Altostratus is a mid-level cloud that appears as gray sheets, composed of ice crystals and snowflakes. It can be thin and semi-transparent or thick and opaque, causing rain or light showers. It can also create a corona or halo around the sun or moon.
Altostratus is a type of cloud that typically appears in the form of featureless gray sheets. These tend to cover most of the sky, making it appear overcast and preventing shadows from forming. Along with the cloud types known as altocumulus and nimbostratus, this cloud is part of the group known as mid-level clouds, which form in the atmosphere at heights between about 6,500 and 20,000 feet (about 2 km and 6 km). Often, following the appearance of the highlands, the meteorological conditions tend to become rainy.
An altostratus cloud is composed primarily of ice crystals and snowflakes and can be thin or thick in nature. The color of the cloud is never white but instead has a gray or bluish tint. Thin altostratus, a type known as translucidus, is most often formed by the thickening and lowering of a high-level cloud layer known as cirrostratus. It is semi-transparent, allowing the moon or sun to shine 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 light, typically reddish, 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 composed primarily of ice crystals and snowflakes, with a relatively small proportion of water droplets. This tends to cause a diffuse, fuzzy-edged appearance, as opposed to the hard contours of clouds that are formed mostly of droplets, such as the well-known fluffy white, almost cauliflower-shaped cumulus cloud.
A thick high-layer cloud that is not transparent is described as opaque. It is darker in color than the thin translucidus version and most of the cloud generally blocks out the moon or sun. This dense cloud can move lower and become denser still, until it completely obscures the sun or moon, and is then classified as a nimbostratus cloud. Nimbostratus appears as a heavy, dark gray layer that produces a constant fall of rain or snow. Altostratus can also cause rain, but typically consists of light to moderate showers rather than the persistent downpours of rain, sleet, or snow associated with nimbostratus.
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