Clouds and Precipitation
Classification
Clouds are grouped by height and form. High clouds (cirrus, cirrostratus, cirrocumulus) are above about 20,000 ft and consist of ice. Middle clouds (altostratus, altocumulus) lie roughly 6500–20,000 ft. Low clouds (stratus, stratocumulus, nimbostratus) lie below about 6500 ft. Clouds with vertical development (cumulus, cumulonimbus) span several levels. The prefix nimbo- or suffix -nimbus indicates precipitation.
Formation and precipitation
The base forms at the condensation level, where rising air cools to its dew point. Precipitation reaches the ground when droplets or ice crystals grow large enough to fall without evaporating. Continuous steady rain suggests nimbostratus; showery rain suggests cumuliform cloud; drizzle suggests stratus.
Aviation significance
Low stratus and fog are the main causes of low ceiling and reduced visibility. Cumulonimbus brings icing, hail, severe turbulence, microbursts and lightning. Lenticular clouds indicate mountain wave and strong turbulence. Freezing rain implies a warm layer aloft over sub-zero air near the surface, a serious icing hazard.
Traps
Questions often ask which cloud type gives continuous rain (nimbostratus) versus intermittent showers (cumulus congestus or cumulonimbus). Do not assume cirrus means fine weather everywhere; thickening cirrostratus often precedes a warm front. Remember that the cloud base is not the same as the freezing level.
Worked example
Surface temperature 24°C, dew point 18°C. Spread 6°C, so cloud base ≈ 6 × 400 = 2400 ft. With a lapse rate near 2°C/1000 ft, the 0°C level is about 12,000 ft, well above the cloud base.