Clouds are visible masses of tiny water droplets or ice crystals suspended in the atmosphere, formed when moist air rises, cools below its dew point, and condenses around dust or salt particles (condensation nuclei).
- The standard classification (after Luke Howard) uses Latin roots: cirrus (wispy, high), cumulus (heaped), stratus (layered), and nimbus (rain-bearing).
- By height: high clouds (above about 6 km) are cirrus, cirrocumulus, and cirrostratus, made of ice crystals; middle clouds (2 to 6 km) are altocumulus and altostratus; low clouds (below 2 km) are stratus, stratocumulus, and nimbostratus.
- Clouds of vertical extent are cumulus (fair-weather, heaped) and cumulonimbus, the tall anvil-topped thunderstorm cloud that brings heavy rain, hail, thunder, and lightning.
- Nimbostratus is the steady continuous-rain cloud; cumulonimbus brings short, intense convective downpours.
- Cirrus clouds, being highest and thin, often signal an approaching warm front or change of weather.
The four base names (cirrus, cumulus, stratus, nimbus), the height bands, and the link of cumulonimbus to thunderstorms and nimbostratus to continuous rain are recurring climatology one-mark facts.
Cirrus (high, icy, wispy) versus stratus (low, layered, grey); nimbostratus (continuous rain) versus cumulonimbus (thunderstorm, convective rain); altostratus is mid-level, stratus is low-level.
Cirrus (high), cumulus (heaped), stratus (layered), nimbus (rain); cumulonimbus is the towering thunderstorm cloud.