Nimbostratus

Nimbostratus is a gray cloud, low (up to 3 km altitude) and large, often shadowy, whose appearance becomes a veil as a result of more or less continuous showers of rain or snow that, in most cases, hit the ground. The thickness of this layer is, in all its extent, sufficient to completely hide the sun.

There are often below this layer ragged clouds, whether or not attached to it. In tropical regions, during rainy breaks, it is possible to observe the Nimbostratus split into distinct clouds and quickly rejoin.

Nimbostratus results from the slow rise of large horizontal air layers to sufficiently high heights; it may come from the thickening of a precipitating Altostratus or the expansion of a Cumulonimbus. They form in unstable air masses, where at the place of their formation the humidity is moderate or high and the temperature is relatively high, and they are usually associated with hot or occluded fronts.

Evaporation of rainwater usually lowers visibility, and a lower layer of cloud or fog may form beneath the Nimbostratus if the air becomes saturated. Although found worldwide, Nimbostratus are most commonly found in the mid latitudes.

The base of a Nimbostratus is usually not clearly visible. In all cases, the Nimbostratus are accompanied by pannus clouds, which develop under the Nimbostratus. Often, when a thick Altostratus cloud descends to lower altitudes, it becomes Nimbostratus.

Typically, Nimbostratus is a sign of moderate to heavy constant precipitation, as opposed to the shorter period of heavier precipitation normally released by a Cumulonimbus. However, precipitation does not occur at ground level in the case of a virgin, and accompanies other cloud types. Precipitation can last several days, depending on the speed of the front. Nimbostratus can be mistaken for thick Altostratus, but Nimbostratus are usually dark gray and you never see the sun through them.

Nimbostratus, unlike Cumulonimbus, are not associated with storms. However, on an exceptionally unstable warm front, Cumulonimbus clouds can be incorporated into the usual Nimbostratus. In this situation, with lightning and rain occurring, it would be difficult to tell which type of cloud would be producing the rain coming to the ground, however Cumulonimbus tend to produce larger drops and more intense rainfall. The occurrence of Cumulonimbus and Nimbostratus together is uncommon, usually only Nimbostratus are found on a warm front.

Subtypes
Nimbostratus praecipitatio

Nimbostratus virga