Altostratus

 

Altostratus is a middle altitude cloud genus belonging to the stratiform physical category characterized by a generally uniform gray to bluish-gray[3] sheet or layer, lighter in color than nimbostratus and darker than highcirrostratus. The sun can be seen through thin altostratus, but thicker layers can be quite opaque.

Altostratus is formed by the lifting of a large mostly stable air mass that causes invisible water vapor to condense into cloud. It can produce light precipitation, often in the form of virga. If the precipitation increases in persistence and intensity, the altostratus cloud may thicken into nimbostratus.[2]

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 signals the approach of a weakened or upper level warm front. THis info was gathered from wikipedia at http://en.wikipedia.org/wiki/Altostratus_cloud