decreases. There is an atmospheric  anomaly that can occur; however, that changes this typical pattern of atmospheric behavior. When  the temperature of the air rises with altitude, a temperature inversion exists. Inversion layers are  commonly shallow layers of smooth, stable air close to the ground. The temperature of the air  increases with altitude to a certain point, which is the top of the inversion. The air at the top of the  layer acts as a lid, keeping weather and pollutants trapped below. If the relative humidity of the air  is high, it can contribute to the formation of clouds, fog, haze, or smoke resulting in diminished  visibility in the inversion layer. Surface-based temperature inversions occur on clear, cool nights when the air close to the ground is  cooled by the lowering temperature of the ground. The air within a few hundred feet of the surface  Remote Pilot – Small Unmanned Aircraft Systems Study Guide  24  Atmospheric Stability Inversion