Thunderstorms, Turbulence, Icing and Other Hazardous Weather
A thunderstorm needs three ingredients: sufficient moisture, conditional instability and a lifting trigger (heating, front, orography or convergence). Its life cycle has three stages: cumulus (updraughts only), mature (updraughts and downdraughts, heaviest rain, hail, lightning, maximum turbulence and icing) and dissipating (downdraughts dominate). Candidates must know the hazards: severe turbulence, hail, microbursts and downbursts, lightning, icing and wind shear. A microburst is a concentrated downdraught producing a headwind then tailwind shear on approach - the classic low-level wind shear accident profile.
Icing
Airframe icing types: rime (small droplets, low liquid water content, rough opaque build-up), clear or glaze (large supercooled droplets, high liquid water content, streamlined and heavy, most dangerous), and mixed. Icing is most likely at 0 to −20 °C, peaking around −2 to −10 °C, in stratiform cloud for rime and in cumuliform cloud for clear ice. Freezing rain gives the most rapid accretion. Pitot heat and carburettor heat are the practical responses; carburettor icing can occur at outside temperatures up to about +20 °C with high humidity.
Turbulence
Mechanical turbulence from surface friction and wind over obstacles, thermal turbulence from convection, and clear air turbulence (CAT) near jet streams, especially on the cold side of the jet core and in wind shear zones.
Worked example: a temperature of −5 °C in a large cumuliform cloud with visible supercooled droplets indicates a high risk of clear ice; avoid the cloud or climb/descend to colder or warmer layers.
Trap: icing risk does not end at 0 °C; supercooled water persists well below freezing, and clear ice is the most serious type.