Meteorological Hazards to Aviation (Icing, Turbulence, Thunderstorms, Windshear)
Icing
Airframe icing occurs roughly between 0°C and −20°C in visible moisture. Clear ice forms in large supercooled droplets, often in cumuliform cloud, and is heavy and dangerous. Rime ice is milky and forms in smaller droplets. Carburettor icing can occur at outside temperatures from about −10°C to +30°C with high humidity. Freezing rain is the most severe hazard.
Turbulence
Mechanical turbulence arises from surface friction and terrain. Convective turbulence occurs in and near cumulus. Clear air turbulence is found near jet streams, especially on the cold side. Mountain wave gives severe turbulence, downdrafts and lenticular cloud.
Thunderstorms and windshear
A thunderstorm has three stages: cumulus, mature and dissipating. The mature stage contains both updrafts and downdrafts, hail, lightning and the strongest windshear. Microbursts are intense, small downdrafts that cause sudden headwind then tailwind changes, a classic low-level windshear hazard. Avoid storms by at least 20 nm in cruise and do not attempt to fly beneath or through a cell.
Traps
Questions often confuse the stage with the greatest hazard (mature) with the stage of greatest vertical growth (cumulus). Remember that icing is most severe in cumuliform cloud. Do not assume that clear air turbulence shows on radar.
Worked example
A 30 kt headwind change to a 30 kt tailwind over a short distance equals a 60 kt loss of airspeed. That is why windshear recovery requires prompt, full control input.