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.
Height of base: 7,000 - 18,000 ft
Shape: Layered and featureless
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.
Last updated: 6 August 2014