Stratus cloud

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Stratus cloud
Stratus cloud
Stratus cloud
Abbreviation St
Symbol CL 6.png
Genus Stratus (layered)
Altitude Below 2,000 m
(Below 6,000 ft)
Classification Family C (Low-level)
Appearance horizontal layers
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".[1] 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 form when a sheet of warm, moist air lifts off the ground and depressurizes, following the lapse rate. This causes the relative humidity to increase due to the adiabatic cooling.[2]

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.[2]


Stratus clouds look like featureless gray to white sheets of cloud.[2] They can be composed of water droplets, supercooled water droplets, or ice crystals depending upon the ambient temperature.[1]


Stratus clouds come in two species: stratus nebulosus and stratus fractus. The nebulosus species has a uniform veil-like appearance, whereas the fractus species are torn and shredded (fractured). Stratus clouds have three varieties: stratus opacus, stratus translucidus, and stratus undulatus. The opacus variety is opaque, the translucidus variety is translucent, and the undulatus variety has undulating layers. Stratus praecipitatio is the only supplementary feature of stratus clouds, and it implies that precipitation is falling from the cloud.[3]


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[edit]

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.[4]

Relation to other clouds[edit]

Cirrostratus clouds[edit]

Milky-white cirrostratus clouds cause the sky to appear lighter and have a milky tint.
A cirrostratus cloud
Main article: Cirrostratus cloud

Cirrostratus clouds, a very high ice-crystal form of stratiform clouds, can appear as a milky sheen in the sky[5] or as a striated sheet.[6] 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.[7] Cirrostratus come in two species, fibratus and nebulosus.[5] 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.[8][9] These clouds commonly produce halos, and sometimes the halo is the only indication that such clouds are present.[10] They are formed by warm, moist air being lifted slowly to a very high altitude.[11] When a warm front approaches, cirrostratus clouds become thicker and descend forming altostratus clouds,[1] and rain usually begins 12 to 24 hours later.[10]

Altostratus clouds[edit]

Main article: Altostratus cloud

Nimbostratus clouds[edit]

Main article: Nimbostratus cloud

Stratocumulus clouds[edit]

Main article: Stratocumulus cloud

A stratocumulus cloud is another type of a cumuliform or stratiform cloud. Like stratus clouds, they form at low levels;[1] 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.[12] Stratocumulus clouds can produce drizzle, which stabilizes the cloud by warming it and reducing turbulent mixing.[13]


  1. ^ a b c d "Cloud Classification". National Weather Service. Retrieved 2 January 2014. 
  2. ^ a b c "Stratus Clouds". Weather. USA Today. 16 October 2005. Retrieved 2 January 2014. 
  3. ^ "WMO classification of clouds" (PDF). World Meteorological Organization. Retrieved 2 January 2014. 
  4. ^ Sednev, Menon & McFarquhar 2009, p. 4747
  5. ^ a b "Common Cloud Names, Shapes, and Altitudes" (PDF). Georgia Institute of Technology. pp. 2, 10–13. Retrieved 12 February 2011. 
  6. ^ Hubbard & Hubbard 2000, p. 340
  7. ^ Day 2005, p. 56
  8. ^ Parungo 1995, p. 254
  9. ^ Parungo 1995, p. 256
  10. ^ a b Ahrens 2006, p. 120
  11. ^ Hamilton, p. 24
  12. ^ Wood 2012, p. 2374
  13. ^ Wood 2012, p. 2398