Nimbostratus (Ns) - the name derives from the Latin nimbus = rain, and stratus = spread out. Nimbostratus is a member of the ten fundamental cloud types (or cloud genera). Although it is classed as a middle-level layer cloud, its base is generally low at 0 to 2 km (0 to 6,500 ft), often very close or even touching the ground. Nimbostratus are also classified as clouds of extreme vertical development.
Nimbostratus are very common clouds in temperate latitudes, forming a dense and extremely extensive grey layer at low altitude. They are thick, dark, amorphous and solid in appearance, often associated with more or less continuous rainfall which makes the cloud base soft and diffuse. Frequently they have ragged edges and small cloud fragments, called scud, developing beneath the main nimbus cloud from recondensation of water saturated air. The Sun is always obscured. Nimbostratus clouds produce dull and gloomy wet days. Precipitation of snow or rain is prolonged and widespread, although not usually heavy and individual small areas may not be producing precipitation at any one moment.
On mountainsides, nimbus clouds often can be seen ascending or descending the slopes as a rainstorm arrives or departs.
Nimbostratus clouds might consist entirely of cloud droplets or raindrops, or of ice crystals and snowflakes. However, the composition of nimbus clouds varies greatly depending on temperature and often they may be mixed with supercooled water droplets and ice crystals present at the same layer.
Extensive layers of nimbus clouds arise through the slow uplift of moist air in a depression, typically at the advancing warm front. Thickening layers of altostratus (As) are frequently preceeding the nimbus cloud as the cloud base lowers towards the surface. Nimbostratus is distinguished from altostratus (As) by its thickness and from altocumulus (Ac) and stratocumulus (Sc) by the fact that they have regular patterns and a well defined cloud base. If the cloud is accompanied by thunder, lightning or hail, then it is d efined as being cumulonimbus.
Varieties of Nimbostratus:
Ns praecipitatio, when precipitation is reaching ground, virga with fall-streaks and pannus, as ragged scud clouds developing between ground surface and cloud base. However, nimbostratus is not subdivided into species, nor does it have very much 'true' varieties. Because of its density, no optical phenomena will appera underneath it. Its just a grey dark blanket.
What do nimbostratus tell about the weather?
Well, firstly nimbostratus are bringing often days of persistent rain or snow. Fall-streaks hanging from the cloud base are heralding imminent rain or snow. Breaking up nimbostratus indicates the passage of a cold front.