Altostratus is a cloud belonging to a class characterized by a generally uniform gray sheet or layer, lighter in color than nimbostratus and darker than cirrostratus. The sun can be seen shining through them, and they frequently cover the whole sky. They are similar to lower altitude stratus clouds. Altostratus is caused by a large air mass that is lifted then condensed, usually by an incoming frontal system and can be found over wide-spread areas. Altostratus clouds are potentially dangerous, because they can cause ice accretion on aircraft. Their altitude is from 6,500-20,000 feet (2000-6,000m). They are made from ice crystals.