Cirrostratus
Latin Translation
Cirrus = “a bird’s tuft”
Stratus = “flattened” or “spread out”
Height
Between 16,500 to 45,000ft (~ 5000 to 13,700 metres) in mid-latitudes.
The sun (or moon) refracting through cirrostratus can form a halo.
About
Cirrostratus is a high-altitude cloud, and part of the high-level cloud family group in the troposphere. It is thin and transparent and can be smooth or fibrous. Light will refract off cirrostratus, which can create a halo around the sun or moon.
How does it form?
When a large area of water vapour is slowly lifted into the extremely cold upper troposphere, it cools and creates a blanket of ice crystals. This icy sheet is cirrostratus. Sometimes it will span thousands of miles!
How to spot it?
Look high into the sky for a widespread transparent/veil-like cloud. If the halo phenomenon is present, then you know it’s cirrostratus.