Cirrostratus (Cs) is a high-level stratiform cloud that develops between 5,000 and 13,000 metres. Composed entirely of ice crystals, it forms a thin, uniform layer that does not completely obscure the sun or moon but reduces their brightness.
Halo indicator
Its most recognisable feature is the production of solar and lunar halos — light rings at 22° from the sun caused by refraction through hexagonal ice crystals. A halo in cirrostratus skies is a classic sign that a warm front is approaching, with rain likely within 12–36 hours.
Formation and varieties
It forms through slow, widespread ascent of moist air ahead of frontal systems, or by the merging of cirrus clouds into a continuous sheet. Varieties include fibratus (fibrous texture) and nebulosus (featureless). Unlike altostratus, cirrostratus always allows the solar disc to be seen clearly and casts sharp shadows on the ground.