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Cirrostratus () is a high-altitude, very thin, and generally uniform stratus cloud genus-type of cloud. It is composed of , which are particles of ice. Cirrostratus is difficult to see and can produce halos. These optical effects are caused when 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 nebulous and turns to fibratus. If the cirrostratus begins as fragmented of clouds in the sky it often means the front is weak. Cirrostratus usually lies above . Its presence indicates a large amount of moisture in the upper troposphere.
Clouds resembling cirrostratus occasionally form in the lower stratosphere over the polar regions of Earth. Polar stratospheric clouds can take on this appearance when composed of tiny supercooling droplets of water or nitric acid.
Cirrostratus clouds sometimes signal the approach 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 within the next 12–24 hours, or as soon as 6–8 hours if the front moves fast. If the cirrostratus is broken fibratus, it can mean that the front is weak and that stratus rather than nimbostratus will be the precipitating cloud (meaning drizzle or snow grains instead of moderate rain or snow).
Cumulus humilis or stratocumulus clouds are often found below cirrostratus formations, due to the stable air associated with cirrostratus creating an inversion and restricting convection, causing cumuliform clouds to become flattened. also tend to spread out and can be visible for up to an hour in cirrostratus. The phrase "milky sunshine" is often, as well as referring to haze or light mist, used to refer to the milky appearance of the sky when cirrostratus is present.
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