When altostratus thickens and becomes rain-bearing, it becomes a nimbostratus cloud.
Height of base: 2,000 - 10,000 ft
Shape: Bands or areas of individual cells
Latin: nimbus - rainy cloud; stratus - flattened or spread out
Precipitation: Continuous rain or snow likely
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.
When there is a dense covering of altostratus or nimbostratus, ragged shreds of stratus clouds are often present. Nimbostratus will often bring precipitation which may last for several hours until the front passes over.
Last updated: 6 August 2014