Explanatory remarks

(Section 2.3.3.8)

Cirrostratus, not completely covering the sky, usually has an irregular border fringed with Cirrus, or may, infrequently, be straight edged and clear cut. 

Cirrostratus is never thick enough to prevent objects on the ground from casting shadows, at least when the Sun is high above the horizon. When the Sun is low (less than about 30°) the relatively longer light path through a Cirrostratus veil may reduce the light intensity so much that shadows do not exist.

The remarks about the colours of Cirrus are, to a great extent, also valid for Cirrostratus.