Nimbostratus clouds are distinguished by a formless cloud layer that is predominantly dark gray in color. “Nimbo” comes from the Latin word “nimbus”, meaning rain. It is a cloud that produces rain, and develops a cloud base between the surface and 10,000 feet. These clouds normally have a thickness of 6,500 to 10,000 feet, but can range from 3,500 to 15,000 feet. In rare cases, nimbostratus can be very thin and be accompanied by a separate layer of altostratus divided by a cloudless layer. Though nimbostratus can be found worldwide, the clouds generally exist around middle latitudes.
The base of the nimbostratus is dimmed by precipitation and is usually not clearly visible. Nimbostratus is almost always accompanied by pannus clouds, which develop underneath nimbostratus. If the pannus layer is overly opaque, the presence of precipitation indicates the presence of nimbostratus. The pannus layer moves slowly and uniformly underneath nimbostratus clouds. Nimbostratus bring more precipitation than other forms of stratus. Sometimes cumulonimbus clouds can be mistaken for nimbostratus clouds as they appear in much the same way, however cumulonimbus clouds bring heavier, less constant precipitation.
Nimbostratus will occur along warm fronts where the slow rising warm air mass creates nimbostratus and stratus clouds. These clouds are preceded by higher-level clouds such as cirrostratus and altostratus clouds. Altostratus will sometimes become a nimbostratus cloud when it thickens and descends into lower altitudes. Nimbostratus can produce rain for several days, mostly depending on the speed of the front it accompanies.