Cirrostratus

Cirrostratus are sheetlike clouds composed of ice crystals. Even though cirrostratus can cover the entire sky, and can be up to several thousand feet deep, they are relatively transparent, as the sun or the moon can be easily seen through them. Sometimes, the only indication of their presence is given by an observed halo (around the sun or moon). Halos result from the refraction of light by the ice crystals which make up the cloud.

Single Halo Multiple Haloes

These high level clouds typically form when a broad layer of air is lifted to its Lifting Condensation Level by large-scale convergence. Cirrostratus clouds, however, tend to thicken as a warm front approaches, signifying an increased production of ice crystals. As a result, the halo is no longer seen and the sun (or moon by night) becomes less visible.