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 may com ...

characterized by horizontal layering with a uniform base, as opposed to convective or

cumuliform Cumulus clouds are clouds which have flat bases and are often described as "puffy", "cotton-like" or "fluffy" in appearance. Their name derives from the Latin ''cumulo-'', meaning ''heap'' or ''pile''. Cumulus clouds are low-level clouds, gener ...

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 Haze is traditionally an atmospheric phenomenon in which dust, smoke, and other dry particulates suspended in air obscure visibility and the clarity of the sky. The World Meteorological Organization manual of codes includes a classification ...

, 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 of the ...

prefix ''strato-'', meaning "layer". Stratus clouds may produce a light

drizzle Drizzle is a light precipitation consisting of liquid water drops smaller than those of rain – generally smaller than in diameter. Drizzle is normally produced by low stratiform clouds and stratocumulus clouds. Precipitation rates from dri ...

or a small amount of

snow Snow comprises individual ice crystals that grow while suspended in the atmosphere—usually within clouds—and then fall, accumulating on the ground where they undergo further changes. It consists of frozen crystalline water throughout ...

. These clouds are essentially above-ground

fog Fog is a visible aerosol consisting of tiny water droplets or ice crystals suspended in the air at or near the Earth's surface. Reprint from Fog can be considered a type of low-lying cloud usually resembling stratus, and is heavily influ ...

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 hail. ...

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 depe ...

to increase due to the

adiabatic cooling In thermodynamics, an adiabatic process (Greek: ''adiábatos'', "impassable") is a type of thermodynamic process that occurs without transferring heat or mass between the thermodynamic system and its environment. Unlike an isothermal process, a ...

. 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 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, 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

of major rain-bearing clouds; these are

nimbostratus A nimbostratus cloud is a multi-level, amorphous, nearly uniform and often dark grey cloud that usually produces continuous rain, snow or sleet but no lightning or thunder.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 Buoyancy, buoyant air ...

clouds, and are classified as pannus clouds. Stratus fractus can also form beside mountain slopes, without the presence of

nimbus cloud A nimbostratus cloud is a multi-level, amorphous, nearly uniform and often dark grey cloud that usually produces continuous rain, snow or sleet but no lightning or thunder.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. Stratus undulatus 1

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 cloud A nimbostratus cloud is a multi-level, amorphous, nearly uniform and often dark grey cloud that usually produces continuous rain, snow or sleet but no lightning or thunder.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 A waterfall is a point in a river or stream where water flows over a vertical drop or a series of steep drops. Waterfalls also occur where meltwater drops over the edge of a tabular iceberg or ice shelf. Waterfalls can be formed in several w ...

, 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 transpi ...

occurring in a forest.

Mutatus mother cloud

Stratus only has one mutatus mother cloud. Stratus stratocumulomutatus clouds occur when

stratocumulus A stratocumulus cloud, occasionally called a cumulostratus, belongs to a genus-type of clouds characterized by large dark, rounded masses, usually in groups, lines, or waves, the individual elements being larger than those in altocumulus, and th ...

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

, which means "

". 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

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 abov ...

conditions. It is common for a stratus to form on a weak warm front, rather than the usual

nimbostratus A nimbostratus cloud is a multi-level, amorphous, nearly uniform and often dark grey cloud that usually produces continuous rain, snow or sleet but no lightning or thunder. 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

, which stabilizes the cloud by warming it and reducing turbulent mixing.

Sources

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