Nimbostratus cloud

A nimbostratus cloud is a multilevel, amorphous, nearly uniform, and often dark-grey cloud that usually produces continuous rain, snow, or sleet, but no lightning or thunder.

Although it is usually a low-based stratiform cloud, it actually forms most commonly in the middle level of the troposphere and then spreads vertically into the low and high levels. Nimbostratus usually produces precipitation over a wide area.

The prefix nimbo- comes from the Latin word nimbus, which means "rain bearing cloud".

Downward-growing nimbostratus can have the same vertical extent as most large upward-growing cumulus, but its horizontal expanse tends to be even greater.

Nimbostratus has a diffuse cloud base generally found near the surface in the lower levels to about 2,000 to 4,000 m (6,600 to 13,100 ft) in the middle levels of the troposphere. Although usually dark at its base, it often appears illuminated from within to a surface observer. Though found worldwide, nimbostratus occurs more commonly in the middle latitudes. It is coded C_M2 on the SYNOP report.

Nimbostratus occurs along a warm front or occluded 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. 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 simultaneous presence of cumulonimbus and nimbostratus clouds is uncommon; nimbostratus clouds are typically associated with warm fronts and occasionally appear along cold fronts. However nimbostratus in Australian east coast low pressure systems (ECL) can bring some of the heaviest rainfall totals on record due to the Tasman sea, which is often warm. For instance, Sydney has recorded^[when?] a 24-hour rainfall total of 327.8 mm (12.91 in) purely from nimbostratus, with 100 mm (3.9 in) falling in the three hours from noon to 3:00 pm.^{[citation needed]} The simultaneous presence of cumulonimbus and nimbostratus clouds is common in eastern Australia, and these storms are called "embedded thunderstorms".

Nimbostratus is generally a sign of an approaching warm or occluded front producing steady moderate precipitation, as opposed to the shorter period of typically heavier precipitation released by a cold-frontal cumulonimbus cloud. Precipitation may last for several days, depending on the speed of the frontal system. A nimbostratus virga cloud is the same as normal nimbostratus, but the rain or snow falls as virga which doesn't reach the ground. Stratus or stratocumulus usually replace the nimbostratus after the passage of the warm or occluded front.