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Altostratus cloud
Altostratus cloud
Altostratus cloud
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An altostratus radiatus cloud showing the characteristic parallel lines of cloud.
Altostratus radiatus cloud showing distinctive parallel bands

Altostratus is a middle-altitude cloud 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.

Altostratus clouds usually predict the arrival of warm fronts. 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

[edit]
The sun shines diumly through a largely-featureless gray altostratus cloud.
Sun shines dimly though the translucidus variant of altostratus clouds

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. The name "altostratus" comes from the conjugation of the Latin words "altum", meaning "high", and "stratus", meaning "flat" or "spread out".[1][2] Altostratus clouds can produce virga, causing the cloud base to appear hazy.[3] While they do not produce significant precipitation, altostratus clouds can cause light sprinkles or even small rain showers.[4] Consistent rainfall and lowering of the cloud base causes altostratus to become nimbostratus.[5]

Unlike most other types of clouds, altostratus clouds are not subdivided into cloud species due to their largely-featureless appearance.[6] However, they still appear in five varieties: Altostratus duplicatus, opacus, radiatus, translucidus, and undulatus.[7] Altostratus duplicatus is a rare form of altostratus clouds composed of two or more layers of cloud.[8] Translucidus is a translucent form of altostratus clouds, meaning that the sun or moon can be seen through the cloud,[9] whereas the opacus variety is opaque.[10] Radiatus is another rare variety. It has parallel bands of cloud that stretch toward the horizon.[11] The undulatus variety has an wavy appearance—the underside of the cloud appears to rise and fall.[12]

Altostratus and altocumulus clouds, both of which are mid-level clouds,[4] are commonly measured together in cloud cover studies. Together, they cover around 25% of the Earth's surface on average[13] based on CALIPSO satellite data.[14] This constitutes roughly one third of the Earth's total cloud cover.[13] By itself, separated from altocumulus, altostratus covers ~16% of the Earth's surface.[13] 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, with tropical regions having vastly fewer altostratus clouds when compared to temperate or polar regions.[15] Altostratus and altocumulus cover roughly 22% of the ocean's surface based on surface measurements, with minimal variation based on season.[16]

Altostratus clouds are warmest at the bottom and coldest at the top,[17] with a fairly consistent[18] 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.[19] Higher lapse rates (i.e. the faster temperature drops with increasing altitude) were associated with colder clouds.[18] The average temperature of altostratus clouds, based on data collected from roughly 45° to 80° latitude, varied from around −16 to −45 °C (3.2 to −49 °F). Warmer temperatures occurred during summer and colder temperatures during winter.[17]

Inside altostratus clouds, the relative humidity 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.[17] Below the bottom of the cloud, the relative humidity drops rapidly.[20]

Microphysical properties

[edit]

Altostratus can be composed of water droplets, supercooled water droplets, and ice crystals,[4] but ice crystals make up the vast majority.[21] In some altostratus clouds made of ice crystals, very thin horizontal sheets of water droplets can appear seemingly at random, but they quickly disappear.[22] 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, scientists noted a halo 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.[23][24] 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.[25]

Formation

[edit]
A labeled diagram showing a warm front and the order in which clouds arrive.
Diagram of a warm front

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.[26] These conditions usually happen at the leading edge of a warm front, where cirrostratus clouds thicken and lower until they transition into altostratus clouds.[2] Alternatively, nimbostratus clouds can thin into altostratus.[27] Altostratus can even form from the spreading of the upper anvil cloud or the middle column of a thunderstorm.[27]

Altostratus clouds are mid-level clouds[4] that form from 2,000 to 4,000 metres (6,600 to 13,000 ft) above sea level in polar regions. In temperate regions, the ceiling increases drastically, allowing altostratus clouds to form between 2,000 to 7,000 metres (6,600 to 23,000 ft). In tropical regions, altostratus can reach even higher, forming from 2,000 to 8,000 metres (6,600 to 26,000 ft).[3] They can range from 1,000 to 5,000 metres (3,300 to 16,000 ft) in thickness[3] and can cover hundreds of kilometers of the Earth's surface.[28]

