On a dull day when it rains or snows gently but ceaselessly, the blanket of grey cloud hiding the sun and any higher cloud, is Nimbostratus.
Nimbus means rain, and stratus means sheet, so nimbostratus is a sheet cloud from which rain or snow is falling - usually continuously. Although this grey, featureless cloud is classified as a medium-level cloud, its base can be quite low: anywhere from 2,000-7,000 ft (600-2,000 m). The layer is thick enough to block out the sun. Nimbostratus forms when warm, moist air is lifted steadily over a large area. This can happen at a warm front or, less commonly, at a cold front. Usually the air is stable and there is little turbulence in the cloud, but local conditions can cause Cumulonimbus to develop inside the Nimbostratus.

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