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Stratus clouds are low-level
clouds In meteorology, a cloud is an aerosol consisting of a visible mass of miniature liquid droplets, frozen crystals, or other particles suspended in the atmosphere of a planetary body or similar space. Water or various other chemicals ma ...
characterized by horizontal layering with a uniform base, as opposed to convective or cumuliform clouds that are formed by rising
thermal A thermal column (or thermal) is a rising mass of buoyant air, a convective current in the atmosphere, that transfers heat energy vertically. Thermals are created by the uneven heating of Earth's surface from solar radiation, and are an example ...
s. More specifically, the term ''stratus'' is used to describe flat, hazy, featureless clouds at low
altitude Altitude or height (also sometimes known as depth) is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The exact definition and reference datum varies according to the context ...
s varying in color from dark gray to nearly white. The word ''stratus'' comes from the
Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power ...
prefix ''strato-'', meaning "layer". Stratus clouds may produce a light drizzle or a small amount of
snow Snow comprises individual ice crystals that grow while suspended in the atmosphere An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet ...
. 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 their fog-like form. While light rain may fall, this cloud does not indicate much meteorological
precipitation In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hai ...
.


Formation

Stratus clouds form when weak vertical currents lift a layer of air off the ground and it depressurizes, following the
lapse rate The lapse rate is the rate at which an atmospheric variable, normally temperature in Earth's atmosphere, falls with altitude. ''Lapse rate'' arises from the word ''lapse'', in the sense of a gradual fall. In dry air, the adiabatic lapse rate is ...
. This causes the
relative humidity Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present. Humidity dep ...
to increase due to the adiabatic cooling. This occurs in environments where atmospheric ''stability'' is abundant.


Description

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


Subforms


Species

Stratus nebulosus Stratus nebulosus is a species of low-level stratus cloud. It is one of only two species that is associated with stratus clouds, other being fractus. Translated from Latin meaning nebulous, their cloud abbreviations can be respectively written as ...
clouds appear as a featureless or nebulous veil or layer of stratus clouds with no distinctive features or structure. They are found at low altitudes, and are a good sign of
atmospheric stability Atmospheric instability is a condition where the Earth's atmosphere is generally considered to be unstable and as a result the weather is subjected to a high degree of variability through distance and time. Atmospheric stability is a measure of th ...
, 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. They mostly appear under the
precipitation In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hai ...
of major rain-bearing clouds; these are nimbostratus and
cumulonimbus Cumulonimbus (from Latin ''cumulus'', "heaped" and ''nimbus'', "rainstorm") is a dense, towering vertical cloud, typically forming from water vapor condensing in the lower troposphere that builds upward carried by powerful buoyant air currents. ...
clouds, and are classified as pannus clouds. Stratus fractus can also form beside mountain slopes, without the presence of nimbus clouds (clouds that precipitate), and their color can be from dark grey to almost white.


Opacity-based varieties

Stratus fractus are not divided into varieties, but stratus nebulosus on the other hand, are divided into two. The Stratus opacus variety appears as a nebulous or milky sheet of the nebulosus species, but are opaque enough to block the sun from view. 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.


Pattern-based variety

Stratus clouds only have one pattern-based variety. This is the ''stratus undulatus'' variety. Mild undulations 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.


Genitus mother clouds

Stratus cumulogenitus clouds occur when the base of cumulus clouds spreads, creating a nebulous sheet of stratiform clouds. This can also occur on nimbostratus clouds (stratus nimbostratogenitus) and on
cumulonimbus cloud Cumulonimbus (from Latin ''cumulus'', "heaped" and ''nimbus'', "rainstorm") is a dense, towering vertical cloud, typically forming from water vapor condensing in the lower troposphere that builds upward carried by powerful buoyant air currents. ...
s (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 aren't typical for the cloud type, for example, Stratus homogenitus, which are stratus formed by human activity, Stratus caractagenitus, which are formed from the spray of waterfalls, and Stratus silvagenitus, which are formed by evaporation or
evapotranspiration Evapotranspiration (ET) is the combined processes by which water moves from the earth’s surface into the atmosphere. It covers both water evaporation (movement of water to the air directly from soil, canopies, and water bodies) and transpir ...
occurring in a forest.


Mutatus mother cloud

Stratus only has one mutatus mother cloud. Stratus stratocumulomutatus clouds occur when stratocumulus opacus patches fuse to create a stratiform layer.


Accessory clouds and supplementary feature

Stratus clouds don't produce accessory clouds, but a supplementary feature praecipitatio is derived from
Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power ...
, which means "
precipitation In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hai ...
". Stratus clouds are too low to produce virga, or rain shears that evaporate before reaching the ground, although higher stratus clouds can produce it.


Forecast

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 An anticyclone is a weather phenomenon defined as a large-scale circulation of winds around a central region of high atmospheric pressure, clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere as viewed from abo ...
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

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 to , the crystals tend to be long, solid, hexagonal columns. Towards the top of the cloud, at temperatures of around to , 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.


Altostratus clouds


Nimbostratus clouds


Stratocumulus clouds

A stratocumulus cloud is another type of a cumuliform or stratiform cloud. Like stratus clouds, they form at low levels; but like cumulus clouds (and unlike stratus clouds), they form via convection. Unlike cumulus clouds, their growth is almost completely retarded by a strong
inversion Inversion or inversions may refer to: Arts * , a French gay magazine (1924/1925) * ''Inversion'' (artwork), a 2005 temporary sculpture in Houston, Texas * Inversion (music), a term with various meanings in music theory and musical set theory * ...
, 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 front A cold front is the leading edge of a cooler mass of air at ground level that replaces a warmer mass of air and lies within a pronounced surface trough of low pressure. It often forms behind an extratropical cyclone (to the west in the Northern ...
s. 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.


Sources

; Footnotes ; Bibliography * * * * * * * * * {{Cloud types