Information on spotting medium-height clouds - altocumulus, altostratus and nimbostratus. The base of these clouds is usually between 6,500 and 20,000 ft over the British Isles.
Height of base: 2,000 - 18,000 ft
Latin: altum - height; cumulus - heap
Altocumulus are small mid-level layers or patches of clouds - called cloudlets - in the shape of rounded clumps. These are white or grey, and the sides away from the Sun are shaded. Mostly found in settled weather, altocumulus are usually composed of droplets, but may also contain ice crystals.
The presence of shading can help tell the difference between altocumulus and cirrocumulus. Cirrocumulus are white but altocumulus can be white or grey, and the sides will be shaded.
Height of base: 7,000 - 18,000 ft
Latin: altum - height; stratus - flattened or spread out
Altostratus are large mid-level thin grey or blue coloured clouds. Usually composed of a mixture of water droplets and ice crystals, they are thin enough in parts to allow you to see the sun weakly through the cloud. The sun cannot cast shadows when shining through altostratus clouds, which is how you can differentiate between altostratus and nimbostratus. They are spread over a large area - up to thousands of square miles - and they are either featureless or can have parallel stripes.
Height of base: 2,000 - 10,000 ft
Latin: nimbus - rainy cloud; stratus - flattened or spread out
Nimbostratus clouds are dark grey or bluish grey featureless layers of clouds, thick enough to block out the sun. These mid-level clouds are often accompanied by continuous heavy rain or snow and cover most of the sky. If there is hail, thunder or lightning it is a cumulonimbus cloud rather than nimbostratus.
For more information on cloud spotting and pictures of different types of clouds, view our Cloud types for observers guide.