STRATUS (ST)

One of the three major cloud types that comprise the ten main groups or genera, stratus clouds organize themselves into sheets or layers. They often form when stable air possessing water vapor is forced to rise (i.e., the cloud is not convective) such as ahead of a synoptic-scale cyclone system over the warm frontal zone. They can cover areas as big as several hundred thousand square kilometers. Stratus (the act of spreading or strewing) clouds form below 2,440 meters (8,000 feet) and are water droplets, unless the freezing level extends lower, as in the wintertime and at higher latitudes. Altostratus is a uniform layer of clouds which form between about 2,400 and 6,100 meters (8,000 and 20,000 feet) and is usually a mixture of water droplets and ice crystals. [See altostratus.] Cirrostratus is a uniform layer of ice-crystal clouds which can be a mile thick and forms above about 6,100 meters (20,000 feet). [ See cirrostratus.]