Nimbostratus occurs as grey or dark grey sheets or layers, most often associated with fronts, particularly warm fronts. It only occurs in opacus form, blotting out the sun or moon, and is normally accompanied by precipitation. Nimbostratus is one of the few major clouds for which there are no species or varieties. The cloud develops through the thickening and lowering of the base of altostratus during the approach of frontal systems, particularly warm fronts and warm occlusions, but it can also develop by the spreading out of the base of cumulonimbus systems, often at cold fronts. If thunder and lightning occur or hail falls from the cloud in such cases, then it would be referred to as Cumulonimbus because there would be proof of convective activity occuring higher up, even though the sky may simply appear as one dark sheet of nimbostratus.
The image was taken on an afternoon of persistent heavy rain caused by a slow moving warm front. The brighter weather on the horizon eventually arrived, but not before our trip to the Farne Islands had been fairly much washed out. Even the kittiwakes, shags, and puffind look rather fed up with it!
Underneath the sheet of dark nimbostratus on the left of the image are a few ragged scuds of the pannus clouds, which frequently occur with Nimbostratus.
Nimbostratus, Inner Farne, Northumberland
25th July, 2002
© Paul Swinhoe
Nimbostratus normally obscures the sky above, but in this example the cloud is present towards the top left of the image, but has broken up and cleared to the right. This was the trailing edge of a cold front, and there had been a spell of heavy rain shortly beforehand. Here, there is a suggestion of cumulus clouds above the nimbostratus, a typical situation at cold fronts. There is every chance that cumulonimbus clouds were also present above the layer of nimbostratus.
Ragged stratus fractus have developed where the nimbostratus has cleared.
Nimbostratus, Wolstanton, Staffordshire
23rd March, 1999
© Paul Swinhoe
This final example is from a situation of unsettled cool showery day, where showers merged to give some longer spells of rain on a WSW wind, along the line of a very poorly defined frontal system. Cumulus clouds are clearly present above, but nimbostratus makes up much of the dark low cloud that dominates this scene.
Some of the very low dark ragged cloud is pannus, underlying, or attached to the base of the main body of nimbostratus.
Nimbostratus over the Cheshire Plain and Mow Cop, Staffordshire
11th July, 2016
© Lionel Burch