Stratus



Stratus clouds (St) are low-altitude clouds which form in horizontal sheets and occur when large areas of air rise, usually due to an incoming frontal system, wind encountering a large land mass, or the lifting of fog (which is, itself, a stratus cloud). The rising airmass must be moist and atmospheric conditions must be stable for a stratus cloud to form. Stratus is the lowest-altitude cloud formation (as fog, its altitude can be as low as 0m above ground level) and typically has a ragged grey appearance. Stratus can cover hundreds of square kilometres but is usually thinner than 450 metres. It is the typical "cloudy day" formation - a sheet of cloud occluding the entire sky. Stratus clouds can appear to be virtually any shade of grey depending upon the thickness of the horizontal sheet. Precipitation from stratus clouds is uncommon though possible in small quantities, especially in thicker stratus clouds. When it does occur, the cloud which produces it is referred to as a nimbostratus cloud. Stratus can be a hazard when it blankets the ground, reducing visibility and potentially causing automobile and aviation accidents.

Stratus clouds were first described by Luke Howard in 1802 as consisting of "a widely extended continuous horizontal sheet, increasing from below upwards."

Subtypes
Stratus nebulosus

Stratus undulatus

Stratus translucidus

Stratus opacus

Stratus praecipitatio

Stratus virga