| Stratus cloud | |
|---|---|
 Typical stratus clouds overSkopje,North Macedonia. The mountains are higher than thecloud top, overlooking asea of clouds. | |
| Abbreviation | St |
| Symbol |  |
| Genus | Stratus (layered) |
| Species |
|
| Variety |
|
| Altitude | 0–2,000 m (0–7,000 ft) |
| Classification | Family C (Low-level) |
| Appearance | Gray, featureless low-altitude cloud capable of ground contact. |
| Precipitation | CommonDrizzle,freezing drizzle,Snow orsnow grains[1] |
Stratus clouds are low-levelclouds characterized by horizontal layering with a uniform base, as opposed toconvective orcumuliform clouds formed by risingthermals. The termstratus describes flat,hazy, featureless clouds at lowaltitudes varying in color from dark gray to nearly white.[2] The wordstratus is derived from the prefixStrato- meaning 'layer'.[3] Stratus clouds may produce a lightdrizzle or a small amount ofsnow. These clouds are essentially above-groundfog formed either through the lifting of morning fog or through cold air moving at low altitudes. Some call these clouds "high fog" for their fog-like form.
Stratus clouds form when weak vertical currents lift a layer of air off the ground and it depressurizes, following thelapse rate. This causes therelative humidity to increase due to theadiabatic cooling.[4] This occurs in environments whereatmosphericstability is abundant.[5]
Stratus clouds look like featureless gray to white sheets of cloud.[4] They can be composed ofwater droplets, supercooled water droplets, or ice crystals depending upon the ambient temperature.
Stratus nebulosus clouds appear as a featureless or nebulous veil or layer of stratus clouds with no distinctive features or structure.[6] They are found at low altitudes, and are a good sign ofatmospheric stability, which indicates continuous stable weather. Stratus nebulosus may produce light rain and drizzle or flakes of snow.Stratus fractus clouds on the other hand, appear with an irregular shape, and forms with a clearly fragmented or ragged appearance.[6] They mostly appear under theprecipitation of major rain-bearing clouds; these arenimbostratus andcumulonimbus clouds, and are classified as types ofpannus clouds. Stratus fractus can also form beside mountain slopes, without the presence ofnimbus clouds (clouds that precipitate), and their color can be from dark grey to almost white.
Stratus fractus are not divided into varieties, but stratus nebulosus on the other hand, are divided into two. TheStratus opacus variety appears as a nebulous or milky sheet of the nebulosus species, but are opaque enough to block the sun from view.[6]Stratus Translucidus is another variety of the nebulosus species. These clouds are considered more thin than the opacus variety because this cloud is rather translucent, allowing the position of the Sun or Moon to be observed from Earth's surface.
Stratus clouds only have one pattern-based variety. This is thestratus undulatus variety. Mildundulations can be observed from this cloud, only associated by the nebulosus species. Though rare, this cloud formation is caused by disturbances on the gentle wind shear. Stratus undulatus clouds are more common on stratus stratocumulomutatus clouds where the wind is stronger as height increases.[6]
Stratus cumulogenitus clouds occur when the base of cumulus clouds spreads, creating a nebulous sheet of stratiform clouds. This can also occur onnimbostratus clouds (stratus nimbostratogenitus) and oncumulonimbus clouds (stratus cumulonimbogenitus). Stratus fractus clouds can also form under the base of precipitation-bearing clouds and are classified as pannus clouds. Stratus clouds may also form from formation mechanisms that are not typical for the cloud type, for example,Stratus homogenitus, which are stratus formed by human activity,Stratus cataractagenitus, which are formed from the spray ofwaterfalls, andStratus silvagenitus, which are formed by evaporation orevapotranspiration occurring in a forest.
Stratus only has one mutatus mother cloud.Stratus stratocumulomutatus clouds occur whenstratocumulus opacus patches fuse to create a stratiform layer.[6]
Stratus clouds do not produce accessory clouds, but a supplementary featurepraecipitatio is derived fromLatin, which means "precipitation". Stratus clouds are generally too low to produce virga, or rain shears that evaporate before reaching the ground, although higher stratus clouds can produce it.
A stratus cloud can form fromstratocumulus 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 inanticyclone conditions. It is common for a stratus to form on a weak warm front, rather than the usualnimbostratus.
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.[7]
Cirrostratus clouds, a very high ice-crystal form of stratiform clouds, can appear as a milky sheen in the sky[8] or as a striated sheet.[9] 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.[10] Cirrostratus come in two species,fibratus andnebulosus.[8] 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.[11][12] These clouds commonly produce halos, and sometimes the halo is the only indication that such clouds are present.[13] They are formed by warm, moist air being lifted slowly to a very high altitude.[14] When a warm front approaches, cirrostratus clouds become thicker and descend forming altostratus clouds,[3] and rain usually begins 12 to 24 hours later.[13]
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Nimbostratus clouds are a type of dark, gray, and flat cloud that produces persistent rain or snow, and does not produce lightning or thunder. It is thick enough to block out sunlight.[15] Elevations that have temperatures below freezing cause water droplets to freeze and turn into ice crystals. The height of the cloud typically reaches between 2–3 kilometres (1.2–1.9 mi).[16] They form when warm, moist air rises, cools down, and condenses into water droplets or ice crystals at a low altitude, which forms the dark gray cloud layer. They are often a sign of a warm or occluded front approaching.[17]
A stratocumulus cloud is another type of a cumuliform or stratiform cloud. Like stratus clouds, they form at low levels;[3] but like cumulus clouds (and unlike stratus clouds), they form via convection. Unlike cumulus clouds, their growth is almost completely retarded by a stronginversion, 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 aftercold fronts. Additionally, stratocumulus clouds reflect a large amount of the incoming sunlight, producing a net cooling effect.[18] Stratocumulus clouds can producedrizzle, which stabilizes the cloud by warming it and reducing turbulent mixing.[19]
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