9th September 2005
Stratus
Source: Excerpt from The Book " Weather "
Distribution: Worldwide, most common near coast and mountains. Height: 0 to 6500 feet. Cause: Lifting of a large air mass, followed by condensation. Associated Weather: Drizzle or slight rain, or snow in sub-zero temperatures. Hazard Warning: Can be an aviation hazard as it sometimes masks terrain. Stratus clouds form in sheets or layers (stratus is the Latin word for layer) and occur when relatively large areas of moist air rise gently in a stable atmosphere to a level where condensation occurs. Normally, the lifting of the air mass is a result of an incoming frontal system or wind encountering a large landmass such as a mountain range. A slightly different form of stratus may occur when a layer of fog that has developed at ground level starts to rise as it is warmed by the Sun. This formation is known as fog stratus.
Stratus is the lowest altitude cloud formation, with condensation occurring any where between ground level and about 6,500 feet. Typically, stratus has a ragged. gray appearance, and varies in thickness from a semi-transparent sheet of a few feet to a deck of around 1,500 feet.
Its horizontal spread is usually far greater and may cover hundreds of square miles. Normally, there is no significant weather associated with this cloud, although light drizzle or rain, or light snow in sub-zero temperatures, may fall if the deck is sufficiently thick. Stratus that gives rise to precipitation is often known as nimbostratus.
When it forms close to the ground, stratus can mask the surrounding terrain, particularly in mountainous areas, and this has been the cause of many aviation accidents. Fortunately, radar equipment fitted to most modern aircraft is reducing the dangers associated with this cloud.
Acknowledgement due: John W. Zillman, William J. Burroughs,
Bob Crowder, Ted Robertson, Eleanor Vallier-Talbot and Richard Whitaker.
Check out Skyscapes for cloud photos taken from the aircraft.
