Altostratus clouds are clouds belonging to a class distinguished by a uniform gray sheet or layer. They are lighter in color than nimbostratus and darker than cirrostratus. The sun is visible and shines through these clouds. Altostratus frequently covers the entire sky. These are similar to lower altitude stratus clouds.
Altostratus is caused when a large mass of air is lifted then condensed, typically by an incoming frontal system. Altostratus can form over wide-spread areas and be massive. Altostratus clouds can be potentially dangerous, especially to aircraft, as they can cause ice to build up on them. These clouds range from 6,500 to 20,000 feet in altitude. They are made up of ice crystals.