Physical constitution

(Section 2.3.3.7)

Cirrostratus is a cloud of moderate vertical extent and is composed of a relatively sparse number of mainly small ice crystals. These factors combine to account for the transparency of this cloud through which the disc (outline) of the Sun is visible, at least when the Sun is not too close to the horizon.

Sometimes Cirrostratus ice crystals are large enough to acquire an appreciable terminal velocity and trailing filaments form. This gives these Cirrostratus clouds a fibrous appearance.

Halo phenomena are often observed in thin Cirrostratus; sometimes the veil of Cirrostratus is so thin that a halo provides the only indication of its presence.