Use in forecasting

[edit]
See also: Weather forecasting

Altostratus clouds tend to form ahead of warm fronts or occluded fronts and herald their arrival.[2] 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.[29] As the frontal system approaches, cirrostratus clouds will thicken into altostratus clouds, which then gradually thicken further into nimbostratus clouds.[2][30] If the frontal system is occluded, cumulonimbus clouds may also be present.[29] Once the altostratus clouds have arrived, rain or snow will usually follow in the next 12 to 24 hours.[30]

Instability in the atmosphere can embed thunderstorms in an altostratus cloud,[3] although altostratus clouds themselves do not produce storms.[4]

Effects on climate

[edit]

Globally, clouds reflect around 50 watts per square meter[a] of short-wave solar radiation back into space, cooling the Earth by around 12 °C (22 °F), an effect largely caused by stratocumulus clouds. 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 7 °C (13 °F)—a process called the greenhouse effect. Cirrus 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 5 °C (9.0 °F).[31][32][33][34]

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).[35]

Optical phenomena

[edit]

Altostratus clouds can produce bright halos when viewed from the air,[3] but not when viewed from the ground.[36] Halos can take the appearance of rings, arcs, or spots of white or multicolored light and are formed by the reflection and refraction of sunlight or moonlight shining through ice crystals in the cloud.[37] 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.[2][38]

Relation to other clouds

[edit]
See also: List of cloud types
A diagram showing clouds at various heights
Heights of various cloud genera including high-, mid-, and low-level clouds

Altostratus and altocumulus clouds are the two genera of mid-level clouds that usually form between 2,000 and 6,100 m (6,500 and 20,000 ft).[4][39] These are given the prefix "alto-". These clouds are formed from ice crystals, supercooled water droplets, or liquid water droplets.[4]

Above the mid-level clouds are three different genera of high-level clouds, cirrus, cirrocumulus, and cirrostratus, all of which are given the prefix "cirro-". High-level clouds usually form above 6,100 m (20,000 ft).[4][39][40] Cirrocumulus and cirrostratus are sometimes informally referred to as cirriform clouds because of their frequent association with cirrus.[41]

Below the mid-level clouds are the low-level clouds, which usually form below 2,000 m (6,500 ft) and do not have a prefix.[4][39] The two genera that are strictly low-level are stratus, and stratocumulus. These clouds are composed of water droplets, except during winter when they are formed of supercooled water 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, and cumulonimbus, and nimbostratus. These are sometimes classified separately as clouds of vertical development, especially when their tops are high enough to be composed of supercooled water droplets or ice crystals.[42][4]

Cirrostratus

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

Cirrostratus clouds can appear as a smooth veil in the sky[43] or as a striated sheet.[40] 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.[44] Cirrostratus come in two species, fibratus and nebulosus.[45] The ice crystals in these clouds vary depending upon the height in the cloud. Towards the bottom, at temperatures of around −35 to −45 °C (−31 to −49 °F), the crystals tend to be long, solid, hexagonal columns. Towards the top of the cloud, at temperatures of around −47 to −52 °C (−53 to −62 °F), the predominant crystal types are thick, hexagonal plates and short, solid, hexagonal columns.[46][47] These clouds commonly produce halos, and sometimes the halo is the only indication that such clouds are present.[30] They are formed by warm, moist air being lifted slowly to a very high altitude.[48] When a warm front approaches, cirrostratus clouds become thicker and descend forming altostratus clouds,[4] and rain usually begins 12 to 24 hours later.[30]

Altocumulus

[edit]
Main article: Altocumulus cloud
Small shreds of white altocumulus clouds against a blue sky.
Altocumulus clouds

