Cirrostratus

Cirrostratus is a high-level cloud genus in the cirrostratus family. It forms a thin, uniform veil that can cover all or most of the sky and is composed primarily of ice crystals. The cloud typically occurs at altitudes of about 5,000 to 13,000 meters (16,500 to 42,000 feet), and its thickness is usually only a few hundred to about a thousand meters.

Formation and appearance: Cirrostratus develops when moist air at high altitude rises and cools, spreading out into a wide, translucent sheet. The cloud often appears as a milky or satin-like veil that diminishes contrast in the sky. Because it is thin, the sun or moon can remain visible through the cloud.

Weather significance: The presence of cirrostratus almost always signals an approaching weather system, especially a warm front. Precipitation can begin within 12 to 24 hours as lower-level clouds develop, though cirrostratus itself may not produce noticeable precipitation. It commonly occurs with other high clouds such as cirrus.

Optical phenomena: Ice crystals in cirrostratus can refract and reflect light to produce halos around the sun or moon. The 22-degree halo is the most common, and sundogs may also appear under favorable conditions.

Relation to other clouds: Cirrostratus differs from cirrus, which are thin, fibrous, and more dispersed, and from altostratus, which is a lower, more uniform gray layer. Cirrostratus forms a continuous veil at high altitude and can be a precursor to lower, precipitation-bearing cloud decks.