Nimbostratus clouds are produced by nearly thermodynamically stable air motions and are deep enough to allow precipitation particles to grow to the sizes of raindrops and snowflakes.
Nimbostratus clouds are dark gray and thick enough to hide the sun completely. Unlike some other clouds, they don’t come in different shapes. You can’t look up at a nimbostratus cloud and guess what the shape of the cloud looks like – it just looks flat and gray, like a big cloud blanket over the whole sky.
Nimbostratus occurs along a warm front or occluded front where the slowly rising warm air mass creates nimbostratus along with shallower stratus clouds producing less rain, these clouds being preceded by higher-level clouds such as cirrostratus and altostratus. Often, when an altostratus cloud thickens and descends into lower altitudes, it will become nimbostratus.
Nimbostratus, unlike cumulonimbus, is not associated with thunderstorms, however at an unusually unstable warm front caused as a result of the advancing warm air being hot, humid and unstable, cumulonimbus clouds may be embedded within the usual nimbostratus. Lightning from an embedded cumulonimbus cloud may interact with the nimbostratus but only in the immediate area around it. In this situation with lightning and rain occurring it would be hard to tell which type of cloud was producing the rain from the ground, however cumulonimbus tend to produce larger droplets and more intense downpours. The occurrence of cumulonimbus and nimbostratus together is uncommon, and usually only nimbostratus is found at a warm front.
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