Explanatory remarks and special clouds

(Section 2.3.8.8)

Stratus forms under the combined effect of cooling in the lower layers of the troposphere and turbulence due to the wind.

Over land, the cooling may be a result of night-time cooling, which is particularly marked when the sky is clear and the wind is weak, or by advection of relatively warm air over colder ground. Over sea, the cooling is mainly due to advection of relatively warm air over the colder water surface.

Stratus is sometimes observed as more or less joined cloud fragments with varying luminance. These Stratus fractus clouds constitute a transitory stage during the formation or the dissipation of the more common extensive uniform Stratus layer. The transitory stage is usually very short.

Stratus fractus clouds may also form as accessory clouds (pannus) under Altostratus, Nimbostratus, Cumulonimbus and precipitating Cumulus. They develop as a result of turbulence in the moistened layers under these clouds. 

Stratus may develop locally in the vicinity of large waterfalls as a consequence of water broken up into spray by the falls. The Stratus will be classified by any appropriate species, variety and supplementary feature, followed by cataractagenitus.

Stratus may develop locally over forests as a result of increased humidity due to evaporation and evapotranspiration from the tree canopy. The Stratus will be classified by any appropriate species, variety and supplementary feature followed by silvagenitus.