Altocumulus clouds are small patches or heaps of white or light gray cloud.[49][4] 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 (rising) processes,[4] whereas altostratus is usually formed by descending and thickening cirrostratus.[2]

Stratus

[edit]
Main article: Stratus cloud
A featureless gray stratus cloud.
Stratus cloud

Stratus are low-level clouds that are usually visually similar to altostratus.[4] Stratus comes in two species: nebulosus, a largely-featureless flat gray cloud sheet, and fractus, shattered fragments of cloud[50] often called "scud".[4] 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 suggests that one of the few ways to distinguish between these clouds is to check what types of clouds came before them.[51] Altostratus clouds, because they tend to form from warm fronts,[2] are usually preceded by high-level cirriform clouds.[51] Stratus clouds tend to form by cooling air masses, often at night,[52] and thus are not usually preceded by other types of clouds.[51]

Nimbostratus

[edit]
Main article: Nimbostratus cloud

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.[4][53] Nimbostratus has no species[54] or varieties.[55] Like altostratus, nimbostratus clouds can be made of ice crystals, supercooled water droplets, or water droplets.[56]

See also

[edit]
Wikimedia Commons has media related to Altostratus clouds.

Notes

[edit]

Sources

[edit]
Footnotes
  1. ^ Cohn et al. 2017, Section 2.3.5.1
  2. ^ a b c d e f g "Altostratus clouds". Meteorological Office of the UK. Retrieved 25 March 2022.
  3. ^ a b c d e "Altostratus". International Cloud Atlas. World Meteorological Organization. Retrieved 25 March 2022.
  4. ^ a b c d e f g h i j k l m n o p Funk, Ted. "Cloud Classifications and Characteristics" (PDF). The Science Corner. NOAA. p. 1. Retrieved 25 March 2022.
  5. ^ Ahrens 2006, p. 194
  6. ^ Cohn et al. 2017, Section 2.3.5.2
  7. ^ Cohn et al. 2017, Section 2.3.5.3
  8. ^ Cohn et al. 2017, Section 2.3.5.3.3
  9. ^ Cohn et al. 2017, Section 2.3.5.3.1
  10. ^ Cohn et al. 2017, Section 2.3.5.3.2
  11. ^ Cohn et al. 2017, Section 2.3.5.3.5
  12. ^ Cohn et al. 2017, Section 2.3.5.3.4
  13. ^ a b c Sassen & Wang 2012, p. 688
  14. ^ Sassen & Wang 2012, p. 679
  15. ^ Sassen & Wang 2012, p. 686
  16. ^ Warren et al. 1988, Table 9b
  17. ^ a b c Yang & Zou 2013, p. 6010
  18. ^ a b Yang & Zou 2013, p. 6013
  19. ^ Yang & Zou 2013, p. 6011
  20. ^ Danne et al. 1999, p. 181
  21. ^ Sassen & Wang 2012, pp. 679–680
  22. ^ Platt 1977, p. 344
  23. ^ Field 1999, p. 1929
  24. ^ Field 1999, p. 1933
  25. ^ Danne et al. 1999, p. 182
  26. ^ "Clouds and How They Form". Center for Science Education. University Corporation for Atmospheric Research. Retrieved 28 March 2022.
  27. ^ a b Cohn et al. 2017, Section 2.3.5.5
  28. ^ Cohn et al. 2017, Section 2.3.5.7
  29. ^ a b "Weather Fronts". Center for Science Education. University Corporation for Atmospheric Research. Retrieved 28 March 2022.
  30. ^ a b c d Ahrens 2006, p. 120
  31. ^ "Cloud Climatology". International Satellite Cloud Climatology Program. National Aeronautics and Space Administration. Retrieved 12 July 2011.
  32. ^ "Cloud Radiative Effect". Geophysical Fluid Dynamics Laboratory. National Oceanic and Atmospheric Administration. Retrieved 29 March 2022.
  33. ^ L'Ecuyer et al. 2019, p. 6213
  34. ^ Riebeek, Holli (3 June 2010). "Global Warming: Feature Articles". Earth Observatory. National Aeronautics and Space Administration. Retrieved 29 March 2022.
  35. ^ L'Ecuyer et al. 2019, p. 6205
  36. ^ Cohn et al. 2017, Section 2.3.5.6.2
  37. ^ Cohn et al. 2017, Section 3.2.3.1
  38. ^ Cohn et al. 2017, Section 3.2.3.2 – 3.2.3.3
  39. ^ a b c "Classifying clouds". World Meteorological Organization. 18 January 2017. Archived from the original on December 18, 2023. Retrieved 14 March 2022.
  40. ^ a b Hubbard 2000, p. 340
  41. ^ "Cirriform – Glossary of Meteorology". American Meteorological Society. Retrieved 23 February 2022.
  42. ^ Koermer, Jim (2011). "Plymouth State Meteorology Program Cloud Boutique". Plymouth State University. Archived from the original on 10 May 2009. Retrieved 2 April 2012.
  43. ^ Cohn et al. 2017, Section 2.3.3.1
  44. ^ Day 2005, p. 56
  45. ^ Cohn et al. 2017, Section 2.3.3.2
  46. ^ Parungo 1995, p. 254
  47. ^ Parungo 1995, p. 256
  48. ^ Hamilton 2007, p. 24
  49. ^ Cohn et al. 2017, Section 2.3.4.1
  50. ^ Cohn et al. 2017, Section 2.3.8.2
  51. ^ a b c Cohn et al. 2017, Section 2.3.5.6.6
  52. ^ Cohn et al. 2017, Section 2.3.8.8
  53. ^ Cohn et al. 2017, Section 2.3.6.1
  54. ^ Cohn et al. 2017, Section 2.3.6.2
  55. ^ Cohn et al. 2017, Section 2.3.6.3
  56. ^ Cohn et al. 2017, Section 2.3.6.7
Bibliography
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Altostratus cloud

