Stratus Cloud - In general, clouds are classified and named according to their appearance and altitude at which they occur, being either high, medium, or low. Stratus clouds are low clouds, and as the root word suggests they are layered in appearance. They occur in broad sheets across most or all of the sky. Generally their formation is due to condensation within layered air that is not subjected to strong vertical movement.
[Journal of the Atmospheric Sciences; v50; 3078-90, 1993.]
[Journal of the Atmospheric Sciences; v43; 90-106; 1986.] 
Atmospheric Chemistry Glossary