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Description Nimbostratus and differences with similar clouds
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Nimbostratus
Grey cloud layer, often dark, the appearance of which is rendered diffuse by more or less
continuously falling Rain or Snow, which in most
cases reaches the ground. It is thick enough
throughout to blot out the sun. Low, ragged clouds frequently occur below the layer, with which
they may or may not merge.
Main differences between Nimbostratus and similar clouds of other genera
Thin Nimbostratus may be confused with thick Altostratus. Nimbostratus generally has a
darker grey colour than Altostratus. By definition, Nimbostratus is sufficiently opaque
throughout to hide the sun or moon, whereas Altostratus hides the luminary only when the
latter is behind the thickest parts. If on dark nights, doubt exists regarding the choice of the
designation Nimbostratus or Altostratus, the cloud is by convention called Nimbostratus when Rain or Snow reaches the ground.
Nimbostratus is distinguished from a thick layer of Altocumulus or Stratocumulus by the lack
of clearly defined elements or its lack of a distinct lower surface.
Nimbostratus is distinguished from thick Stratus by the fact that it is a dense cloud producing
rain, snow or ice pellets ; the precipitation which may fall from Stratus is in the form of Drizzle, Ice prisms or Snow grains.
When the observer is beneath a cloud having the appearance of a Nimbostratus, but
accompanied by Lightning, Thunder or Hail, the cloud should by convention be called
Cumulonimbus.