altostratus clouds
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Altostratus is a middle-altitude
cloud 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 ...
genus made up of water droplets, ice crystals, or a mixture of the two. Altostratus clouds are formed when large masses of warm, moist air rise, causing water vapor to condense. Altostratus clouds are usually gray or blueish featureless sheets, although some variants have wavy or banded bases. The sun can be seen through thinner altostratus clouds, but thicker layers can be quite
opaque Opacity or opaque may refer to: * Impediments to (especially, visible) light: ** Opacities, absorption coefficients ** Opacity (optics), property or degree of blocking the transmission of light * Metaphors derived from literal optics: ** In lingu ...
. Altostratus clouds usually predict the arrival of
warm front A warm front is a density discontinuity located at the leading edge of a homogeneous warm air mass, and is typically located on the equator-facing edge of an isotherm gradient. Warm fronts lie within broader troughs of low pressure than cold f ...
s. Once altostratus clouds associated with a warm front arrive, continuous rain or snow will usually follow in the next 12 to 24 hours. Although altostratus clouds predict the arrival of warmer, wetter weather, they themselves do not produce significant precipitation. Thunderstorms can be embedded in altostratus clouds; however, bringing showers. Because altostratus clouds can contain ice crystals, they can produce some optical phenomena like iridescence and coronas.


Description

Altostratus clouds are generally gray or blue-tinged with a largely-uniform blanket-like appearance. They do not have distinct features, and usually do not produce
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. ...
. The name "altostratus" comes from the conjugation of 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 ...
words "altum", meaning "high", and "stratus", meaning "flat" or "spread out". Altostratus clouds can produce
virga In meteorology, a virga, also called a dry storm, is an observable streak or shaft of precipitation falling from a cloud that evaporates or sublimates before reaching the ground. A shaft of precipitation that does not evaporate before rea ...
, causing the cloud base to appear hazy. While they do not produce significant precipitation, altostratus clouds can cause light sprinkles or even small rain showers. Consistent rainfall and lowering of the cloud base causes altostratus to become
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. Unlike most other types of clouds, altostratus clouds are not subdivided into cloud species due to their largely-featureless appearance. However, they still appear in five varieties: ''Altostratus duplicatus'', ''opacus'', ''radiatus'', ''translucidus'', and ''undulatus''. ''Altostratus duplicatus'' is a rare form of altostratus clouds composed of two or more layers of cloud. ''Translucidus'' is a
translucent In the field of optics, transparency (also called pellucidity or diaphaneity) is the physical property of allowing light to pass through the material without appreciable scattering of light. On a macroscopic scale (one in which the dimensions ...
form of altostratus clouds, meaning that the sun or moon can be seen through the cloud, whereas the ''opacus'' variety is opaque. ''Radiatus'' is another rare variety. It has parallel bands of cloud that stretch toward the horizon. The ''undulatus'' variety has an wavy appearance—the underside of the cloud appears to rise and fall. Altostratus and
altocumulus cloud Altocumulus (From Latin ''Altus'', "high", ''cumulus'', "heaped") is a middle-altitude cloud genus that belongs mainly to the ''stratocumuliform'' physical category characterized by globular masses or rolls in layers or patches, the individual ele ...
s, both of which are mid-level clouds, are commonly measured together in cloud cover studies. Together, they cover around 25% of the Earth's surface on average based on CALIPSO satellite data. This constitutes roughly one third of the Earth's total cloud cover. By itself, separated from altocumulus, altostratus covers ~16% of the Earth's surface. Altostratus cloud cover varies seasonally in temperate regions, with significantly less coverage in the summer months as compared to the other seasons. Additionally, altostratus cloud cover varies by
latitude In geography, latitude is a coordinate that specifies the north– south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from –90° at the south pole to 90° at the north pol ...
, with tropical regions having vastly fewer altostratus clouds when compared to temperate or polar regions. Altostratus and altocumulus cover roughly 22% of the ocean's surface based on surface measurements, with minimal variation based on season. Altostratus clouds are warmest at the bottom and coldest at the top, with a fairly consistent lapse rate of 5 to 7 °C per kilometer (14 to 20 °F per mile) inside the cloud. The lapse rate is the rate at which the temperature decreases with altitude. Higher lapse rates (i.e. the faster temperature drops with increasing altitude) were associated with colder clouds. The average temperature of altostratus clouds, based on data collected from roughly 45° to 80° latitude, varied from around . Warmer temperatures occurred during summer and colder temperatures during winter. Inside altostratus clouds, 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 ...
is generally greatest towards the top of the cloud decreasing slowly and roughly linearly towards the bottom. The lowest part of the cloud has the lowest relative humidity. Below the bottom of the cloud, the relative humidity drops rapidly.


