Clouds, Precipitation and Fog
Cloud families
Clouds are classified by height and form. High cloud: cirrus, cirrostratus, cirrocumulus. Middle: altostratus, altocumulus. Low: stratus, stratocumulus, nimbostratus. Vertical development: cumulus and cumulonimbus. Candidates must link cloud type to stability: heap clouds (cumulus, cumulonimbus) indicate instability and convection; layer clouds (stratus, altostratus, nimbostratus) indicate stable air and gentle lift. Cirrus warns of an approaching warm front.
Precipitation
Precipitation forms by ice-crystal growth and coalescence. Continuous rain from nimbostratus, showers from cumulus/cumulonimbus, drizzle from stratus. Freezing rain occurs when snow melts in a warm layer and refreezes on impact - a serious icing hazard.
Fog
Fog is cloud at ground level, classified by formation: radiation fog (clear, calm, humid nights, forms over land, burns off with sun), advection fog (warm moist air over cold surface, can occur at sea and in wind), upslope fog, frontal fog and steam fog. Radiation fog requires light wind, clear skies and high humidity; advection fog requires a temperature difference between air and surface and can persist with wind. Visibility criteria: fog < 1000 m, mist 1000–5000 m.
Traps
Questions confuse radiation and advection fog conditions, or call nimbostratus a shower cloud. Remember that freezing rain implies an inversion with a warm layer aloft.
Rule of thumb
If temperature–dewpoint spread is ≤ 2°C and wind is light, expect fog or low stratus overnight.