Altostratus is a type of cloud which typically appears in the form of featureless gray sheets. These tend to cover most of the sky, making it look overcast and preventing shadows from forming. Together with cloud types known as altocumulus and nimbostratus, this cloud is a member of the group known as middle level clouds, which are formed in the atmosphere at heights of between around 6,500 and 20,000 feet (about 2 km and 6 km). Often, following the appearance of altostratus clouds, weather conditions tend to become rainy.
An altostratus cloud is composed mainly of ice crystals and flakes of snow, and may be thin or thick in nature. The color of the cloud is never white but has a gray or blueish hue instead. Thin altostratus, a type known as translucidus, is most often formed from the thickening and lowering of a sheet of high level cloud known as cirrostratus. It is semi-transparent, enabling the moon or sun to shine through with an appearance similar to that seen through a piece of ground glass. Sometimes this gives rise to what is called a corona, where light is scattered by the tiny particles that make up the cloud, and rings of colored, typically reddish light are seen around the sun or moon.
A larger ring, known as a halo, is occasionally seen when the sun or moon shines through a cloud, and this usually indicates that the cloud is cirrostratus rather than altostratus. Both cirrostratus and altostratus are examples of clouds that are mostly composed of ice crystals and snowflakes, with a relatively small proportion of water droplets. This tends to cause a diffuse, fuzzy-edged appearance, unlike the sharp outlines of clouds formed mainly of droplets, such as the well-known white and fluffy, almost cauliflower-shaped cumulus.
A thick altostratus cloud that is not transparent is described as being opacus in type. It is darker in color than the thin translucidus version, and most parts of the cloud generally block out the moon or sun. This thick cloud may move lower and become even more dense, until the whole of it obscures the sun or moon, and it is then classified as a nimbostratus cloud. Nimbostratus appears as a heavy, dark gray layer, producing a steady fall of rain or snow. Altostratus can also cause rain, but this generally consists of light to moderate showers rather than the persistent downpour of rain, sleet or snow associated with nimbostratus.