Cirrostratus Cloud

Cirrostratus /ˌsɪroʊˈstrɑːtəs/ clouds are thin, generally uniform clouds, composed of ice-crystals. They are difficult to detect and if capable of forming halos the cloud takes the form of thin cirrostratus nebulosus. The cloud has a fibrous texture with no halos if it is thicker cirrostratus fibratus. On the approach of a frontal system, the cirrostratus often begins as nebulosus and turns to fibratus. If the cirrostratus begins as fragmented fibratus it often means the front is weak. Cirrostratus is usually located above 5.5 km (18,000 ft). Its presence indicates a large amount of moisture in the upper atmosphere.

Cirrostratus clouds sometimes signal the beginning of a warm front if they form after cirrus and spread from one area across the sky and thus may be signs that precipitation might follow in the next 12 to 24 hours or as soon as 6–8 hours if the front is fast moving. If the cirrostratus is broken fibratus it can mean that the front is weak and that stratus rather than nimbostratus will be the rain cloud (meaning drizzle instead of moderate rain). Cumulus humilis or stratocumulus clouds are often found below cirrostratus formations, this being due to the stable air associated with cirrostratus creating an inversion and restricting convection, causing cumuliform clouds to become flattened. Contrails also tend to spread out and can be visible for up to an hour in cirrostratus.

The phrase "hazy sunshine" is often, as well as referring to haze or light mist, used to refer to the milky look of the sky when cirrostratus is present.

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