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Altostratus clouds are mid-level atmospheric formations classified as a principal cloud by the , appearing as a greyish or bluish sheet or layer with a striated, fibrous, or uniform structure that totally or partly covers the sky. Thinner portions of these clouds allow the sun to be faintly visible as through , without halo phenomena, while thicker sections obscure it entirely. Composed primarily of a mixture of water droplets and ice crystals, they often exhibit variations in optical thickness or color across their extent. These clouds typically develop at altitudes ranging from 2 to 7 kilometers (6,500 to 23,000 feet) in temperate regions and slightly lower in polar areas, positioning them in the middle . Altostratus forms through several processes, including the thickening of cirrostratus veils, the thinning and spreading of nimbostratus layers, or the expansion of cumulonimbus or altocumulus sheets, often as a result of large-scale lifting of moist air ahead of weather fronts. They are lighter in color than the denser nimbostratus but darker than the higher cirrostratus, and may include supplementary features such as (trailing precipitation that evaporates before reaching the ground), praecipitatio (actual ), or clouds beneath them. Altostratus serves as a key indicator of approaching systems, frequently preceding warm or occluded fronts and gradually deepening into rain- or snow-producing nimbostratus. Varieties include translucidus (sufficiently translucent to reveal the sun's position), opacus (opaque enough to fully mask the sun), and undulatus (wavy appearance). In and , their presence signals potential weather changes, influencing forecasts for and .

