High Clouds
Above 20,000 ft
Cirrus (Ci), Cirrocumulus (Cc), Cirrostratus (Cs). Ice crystal clouds — thin, wispy. Generally not a ceiling concern but Cs can reduce visibility.
Mid Clouds
6,500 – 20,000 ft
Altocumulus (Ac), Altostratus (As). Mixed ice and water. Altostratus can produce continuous rain reaching the surface. Common over Scotland.
Low Clouds
Below 6,500 ft
Stratus (St), Stratocumulus (Sc), Nimbostratus (Ns). Most significant for approach. Stratocumulus is the most common UK cloud type. Ns brings persistent rain.
Cumulonimbus (Cb)
All levels — reported in METAR
Thunderstorm cloud. Extreme turbulence, icing, lightning, hail. Avoided by all aircraft. Explicitly reported in METAR as CB suffix on a cloud group.
Towering Cumulus (TCU)
Low to mid — reported in METAR
Developing Cb. Significant turbulence and rapidly deteriorating conditions. Treated with the same caution as CB. Reported in METAR as TCU suffix.
CAVOK
No cloud below 5,000 ft
Ceiling And Visibility OK. Visibility ≥10km, no cloud below 5,000ft or MSA, no CB/TCU, no significant weather. The best possible METAR descriptor.