Nimbostratus

(Section 2.5.2.6)

The main body of Nimbostratus almost invariably occurs at altitudes between 2 km and 4 km (6 500 ft and 13 000 ft) in polar regions, between 2 km and 7 km (6 500 ft and 23 000 ft) in temperate regions and between 2 km and 8 km (6 500 ft and 25 000 ft) in tropical regions. However, the base is often below these limits, and the upper surface is above them. Nimbostratus is generally thicker than Altostratus, having a vertical thickness of typically 2 km to 8 km (6 500 ft to 25 000 ft).

Below the cloud. Viewed from below, Nimbostratus is grey and often dark. Rain or snow fall from its base, usually reaching the ground. Therefore, its base appears diffuse or indefinite, or it may not even be visible through heavy precipitation. Pannus is often encountered under Nimbostratus. Turbulence is stronger in the pannus than in the Nimbostratus immediately above it.

Within the cloud. The constituent particles of Nimbostratus are similar to that of Altostratus, but are generally larger and more numerous. This, and the typically great vertical extent of Nimbostratus, causes it to be somewhat dark within the lower parts of the cloud. While Nimbostratus is essentially a layer cloud, cumuliform convective clouds with considerable vertical extent may form within it. In Nimbostratus, visibility is poor, often less than 50 m, and icing may occur. Turbulence is generally moderate, but may become fairly strong when there is internal convection.

Above the cloud. Viewed from above, the upper surface of Nimbostratus is often similar to Cirrostratus and Altostratus. It is diffuse and fairly smooth, and it sometimes appears flat, undulated or fleecy. In unstable air masses, Cumulus congestus or Cumulonimbus may be embedded in the Nimbostratus, and may rise above its upper surface. Optical phenomena such as a glory, fog bow and subsun (undersun) may be visible.