Microphysical properties

Altostratus can be composed of water droplets, supercooled water droplets, and ice crystals, but ice crystals make up the vast majority. In some altostratus clouds made of ice crystals, very thin horizontal sheets of water droplets can appear seemingly at random, but they quickly disappear. The sizes of the ice crystals in the cloud tended to increase as altitude decreased. However, close to the bottom of the cloud, the particles decreased in size again. During the sampling of one cloud, the scientists noted a
halo Halo, halos or haloes usually refer to: * Halo (optical phenomenon) * Halo (religious iconography), a ring of light around the image of a head HALO, halo, halos or haloes may also refer to: Arts and entertainment Video games * ''Halo'' (franch ...
while flying near the top of the cloud, which indicated that the ice crystals were hexagonal near the top. However, farther down, the ice crystals became more conglomerated. Mixed-phase (containing both ice and water) altostratus clouds contain a "melt layer", below which the ice crystals tend to melt into water droplets. These water droplets are spheres and thus fall much faster than ice crystals, collecting at the bottom of the cloud.


Formation

Altostratus clouds form when a large mass of warm air rises, causing water vapor in the atmosphere to condense onto nuclei (small dust particles), forming water droplets and ice crystals. These conditions usually happen at the leading edge of a warm front, where
cirrostratus cloud Cirrostratus is a high-level, very thin, generally uniform ''stratiform'' genus-type of cloud. It is made out of ice-crystals, which are pieces of frozen water. It is difficult to detect and it can make halos. These are made when the cloud takes ...
s thicken and lower until they transition into altostratus clouds. Alternatively, nimbostratus clouds can thin into altostratus. Altostratus can even form from the spreading of the upper anvil cloud or the middle column of a thunderstorm. Altostratus clouds are mid-level clouds that form from above sea level in
polar regions The polar regions, also called the frigid zones or polar zones, of Earth are the regions of the planet that surround its geographical poles (the North and South Poles), lying within the polar circles. These high latitudes are dominated by floa ...
. In
temperate regions In geography, the temperate climates of Earth occur in the middle latitudes (23.5° to 66.5° N/S of Equator), which span between the tropics and the polar regions of Earth. These zones generally have wider temperature ranges throughout t ...
, the ceiling increases drastically, allowing altostratus clouds to form between . In
tropical regions The tropics are the regions of Earth surrounding the Equator. They are defined in latitude by the Tropic of Cancer in the Northern Hemisphere at N and the Tropic of Capricorn in the Southern Hemisphere at S. The tropics are also referred to ...
, altostratus can reach even higher, forming from . They can range from in thickness and can cover hundreds of kilometers of the Earth's surface.


Use in forecasting

Altostratus clouds tend to form ahead of
warm front A warm front is a density discontinuity located at the leading edge of a homogeneous warm air mass, and is typically located on the equator-facing edge of an isotherm gradient. Warm fronts lie within broader troughs of low pressure than cold f ...
s or
occluded front In meteorology, an occluded front is a type of weather front formed during cyclogenesis. The classical and usual view of an occluded front is that it initiates when a cold front overtakes a warm front near a cyclone, such that the warm air is separ ...
s and herald their arrival. These warm fronts bring warmer air into the region. Occluded fronts form when a faster-moving cold front catches up to a warm front, and the temperature after the frontal system passes may rise or fall. As the frontal system approaches, cirrostratus clouds will thicken into altostratus clouds, which then gradually thicken further into nimbostratus clouds. If the frontal system is occluded,
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 may also be present. Once the altostratus clouds have arrived, rain or snow will usually follow in the next 12 to 24 hours. Instability in the atmosphere can embed
thunderstorms A thunderstorm, also known as an electrical storm or a lightning storm, is a storm characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere, known as thunder. Relatively weak thunderstorms are somet ...
in an altostratus cloud, although altostratus clouds themselves do not produce storms.


Effects on climate

Globally, clouds reflect around 50
watt The watt (symbol: W) is the unit of power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantify the rate of energy transfer. The watt is named after James ...
s per square meter of short-wave solar radiation back into space, cooling the Earth by around , an effect largely caused by
stratocumulus cloud 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 the ...
s. However, at the same time, they reflect around 30 watts per square meter of long-wave (infrared) black body radiation emitted by the Earth back to Earth's surface, heating the Earth by around —a process called the
greenhouse effect The greenhouse effect is a process that occurs when energy from a planet's host star goes through the planet's atmosphere and heats the planet's surface, but greenhouse gases in the atmosphere prevent some of the heat from returning directly ...
.
Cirrus Cirrus may refer to: Science *Cirrus (biology), any of various thin, thread-like structures on the body of an animal *Cirrus (botany), a tendril * Infrared cirrus, in astronomy, filamentary structures seen in infrared light *Cirrus cloud, a typ ...
and altostratus clouds are the top two sources of this heating effect. This combination of heating and cooling sums out to a net loss of 20 watts per square meter globally, cooling the Earth by roughly . Altostratus clouds are the only cloud genus besides cirrus clouds to exhibit a net global heating effect on Earth and its atmosphere; however, cirrus have a heating effect that is four times as potent as altostratus (2 watts per square meter versus only 0.5 watts per square meter).


