Cloud types and what they tell you
1The lesson in plain words
Clouds have ten main types, called genera, grouped by height. Their shapes and changing sequence help you read the weather.
How clouds form
Clouds form when air cools to saturation, usually as it rises. Convection is rising air heated from below. Orographic lifting forces air up over high ground. Frontal lifting carries warmer air over colder air. Convergence occurs where air streams meet near the surface and force air upward, often around a low-pressure centre. Cooling from below can also produce fog or low stratus.
Names and levels
The WMO Cloud Atlas lists ten genera in three levels. In middle latitudes: high 5–13 km (cirrus, cirrocumulus, cirrostratus), middle 2–7 km (altocumulus, altostratus, often nimbostratus), low below 2 km (stratus, stratocumulus). Cumulus and cumulonimbus have low bases but can grow far upwards. The name parts help: cirro- high, alto- middle, strato- layer, cumulo- heap, nimbo- rain.
Layers or heaps
Layer clouds (stratus, altostratus) usually mean stable air, lifted gently over a wide area or cooled from below. Heap clouds (cumulus) mean unstable, rising air. Small flat cumulus on a sunny day mean fair weather. Cumulus that keep growing tall by late morning warn of showers in the afternoon.
A front coming
A classic sign: cirrus thickening into a milky cirrostratus with a halo round the sun, then a grey altostratus, then a low dark nimbostratus with steady rain. With a falling barometer, this sequence often comes ahead of a warm front, sometimes a day in advance. The Pressure systems unit follows the whole front.
Warning clouds
A cumulonimbus, a huge tower often with an anvil top, brings heavy showers, squalls (sudden strong winds), lightning and sometimes hail or waterspouts. Turreted altocumulus castellanus in the morning warn of thunderstorms later. Smooth, lens-shaped lenticular clouds over high ground mark waves in stable air and often warn of strong wind aloft. Practise recognition in the Weather lab: clouds.
2Key data
- High clouds (5–13 km)
- Cirrus, cirrocumulus, cirrostratus
- Middle clouds (2–7 km)
- Altocumulus, altostratus; nimbostratus spreads above and below
- Low clouds (below 2 km)
- Stratus, stratocumulus; cumulus and cumulonimbus have low bases
- Warm front sign
- Cirrus → cirrostratus (halo) → altostratus → nimbostratus
- Danger cloud
- Cumulonimbus: squalls, lightning, hail, waterspouts
3Key points
- 1
Clouds form when air cools to saturation, usually by rising.
- 2
Ten genera in three levels; name parts tell height and shape.
- 3
Layer cloud: stable air. Heap cloud: unstable air.
- 4
Cirrus → cirrostratus halo → altostratus → nimbostratus often means a warm front.
- 5
Cumulonimbus warns of heavy showers, sudden strong winds and lightning.
4Frequent mistakes
Wrong: Any cirrus means rain tomorrow.
Right: Cirrus alone does not forecast rain. Watch for it thickening into a veil, a halo and lower cloud, with the barometer falling.
Wrong: Cumulus clouds always mean showers.
Right: Small flat cumulus mean fair weather. Watch for cumulus that keep growing tall.
Wrong: A halo around the sun is just a pretty effect.
Right: It shows ice crystals, often in cirrostratus, part of the cloud sequence ahead of a warm front.
5Practice case & exam keys
At anchor you see thin cirrus at breakfast. By noon the sky is a milky veil with a halo round the sun, and the barometer has fallen 3 hPa since morning. What may be coming, and what do you do?
Show the answer
The thickening high cloud, halo and falling pressure suggest an approaching warm front. These signs do not fix its arrival time or wind direction. Check the latest forecast for timing and wind changes, record the pressure regularly, and check that your anchorage will remain sheltered.
Exam keys
- Ten cloud genera: cirro- means high, alto- middle, strato- layer, cumulo- heap and nimbo- rain.
- Layer clouds mean stable air; heap clouds mean unstable air.
- Cirrus → cirrostratus with halo → altostratus → nimbostratus and a falling barometer: a warm front often approaching.
Finish this unit
When you've read it, match the terms, then answer three questions. Get them all right to finish the unit; opening the page alone doesn't count.
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