|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.
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.
- Species: Stratus clouds come in two species: stratus nebulosus and stratus fractus. The nebulosus species (St neb) is a featureless veil of low cloud sometimes producing light drizzle that is coded CL6 in the SYNOP report. Stratus fractus (St fra) appears as a ragged broken up sheet that often forms as an accessory cloud in precipitation falling from a higher cloud deck. It is coded CL7 when associated with bad weather. This species may also result from a continuous sheet of stratus in clear air becoming broken up by the wind, in which case it is coded CL6, the same as for stratus nebulosis not associated with bad weather.
- Opacity-based varieties: Stratus nebulosus can appear as translucidus or opacus depending on its thickness. The opacus variety is opaque, the translucidus variety is translucent The perlucidus variety is not usually associated with this species because of its relatively even structure.
- Pattern-based variety: Stratus nebulosus usually shows no patterns. However a slightly disturbed gentle wind current can create an undulatus variety which has mild undulating layers, but this is rarely seen. Varieties of any kind are not commonly associated with stratus fractus because of the highly fragmented structure that identifies this species.
- Precipitation-based supplementary feature: Stratus cloud is too low to produce virga, but the praecipitatio feature can be seen in the form of drizzle or snow grains.
- Accessory clouds: Stratus does not have any accessory clouds as such, but ragged stratus may form in precipitation as pannus associated with other precipitating clouds.
- Genitus mother clouds: Stratus can form from the spreading or thinning of the base of clouds with significant vertical development, particularly nimbostratus, cumulus, or cumulonimbus.
- Mutatus mother clouds: This genus type can form as the result of the fusing of stratocumulus elements into an even featureless sheet.
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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