Optical phenomena

Altostratus clouds can produce bright halos when viewed from the air, but not when viewed from the ground. Halos can take the appearance of rings, arcs, or spots of white or multicolored light and are formed by the
reflection Reflection or reflexion may refer to: Science and technology * Reflection (physics), a common wave phenomenon ** Specular reflection, reflection from a smooth surface *** Mirror image, a reflection in a mirror or in water ** Signal reflection, in ...
and
refraction In physics, refraction is the redirection of a wave as it passes from one medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commonly observed phenome ...
of sunlight or moonlight shining through ice crystals in the cloud. Light diffraction through altostratus clouds can also produce coronas, which are small, concentric pastel-colored rings of light around the sun or moon. They can also be iridescent, with often-parallel bands of bright color projected on a cloud. Unlike the halos, the coronas and iridescence can be seen from Earth's surface.


Relation to other clouds

Altostratus and
altocumulus cloud Altocumulus (From Latin ''Altus'', "high", ''cumulus'', "heaped") is a middle-altitude cloud genus that belongs mainly to the ''stratocumuliform'' physical category characterized by globular masses or rolls in layers or patches, the individual ele ...
s are the two genera of mid-level clouds that usually form between . These are given the prefix "alto-". These clouds are formed from ice crystals, supercooled water droplets, or liquid water droplets. Above the mid-level clouds are three different genera of high-level clouds,
cirrus Cirrus may refer to: Science *Cirrus (biology), any of various thin, thread-like structures on the body of an animal *Cirrus (botany), a tendril * Infrared cirrus, in astronomy, filamentary structures seen in infrared light *Cirrus cloud, a typ ...
,
cirrocumulus Cirrocumulus is one of the three main genus-types of high-altitude tropospheric clouds, the other two being cirrus and cirrostratus. They usually occur at an altitude of . Like lower-altitude cumuliform and stratocumuliform clouds, cirrocumulus s ...
, and cirrostratus, all of which are given the prefix "cirro-". High-level clouds usually form above . Cirrocumulus and cirrostratus are sometimes informally referred to as ''cirriform clouds'' because of their frequent association with cirrus. Below the mid-level clouds are the low-level clouds, which usually form below and do not have a prefix. The two genera that are strictly low-level are
stratus Stratus may refer to: Weather *Stratus cloud, a cloud type **Nimbostratus cloud, a cloud type **Stratocumulus cloud, a cloud type **Altostratus cloud, a cloud type **Altostratus undulatus cloud, a cloud type **Cirrostratus cloud, a cloud type Mus ...
, and
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 ...
. These clouds are composed of water droplets, except during winter when they are formed of
supercooled water Supercooling, also known as undercooling, is the process of lowering the temperature of a liquid or a gas below its melting point without it becoming a solid. It achieves this in the absence of a seed crystal or nucleus around which a crystal ...
droplets or ice crystals if the temperature at cloud level is below freezing. Three additional genera usually form in the low altitude range, but may be based at higher levels under conditions of very low humidity. They are the genera
cumulus 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 ...
, 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. ...
, and
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

Cirrostratus clouds can appear as a smooth veil 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 , the crystals tend to be long, solid, hexagonal columns. Towards the top of the cloud, at temperatures of around , 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.


Altocumulus

Altocumulus clouds are small patches or heaps of white or light gray cloud. Like altostratus, altocumulus are composed of a mixture of water droplets, supercooled water droplets, and ice crystals. Although altocumulus clouds are mid-level clouds that form at roughly the same altitude as altostratus clouds, their formation methods are completely different. Altocumulus forms from
convective Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the convec ...
(rising) processes, whereas altostratus is usually formed by descending and thickening cirrostratus.


Stratus

Stratus are low-level clouds that are usually visually similar to altostratus. Stratus comes in two species: ''nebulosus'', a largely-featureless flat gray cloud sheet, and ''fractus'', shattered fragments of cloud often called "scud". Opaque varieties of altostratus and ''stratus nebulosus'' clouds can be virtually indistinguishable from each other to the naked eye, to the point that the
World Meteorological Organization The World Meteorological Organization (WMO) is a specialized agency of the United Nations responsible for promoting international cooperation on atmospheric science, climatology, hydrology and geophysics. The WMO originated from the Intern ...
suggests that one of the few ways to distinguish between these clouds is to check what types of clouds came before them. Altostratus clouds, because they tend to form from warm fronts, are usually preceded by high-level cirriform clouds. Stratus clouds tend to form by cooling air masses, often at night, and thus are not usually preceded by other types of clouds.


Nimbostratus

Nimbostratus are low-level (sometimes classified as vertical) rain-bearing stratus clouds. Unlike the sprinkles or light drizzles that altostratus or stratus can produce, nimbostratus produces heavy, continuous rain or snow. These clouds are thick and dark enough to entirely blot out the sun. Nimbostratus has no species or varieties. Like altostratus, nimbostratus clouds can be made of ice crystals, supercooled water droplets, or water droplets.


See also

*
List of cloud types The list of cloud types groups all genera as ''high'' (cirro-, cirrus), ''middle'' (alto-), ''multi-level'' (nimbo-, cumulo-, cumulus), and ''low'' (strato-, stratus). These groupings are determined by the altitude level or levels in the troposphe ...


Notes


Sources

; Footnotes ; Bibliography * * * * * * * * * * * * * * * {{DEFAULTSORT:Altostratus Cloud Stratus Cloud types