Physical Characteristics

Appearance and Varieties

Altostratus clouds present as a grayish or sheet or layer with a uniform, fibrous, or striated appearance, often covering the entire or large portions of it. This featureless or subtly textured veil creates a diffused, watery that partially reveals the or in thinner sections, while thicker areas obscure celestial bodies entirely. The overall effect is a somber, without distinct edges or billows, distinguishing altostratus from more turbulent forms. These clouds form at mid-level altitudes, typically between 2 and 7 kilometers (6,500 to 23,000 feet) in temperate regions, though they occur lower at 2 to 4 kilometers (6,500 to 13,000 feet) in polar areas. Their vertical thickness varies from about 1 kilometer to more than 5 kilometers (3,300 to over 16,500 feet), allowing for substantial that influences visibility. According to the (WMO) classification, altostratus exhibits several varieties based on transparency, layering, and arrangement. The translucidus variety features thinner sections that permit a translucent, watery view of the sun, while the opacus variety is denser and opaque, fully blocking solar or lunar discs. The duplicatus variety appears as two or more superimposed layers with uneven bases, and radiatus shows parallel bands converging toward the horizon. Additionally, undulatus displays wavy, undulating bases across the layer. Supplementary features enhance identification of altostratus. Praecipitatio indicates ongoing reaching the ground, while virga consists of trailing streaks that evaporate before impact. The translucency in varieties such as translucidus arises partly from embedded crystals scattering light.

Microphysical Composition

Altostratus clouds consist primarily of water droplets in their lower portions, with typical diameters of 10 to 20 μm, transitioning to predominantly crystals in the upper, colder regions where crystal diameters can extend up to 100 μm or more. The combined liquid and ice water content in these clouds generally ranges from 0.1 to 0.5 g/m³, reflecting their stratiform nature with moderate levels. These clouds exhibit mixed-phase characteristics, where supercooled droplets coexist with ice crystals—such as aggregates, dendrites, and hexagonal plates—at temperatures below 0°C, often spanning -1°C to -31°C. This coexistence facilitates processes like the Bergeron-Findeisen mechanism, promoting gradual cloud thickening as ice crystals grow at the expense of surrounding vapor and droplets. Particle distribution within altostratus forms uniform horizontal layers with minimal vertical turbulence, differing markedly from the disorganized structures in convective clouds; liquid water content often increases with height in single-layer formations.

Formation Processes

Atmospheric Conditions

Altostratus clouds form in stable, moist air masses at mid-tropospheric levels, typically requiring high relative humidity within the layer to support widespread condensation without significant vertical development. The temperature lapse rate in these environments approximates the moist adiabatic rate, ranging from 6 to 9°C per kilometer, which promotes gradual cooling and uniform cloud layer formation rather than convective instability. In synoptic-scale settings, altostratus often develops ahead of warm or occluded fronts in mid-latitudes, where isentropic lift elevates moist air parcels along surfaces of constant potential temperature. This uplift mechanism arises from large-scale convergence in baroclinic zones, fostering the slow ascent necessary for stratiform cloud decks spanning hundreds of kilometers. The characteristic air masses involve warm, moist tropical or maritime air overriding denser, cooler polar air at frontal boundaries, resulting in frontal convergence that supplies the moisture for cloud initiation. Vertical remains minimal, allowing the cloud layer to maintain its horizontal uniformity without disruption from differential horizontal motions. In polar regions, altostratus can also form within 2-4 km thick layers during development, driven by cyclone-induced uplift in cold marine air outbreaks, a highlighted in pre-2023 studies as understudied due to limited observations in high-latitude environments.

Developmental Stages

Altostratus clouds initiate their lifecycle as a thin layer of altostratus translucidus, forming through the gradual thickening of cirrostratus via sustained atmospheric uplift associated with approaching frontal systems. This initial stage often emerges ahead of , as the veil-like cirrostratus descends and condenses further, transitioning into a uniform, grayish-blue sheet that partially veils the sun. In the maturation phase, the cloud layer progressively thickens to altostratus opacus over several hours to a day, driven by continued uplift that promotes droplet coalescence and enhances the cloud's , ultimately obscuring solar visibility. This evolution reflects increasing water content and droplet size, with the layer expanding vertically while maintaining a featureless, stratified appearance. During the advanced stage, if uplift intensifies, altostratus opacus can transform into nimbostratus, marking the onset of widespread, steady as the lowers and or begins to fall. Conversely, occurs through atmospheric drying or , which evaporates the cloud layer or disperses it, depending on synoptic persistence. Altostratus may also form through other processes, including the thinning of nimbostratus layers or the spreading out of the upper parts of cumulonimbus or altocumulus sheets.

