|Altitude||Below 2,000 m
(Below 6,000 ft)
|Classification||Family C (Low-level)|
|Precipitation cloud?||Drizzle, freezing drizzle or snow grains|
Stratus clouds are a genus of low-level cloud characterized by horizontal layering with a uniform base, as opposed to convective or cumuliform clouds that are formed by rising thermals. More specifically, the term stratus is used to describe flat, hazy, featureless clouds of low altitude varying in color from dark gray to nearly white. The word "stratus" comes from the Latin prefix "strato-", meaning "layer". Stratus clouds may produce a light drizzle or a small amount of snow. These clouds are essentially above-ground fog formed either through the lifting of morning fog or through cold air moving at low altitudes over a region. Some call these clouds "high fog" for the fog-like cloud. While light rain may fall, this cloud does not indicate much meteorological activity.
- 1 Formation
- 2 Description
- 3 Species
- 4 Opacity-based Varieties
- 5 Pattern-based Variety
- 6 Precipitation-based Supplementary Feature
- 7 Accessory Clouds
- 8 Genitus Mother Clouds
- 9 Mutatus Mother Clouds
- 10 Forecast
- 11 Relation to other clouds
- 12 Sources
Stratus clouds can also form in a similar manner to fog when the ambient air temperature decreases, increasing the relative humidity. Once the temperature drops below the dew point, a stratus cloud can form.
Stratus Nebulosus cloud appears as a featureless sheet of foglike strcture. It is coded CL7 on the SYNOP report. It usually produces precipitation of light rain or snow, and it is a good indicator of atmospheric stability.
Stratus Fractus cloud appears with a ragged or shredded appearance, easily seen on mountains during an overcast of cirrostratus clouds or altostratus clouds. It is coded CL7 on the SYNOP report. It also appears under precipitation-bearing cloud such as the cumulonimbus cloud or the nimbostratus cloud, classified as pannus clouds. The difference between cumulus fractus and stratus fractus can be distinguished easily, by the speed of movement and the height from the ground (if seen from a farther view).
Stratus Translucidus appear as a translucent sheet of stratus nebulosus in which the sun or moon can be seen through. It might be blurry. Stratus fractus clouds do not have varieties due to its ragged, shredded appearance, much like the cumulus fractus cloud.
Stratus Undulatus appears with mild undulating layers. This is due to disturbances on the gentle wind which causes these very rare undulating layers. This variety is only associated by stratus nebulosus cloud.
Precipitation-based Supplementary Feature
Praecipitatio is derived from Latin which means "precipitation" or rain or snow that reaches the ground. Stratus clouds are too low to produce virga, and can further only produce light rains or flakes of snow.
Genitus Mother Clouds
Stratus Cumulogenitus, Nimbostratogenitus, or Cumulonimbogenitus are stratus clouds formed by the thinning of the bases of the mentioned genitus mother clouds. Stratus fractus can also form under precipitation-bearing clouds such as nimbostratus and cumulonimbus.
Mutatus Mother Clouds
A stratus cloud can form from stratocumulus spreading out under an inversion, indicating a continuation of prolonged cloudy weather with drizzle for several hours and then an improvement as it breaks into stratocumulus. Stratus clouds can persist for days in anticyclone conditions. It is common for a stratus to form on a weak warm front, rather than the usual nimbostratus.
Effects on climate
According to Sednev, Menon, and McFarquhar, Arctic stratus and other low-level clouds form roughly 50% of the annual cloud cover in Arctic regions, causing a large effect on the energy emissions and absorptions through radiation.
Relation to other clouds
Cirrostratus clouds, a very high ice-crystal form of stratiform clouds, can appear as a milky sheen in the sky or as a striated sheet. They are sometimes similar to altostratus and are distinguishable from the latter because the sun or moon is always clearly visible through transparent cirrostratus, in contrast to altostratus which tends to be opaque or translucent. Cirrostratus come in two species, fibratus and nebulosus. The ice crystals in these clouds vary depending upon the height in the cloud. Towards the bottom, at temperatures of around −35 °C (−31 °F) to −45 °C (−49 °F), the crystals tend to be long, solid, hexagonal columns. Towards the top of the cloud, at temperatures of around −47 °C (−53 °F) to −52 °C (−62 °F), the predominant crystal types are thick, hexagonal plates and short, solid, hexagonal columns. These clouds commonly produce halos, and sometimes the halo is the only indication that such clouds are present. They are formed by warm, moist air being lifted slowly to a very high altitude. When a warm front approaches, cirrostratus clouds become thicker and descend forming altostratus clouds, and rain usually begins 12 to 24 hours later.
A stratocumulus cloud is another type of a cumuliform or stratiform cloud. Like stratus clouds, they form at low levels; but like cumulus clouds, they form via convection. Unlike cumulus clouds, their growth is almost completely retarded by a strong inversion, causing them to flatten out like stratus clouds and giving them a layered appearance. These clouds are extremely common, covering on average around twenty-three percent of the earth's oceans and twelve percent of the earth's continents. They are less common in tropical areas and commonly form after cold fronts. Additionally, stratocumulus clouds reflect a large amount of the incoming sunlight, producing a net cooling effect. Stratocumulus clouds can produce drizzle, which stabilizes the cloud by warming it and reducing turbulent mixing.
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