Clouds and precipitation
This domain tests cloud classification, formation, microstructure and precipitation type, and the operational meaning of cloud reports.
Formation and classification
Clouds form when air cools to saturation by ascent, radiative cooling or mixing. Genera are cirrus, cirrocumulus and cirrostratus (high); altocumulus and altostratus (middle); stratus, stratocumulus and nimbostratus (low); cumulus and cumulonimbus (vertical). Cumuliform clouds indicate instability and convective currents; stratiform clouds indicate stable, widespread ascent. Cloud base from surface temperature and dew point is about 125 m per °C spread (400 ft/°C).
Precipitation
Warm-cloud rain grows by collision–coalescence; cold-cloud precipitation involves ice crystals, aggregation and riming. Rain, drizzle, snow, sleet, freezing rain and hail have distinct formation paths. Freezing rain occurs when a warm layer aloft melts snow and a subfreezing layer exists near the surface; it is a severe icing hazard. Hail grows in strong cumulonimbus updraughts.
Worked example
Surface temperature 30°C, dew point 22°C, convective cloud reported. Spread 8°C gives a cumulus base near 8 × 125 = 1,000 m (about 3,300 ft). If the ELR is steep and the air is conditionally unstable, expect towering cumulus or CB, showers and possible hail. If the layer is stable, expect stratocumulus instead. The base calculation does not give cloud top.
Traps
Do not equate BKN with overcast; cloud amount and height are separate. Nimbostratus gives steady precipitation, not showery precipitation. CB is both a cloud type and a hazard. Virga may be reported as precipitation but not reach the ground.