Weather Hazards (icing, turbulence, thunderstorms, visibility)
Weather Hazards tests the aircraft-relevant side of meteorology: icing, turbulence, thunderstorms and low visibility.
Icing
Supercooled water is the key ingredient. Rime ice forms from small droplets at roughly −10 to −20 °C, rough and milky; clear ice forms from large droplets around 0 to −10 °C, smooth, heavy and fast-accreting, with serious aerodynamic effects. Freezing rain implies severe clear ice. Stratiform cloud gives widespread icing; cumuliform cloud gives more intense but local icing. Icing is most likely between 0 and −20 °C and near the freezing level in stratiform cloud.
Turbulence
Mechanical (surface friction, hills, buildings), thermal (convection), frontal, and clear-air turbulence near jets and in mountain waves. Mountain waves produce lenticular cloud, rotors and severe low-level turbulence on the lee side. Low-level wind shear is critical near microbursts, gust fronts and frontal passages.
Thunderstorms
Life cycle: cumulus (updraughts), mature (rain, downdraught, lightning, hail, downburst), dissipating (downdraughts, evaporation). Hazards: hail, severe turbulence, icing, lightning, microbursts, wind shear and torrential rain - all present in the mature stage.
Visibility
Fog types: radiation (clear, calm, cool, autumn or winter, clearing after sunrise), advection (warm moist air over a cold surface, persistent), frontal, upslope and steam fog. Mist and haze reduce visibility without saturating the air.
Worked example
An aerodrome reports 20 °C/18 °C just after sunset, clear sky, 3 kt wind, in autumn. The 2 °C spread suggests saturation within hours as the ground cools radiatively, so radiation fog is likely before dawn, with falling RVR and a low ceiling.
Traps
- Reversing rime and clear ice.
- Assuming fog needs precipitation.
- Ignoring why advection fog persists while radiation fog burns off.
- Forgetting that the mature thunderstorm stage carries the most hazards.