An altostratus cloud is a type of cloud found in the middle layer: at temperate latitudes its base is located between 1,500 and 5,000 m above the ground and its thickness is in the order of 2,000 m. It is a greyish or bluish sheet or layer with a streaked, fibrous or uniform appearance. It may partially or totally cover the sky, and may produce precipitation. An important detail is that often parts of this type of cloud are thin enough to let the sun or moon shine through, creating a frosted glass effect.
Altostratus clouds develop in stable atmospheres. They generally form during the ascent and condensation of a large air mass at the approach of a disturbance, and in this case are the result of the thickening of a cirrostratus cloud cover. Sometimes, the cloud may form after a layer of nimbostratus thins, or even from the trails of ice crystals falling from an altocumulus.