Clouds, Precipitation and Fog
Clouds are classified by height and form: high (cirrus, cirrostratus, cirrocumulus), middle (altostratus, altocumulus), low (stratus, stratocumulus, nimbostratus) and clouds of vertical development (cumulus, cumulonimbus). Candidates must link cloud type to stability and to the associated weather: stratiform cloud means stable air and steady, often light precipitation; cumuliform cloud means instability, showers and turbulence. Nimbostratus gives continuous rain; cumulonimbus gives heavy showers, hail, lightning and icing.
Precipitation types
Rain, drizzle (droplets under 0.5 mm, typically from stratus), snow, sleet, hail and freezing rain. Freezing rain occurs when rain falls through a warm layer into a sub-zero layer and freezes on impact - an extreme icing hazard.
Fog
Fog is cloud at the surface with visibility under 1000 m. Radiation fog forms on clear, calm nights with a small temperature-dew point spread and moist air, typically in autumn/winter and in valleys; it dissipates with insolation and wind mixing. Advection fog forms when warm moist air moves over a cold surface and needs wind; sea fog is of this type. Upslope, frontal and steam fog are the other main types.
Worked example: clear night, wind 3 kt, temperature 8 °C, dew point 6 °C, high relative humidity - expect radiation fog by dawn, clearing a few hours after sunrise.
Trap: radiation fog needs light wind, not calm; perfectly calm air limits vertical mixing and produces dew or mist instead.