altostratus

Altostratus is a mid-level cloud type in the stratiform family. It generally forms a uniform gray or blue-gray sheet that spreads across large portions of the sky and often diminishes the brightness of the sun. Unlike puffy cumulus, altostratus lacks well-defined individual cloud elements, though the sun may be seen as a dim disk through a very thin veil.

Formation and altitude: Altostratus usually develops at mid-altitude, roughly 2,000 to 7,000 meters (6,500 to 23,000 feet). It forms as air rises and moisture condenses in the middle troposphere, frequently ahead of a warm front or in the approach of a storm system. In many cases it replaces cirrostratus as an approaching front thickens, and it may transition to nimbostratus as precipitation begins.

Weather implications and subtypes: When thin, altostratus may produce little to no visible precipitation, but thicker layers can yield light rain or drizzle, sometimes persistent with frontal activity. The main named subtypes are altostratus translucidus (sun disk faintly visible through the veil) and altostratus opacus (sun not visible through a dense veil). Altostratus undulatus refers to wavy patterns that can appear in the layer. Halos around the sun or moon are possible in very thin cases.

Etymology and classification: The name derives from Latin altus meaning high and stratus meaning spread out. Altostratus is a standard mid-level cloud type in the International Cloud Atlas and the World Meteorological Organization classification, situated between cirrostratus and nimbostratus in altitude and behavior.