Altostratus is a middle-altitude cloud genus made up of water droplets, ice crystals, or a mixture of the two. Altostratus clouds are formed when large masses of warm, moist air rise, causing water vapor to condense. Altostratus clouds are usually gray or blueish featureless sheets, although some variants have wavy or banded bases. The sun can be seen through thinner altostratus clouds, but thicker layers can be quite opaque.

Altostratus clouds usually predict the arrival of warm fronts. Once altostratus clouds associated with a warm front arrive, continuous rain or snow will usually follow in the next 12 to 24 hours. Although altostratus clouds predict the arrival of warmer, wetter weather, they themselves do not produce significant precipitation. Thunderstorms can be embedded in altostratus clouds, however, bringing showers.

Because altostratus clouds can contain ice crystals, they can produce some optical phenomena like iridescence and coronas.

Altostratus clouds are generally gray or blue-tinged with a largely-uniform blanket-like appearance. They do not have distinct features, and usually do not produce precipitation. The name "altostratus" comes from the conjugation of the Latin words "altum", meaning "high", and "stratus", meaning "flat" or "spread out". Altostratus clouds can produce virga, causing the cloud base to appear hazy. While they do not produce significant precipitation, altostratus clouds can cause light sprinkles or even small rain showers. Consistent rainfall and lowering of the cloud base causes altostratus to become nimbostratus.

Unlike most other types of clouds, altostratus clouds are not subdivided into cloud species due to their largely-featureless appearance. However, they still appear in five varieties: Altostratus duplicatus, opacus, radiatus, translucidus, and undulatus. Altostratus duplicatus is a rare form of altostratus clouds composed of two or more layers of cloud. Translucidus is a translucent form of altostratus clouds, meaning that the sun or moon can be seen through the cloud, whereas the opacus variety is opaque. Radiatus is another rare variety. It has parallel bands of cloud that stretch toward the horizon. The undulatus variety has an wavy appearance—the underside of the cloud appears to rise and fall.

Altostratus and altocumulus clouds, both of which are mid-level clouds, are commonly measured together in cloud cover studies. Together, they cover around 25% of the Earth's surface on average based on CALIPSO satellite data. This constitutes roughly one third of the Earth's total cloud cover. By itself, separated from altocumulus, altostratus covers ~16% of the Earth's surface. Altostratus cloud cover varies seasonally in temperate regions, with significantly less coverage in the summer months as compared to the other seasons. Additionally, altostratus cloud cover varies by latitude, with tropical regions having vastly fewer altostratus clouds when compared to temperate or polar regions. Altostratus and altocumulus cover roughly 22% of the ocean's surface based on surface measurements, with minimal variation based on season.

Altostratus clouds are warmest at the bottom and coldest at the top, with a fairly consistent lapse rate of 5 to 7 °C per kilometer (14 to 20 °F per mile) inside the cloud. The lapse rate is the rate at which the temperature decreases with altitude. Higher lapse rates (i.e. the faster temperature drops with increasing altitude) were associated with colder clouds. The average temperature of altostratus clouds, based on data collected from roughly 45° to 80° latitude, varied from around −16 to −45 °C (3.2 to −49 °F). Warmer temperatures occurred during summer and colder temperatures during winter.

Inside altostratus clouds, the relative humidity is generally greatest towards the top of the cloud decreasing slowly and roughly linearly towards the bottom. The lowest part of the cloud has the lowest relative humidity. Below the bottom of the cloud, the relative humidity drops rapidly.