Altostratus clouds

Altostratus evolves as a thin layer from a gradually thickening veil of cirrostratus and is usually grey or blue in colour with very few features.

Symbol: Altocumulus symbol

Height of base: 7,000 - 18,000 ft

Shape: Layered and featureless

Latin: altum - height; stratus - flattened or spread out

Precipitation: None

Altostratus cloud (Photo: Jim Galvin) Altostratus cloud (Photo: Jim Galvin)

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.

Last updated: 6 August 2014