ALTOSTRATUS
This middle cloud genus is composed of water droplets, and sometimes ice crystals. In the mid-latitudes, cloud bases are generally found between 15,000 and 20,000 feet. White to gray in color, it can create a fibrous veil or sheet, sometimes obscuring the sun or moon. It is a good indicator of precipitation, as it often precedes a storm system. Virga often falls from these clouds.
High, sheet-like (layered) clouds which are found at an average height of about 3,000 meters (10,000 feet) or 1,700 to 6,000 meters (6,000 to 20,000 feet). They appear fibrous, striated, or lightly striped. The clouds are composed of water droplets and/or ice crystals (unlike the higher cirrostratus which are primarily made up of ice crystals). [See also stratus.]
