HOW Do altostartus clouds form?

Altostratus is caused by a large air mass that is lifted then condensed, usually by an incoming frontal system and can be found over widespread areas. Altostratus clouds are potentially dangerous, because they can cause ice accretion on aircraft. Their altitude is from 6,500-20,000 feet (2,400-6,100 m). They are primarily composed of water droplets.