Nimbostratus with fractus
|Genus||Nimbostratus (rain, layered)|
|Altitude||below 3,000 m
(below 10,000 ft)
|Classification||Family D (Vertically developed)|
|Appearance||Dark, widespread, formless layer|
|Precipitation cloud?||Yes, but may be virga|
Nimbostratus is a low-to-middle altitude cloud that has considerable vertical and horizontal extent and produces precipitation over a wide area. "Nimbo" is from the Latin word "nimbus", which denotes precipitation. It is a multilevel stratiform layer with a diffuse cloud base generally found anywhere from near surface and about 10000 ft (3000 m). This cloud typically forms from altostratus in the middle altitude range, but it tends to thicken into the low altitude range during precipitation. Although usually dark at its base, it often appears illuminated from within to a surface observer. Nimbostratus usually has a thickness of about 2000 m. Though found worldwide, nimbostratus occurs more commonly in the middle latitudes.
Nimbostratus will occur along a warm front where the slowly rising warm air mass creates nimbostratus along with shallower stratus clouds producing less rain, these clouds being preceded by higher-level clouds such as cirrostratus and altostratus clouds. Often, when an altostratus cloud thickens and descends into lower altitudes, it will become nimbostratus.
Nimbostratus, unlike cumulonimbus, is not associated with thunderstorms, however at an unusually unstable warm front caused as a result of the advancing warm air being hot, humid and unstable, cumulonimbus clouds may be embedded within the usual nimbostratus. Lightning from an embedded cumulonimbus cloud may interact with the nimbostratus but only in the immediate area around it. In this situation with lightning and rain occurring it would be hard to tell which type of cloud was producing the rain from the ground, however cumulonimbus tend to produce larger droplets and more intense downpours. The occurrence of cumulonimbus and nimbostratus together is uncommon, and usually only nimbostratus is found at a warm front.
Nimbostratus fractus is a variation of nimbostratus and will sometimes occur as a weather front clears, these being areas of dark rain bearing clouds with lighter altostratus between.
Usually, nimbostratus is a sign of steady moderate to heavy precipitation, as opposed to the shorter period of typically heavier precipitation released by a cumulonimbus cloud. However, precipitation does not occur at ground level in case of virga and accompanies other cloud types. Precipitation may last for several days, depending on the speed of the occluded front it accompanies. A nimbostratus virga cloud is the same as a normal nimbostratus cloud, but the precipitation is virga and it never reaches the ground. Stratus or stratocumulus (comprising the warm sector of a frontal system) usually forms when it clears.
- World Meteorological Organization International Cloud Atlas
- Pretor-Pinney, Gavin (2007). The Cloudspotter's Guide: The Science, History, and Culture of Clouds. Perigee. ISBN 0-399-53345-1.
- Dunlop, Storm (2003). Weather Identification Handbook: The Ultimate Guide for Weather Watchers. Guilford, Connecticut: Globe Pequot. ISBN 1-58574-857-9.
- Thompson, Graham; Turk, Jonathan (1993). Earth Science and the Environment. Fort Worth: Saunders College Publishing. ISBN 0-03-075446-1.
- Lankford, Terry (2000). Aviation Weather Handbook. McGraw-Hill Professional. ISBN 978-0-07-136103-3.
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