Cirrostratus

(Section 2.5.2.3)

Cirrostratus most frequently occurs above 3 km (10 000 ft) in polar regions, 5 km (16 500 ft) in temperate regions and 6 km (20 000 ft) in tropical regions.

Below the cloud. Viewed from below, Cirrostratus appears as a transparent, fairly homogeneous whitish veil, totally or partly covering the sky. It usually produces halo phenomena, and it is difficult to identify the base.

Within the cloud. Cirrostratus often occurs in several layers. Its constituent ice particles often glitter in the sunlight. Many kinds of halo phenomenon may be observed. Slight turbulence may be encountered in the lower portion of the cloud, particularly near the base.

Above the cloud. Viewed from above, Cirrostratus appears practically the same as Cirrus, except for its greater breadth of continuous sky cover. The upper surface may be well defined and flat, or can be diffuse with bulges resembling Cirrocumulus. The ground is usually visible through a thin Cirrostratus veil, though seldom through a thick cloud veil. A subsun (undersun) may be observed, but other halo phenomena are rare.