|Weather Reference - Clouds - Stratus
Stratus | Orographic Stratus | Stratocumulus | Disclaimer
Stratus clouds are grey, nearly featureless blankets of cloud, sometimes ragged at the top or bottom, and are low and sometimes even on the ground. They can produce slight drizzle, a few snowflakes or ice crystals.
Stratus clouds are formed in sheets or layers and occur when large areas of moist air rise gently in a stable atmosphere to a level when condensation occurs. The lifting of air mass is a result of an incoming frontal system or wind encountering a large landmass such as a mountain range.
Orographic Stratus clouds are formed when moisture is carried on a prevailing wind is lifted by an elevated landform, such as a mountain range, to a level where condensation takes place.
This is a low level formation which occurs most frequently in coastal regions where the air flow is heavily moisture laden.
Generally the landform must be at least 500 feet high to generate the cloud and higher in areas of clean, dry air such as desserts.
Orographic Stratus tend to remain stationary.
The extent of this type of cloud depends on the humidity of the surrounding air mass and also by the steepness and elevation of the landform, the strength of the wind and the direction of the wind relative to the landform.
Stratocumulus clouds have a distinct layer of regular heaps or rolls of cloud, with dark shading and are normally seen at a height of below 2,000 feet. Precipitation is light, if any and may not even reach the ground.
Two processes may give rise to Stratocumulus, either separately or together:
1. A large moist air mass is lifted by a frontal system or a landmass to a level where condensation occurs, slight instability at cloud level then creates the clouds cumuliform shape.
2. Involves a pocket of warm air rising from the ground as a result of weak convection, giving rise to condensation at the same level over a wide area.
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