Weather hazards: thunderstorms, icing, fog and wind shear

Thunderstorms

A thunderstorm needs three ingredients: enough water vapor, an unstable lapse rate, and a lifting force (like a front, terrain or surface heating). It goes through three stages:

1. Cumulus: strong updrafts as the cloud builds.
2. Mature: precipitation begins falling from the base; updrafts and downdrafts side by side. This is the most violent stage.
3. Dissipating: downdrafts take over and the storm rains itself out.

Thunderstorms bring severe turbulence, hail, lightning, icing and microbursts (intense, small downdrafts that can exceed a small airplane's climb ability). The standard advice is to avoid severe thunderstorms by at least 20 NM, and never fly under one or try to fly between closely spaced storms.

Icing

Structural icing needs two things: visible moisture (clouds or rain) and an airframe temperature at or below freezing. Ice destroys lift, adds drag and weight, and can block the pitot tube. Freezing rain means there's warmer air above you (rain falling from a warm layer into freezing air), and it's one of the most dangerous icing conditions. Training airplanes are not certified for flight into known icing; avoid it completely.

Fog

• Radiation fog: clear, calm nights when the ground cools the air to its dewpoint. Common in valleys; usually burns off in the morning.
• Advection fog: warm, moist air moving over a colder surface, needs wind. Common along coasts and can cover big areas.
• Upslope fog: moist air pushed up a slope and cooled.
• Steam fog: cold air moving over warmer water.
• Precipitation-induced fog: warm rain falling through cooler air, often with warm fronts.

Wind shear, turbulence and mountain waves

Wind shear is a sudden change in wind speed or direction, dangerous near the ground on takeoff and landing. It's associated with thunderstorms, fronts and temperature inversions. Strong winds over mountains can create mountain waves and violent rotor turbulence on the downwind side; lens-shaped lenticular clouds are a visible warning.