Cirrostratus

Cirrostratus are tall clouds (over 6 km) with the appearance of a very thin, whitish and transparent veil, a few hundred meters thick, that can cover the whole sky. They develop from the fusion of Cirrus or Cirrocumulus elements or the expansion of a Cumulonimbus anvil, and are formed by ice crystals.

When approaching a frontal system, Cirrostratus often begin as nebulosus and later become fibratus. If Cirrostratus starts as fibratus, it often means that the frontal system is weak. Its presence indicates a large amount of moisture in the upper part of the atmosphere.

Cirrostratus sometimes signal the approach of a warm front if they form after Cirrus and spread throughout the sky and thus may be signs that precipitation may continue in the next 12 to 24 hours, or the next 6. 8 hours if the front moves fast. Cumulus humilis or Stratocumulus are often found below Cirrostratus.

When followed by light clouds, they often announce 1 to 2 days in advance of thunderstorms.

Sometimes they are almost invisible and are revealed only by a milky sky and a halo around the moon or sun resulting from the refraction of light on the ice crystals.

Subtypes
Cirrostratus fibratus

Cirrostratus nebulosus

Cirrostratus undulatus

Cirrostratus duplicatus