Altostratus clouds

Altostratus (As) is a middle altitude cloud (height 2000-7000m; 7000-23000ft) genus belonging to the stratiform physical category.

Altostratus most often takes the form of a featureless sheet of cloud but can be wavy (undulatus) as a result of wind shear through the cloud. It can also be fragmented (fibratus) with clear sky visible, which often precedes the approach of a weakened or upper level warm front.