Clouds, Precipitation and Fog
This domain tests cloud identification, the weather each genus implies, and the conditions that bring visibility down.
Cloud families
High cloud (above about 6 km): cirrus, cirrostratus, cirrocumulus - ice cloud with no significant precipitation. Middle cloud (2-6 km): altostratus and altocumulus. Low cloud (below 2 km): stratus, stratocumulus, nimbostratus. Convective cloud: cumulus and cumulonimbus, the latter spanning all levels and producing thunderstorms, hail and severe icing. Estimate base from the surface temperature-dew point spread: roughly 125 m per 1°C.
Fog
Fog is visibility below 1,000 m; mist is 1,000-5,000 m. Radiation fog needs clear skies, light wind, moist surface air and overnight cooling, forms near dawn and burns off with insolation. Advection fog needs warm moist air moving over a cold surface with moderate wind, and can persist day and night. Other types are upslope, frontal, steam fog and precipitation-induced fog.
Precipitation
Drizzle falls from stratus, continuous rain from nimbostratus, showers from cumulus and cumulonimbus; also snow, sleet, hail and virga.
Worked example
At 1800 local, temperature 18°C, dew point 17°C, clear sky, wind 3 kt. The 1°C spread means only about 1°C of cooling is needed for saturation; with radiative cooling near 1°C per hour, fog forms within one to two hours and visibility can fall below 400 m by dawn. It is reported as FG because visibility is under 1,000 m.
Traps
Using 5 km as the fog threshold; assuming radiation fog needs calm or strong wind; confusing nimbostratus (steady rain) with cumulonimbus (showery rain with lightning).