Turbulence, Icing, Thunderstorms and Other Aviation Hazards
Turbulence and wind shear
Turbulence types include mechanical (terrain, buildings), thermal (convection), frontal and clear-air turbulence (CAT), which occurs near jet streams and is invisible. Low-level wind shear and microbursts are the most dangerous near the ground: a sudden headwind-to-tailwind change during approach can cause a severe performance loss. Thunderstorm gust fronts and downbursts are classic triggers. Wake turbulence is a separate category; stay above and upwind of heavy aircraft.
Icing
Structural icing needs visible moisture and temperatures between 0°C and about -20°C, with the worst around -2°C to -10°C. Clear (glaze) ice forms in large supercooled droplets, often in cumuliform cloud and freezing rain; it is heavy, fast-growing and hard to remove. Rime ice is milky and forms in smaller droplets in stratiform cloud. Mixed ice combines both. The traps: icing is rare below -20°C because droplets freeze, and freezing rain is extremely hazardous.
Thunderstorms and other hazards
A thunderstorm needs moisture, instability and a lifting trigger. Its life cycle is cumulus, mature (updraught and downdraught, lightning, hail) and dissipating. Avoid the mature stage. Other hazards include hail, lightning, downbursts, tornadoes and volcanic ash, which is hazardous to jet engines. In the northern hemisphere, avoid the right-hand flank of a moving storm where hail is likely.
Worked example
An aircraft climbs through a stratiform layer at -5°C with moderate icing and no radar returns. Expect rime or mixed ice, and consider a climb to colder air or a descent to warmer air, requesting a change of level early.