Hazardous weather: thunderstorms, icing, turbulence and visibility-reducing phenomena
This domain tests recognition and avoidance of convective, icing, turbulence and visibility hazards.
Thunderstorms and turbulence
A thunderstorm has cumulus, mature and dissipating stages. Hazards include lightning, hail, microbursts and downbursts, gust fronts, low-level wind shear, tornadoes and severe turbulence. Microbursts are small, intense downdraughts producing divergent surface winds and are most dangerous near the ground. Turbulence types include mechanical, thermal, frontal and clear-air turbulence, often near jet streams.
Icing and visibility
Structural icing requires visible moisture and temperatures near or below 0°C; risk is greatest in stratiform cloud at about 0 to −10°C and in freezing rain or drizzle. Clear ice forms in large supercooled drops; rime ice forms in small drops. Visibility reducers include fog, mist, haze, dust, smoke, blowing snow and heavy precipitation.
Worked example
A flight descends through stratocumulus at −4°C with visible moisture. The air is below freezing and the cloud is supercooled, so expect airframe icing. If the cloud is thin, climb or descend out of it; if freezing rain is reported, treat it as severe. At the same time, a surface temperature of 2°C, dew point 1°C and light wind suggests radiation fog rather than drizzle.
Traps
Do not use temperature alone to rule out icing. Freezing rain is not just rain. Hail can be thrown out ahead of a storm. Fog needs saturation, not just high relative humidity. Clear-air turbulence is not visible on radar.