Meteorological Applications

Weather Forecasting

Altostratus clouds serve as key indicators in short-term weather forecasting, often signaling the impending arrival of precipitation. Their presence typically precedes steady light to moderate rain or snow within 6 to 24 hours, as the mid-level cloud layer advances ahead of warm fronts or low-pressure systems. As altostratus thickens, a lowering of the cloud base—often from around 3 km to 1-2 km—heralds the transition to more persistent rainfall, eventually evolving into nimbostratus if conditions intensify. Observers rely on ground-based visual cues for real-time assessment, particularly noting the translucidus variety where the sun appears diffused or "watery" through the semi-transparent sheet, indicating thinner sections less likely to produce immediate . reflectivity provides quantitative support, with values of 10-20 dBZ commonly associated with the onset of light from altostratus, allowing meteorologists to track the cloud's precipitation potential as it approaches. In models like the European Centre for Medium-Range Weather Forecasts (ECMWF) Integrated Forecasting System and the (GFS), altostratus is represented through cloud parameterization schemes that incorporate mid-level moisture and humidity profiles to simulate layer formation and evolution. These schemes enhance the accuracy of frontal system timing by better resolving moisture , contributing to improved short-range forecasts. Recent advancements in AI-driven using all-sky camera have boosted identification of precipitating clouds, achieving up to 99% accuracy in determining rainfall potential.

Indicators of Frontal Systems

Altostratus clouds serve as a primary indicator of an approaching , typically appearing 300 to 600 km ahead of the frontal boundary in mid-level layers formed by the gradual lifting of warm, moist air over cooler air masses. As the front advances, these clouds thicken progressively, often leading to light precipitation and providing an early warning of steady or . In occluded fronts, where a overtakes a , altostratus exhibits faster thickening due to intensified uplift and convergence at the occlusion line. The typical sequence of clouds in advancing frontal systems positions altostratus after the initial high-level cirrostratus and before the lower, rain-bearing nimbostratus, reflecting the descending cloud bases as the front nears. A noticeable lowering of the altostratus base further signals the transition to occlusion, as the cloud layer merges with lower stratus formations and intensifies precipitation. In mid-latitudes, altostratus presence commonly forecasts within 12 to 36 hours, allowing for preparation against prolonged conditions and . Regionally, variations occur; in tropical zones, altostratus links to onset, with bases elevated at 4 to 6 km owing to the warmer, more expansive that supports broader stratiform development.

Environmental and Climatic Roles

Radiation and Climate Effects

Altostratus clouds influence Earth's radiative balance by reflecting shortwave and trapping longwave . Mid-level clouds like altostratus have albedos typically ranging from 0.4 to 0.8, reflecting incoming solar and contributing to a cooling effect at the top of the atmosphere. Globally, cloud shortwave forcing is around -50 W/m², with mid-level clouds playing a role in this reflection. Conversely, they emit downward longwave , providing a warming effect at the surface, estimated at 20-30 W/m² under conditions in mid-latitudes. The net radiative effect of mid-level clouds is generally cooling in mid-latitudes due to greater shortwave reflection compared to trapping. They contribute to the overall global radiative forcing of approximately -20 to -30 W/m². In polar regions, the warming may dominate during winter. Altostratus enhances the planetary relative to clear-sky conditions. Cloud feedbacks involving mid-level clouds like altostratus are positive, amplifying warming through changes in cover and thickness with increases. Aerosol-cloud interactions increase droplet numbers, enhancing and contributing to the global effective radiative forcing from aerosol-cloud interactions of -1.0 ± 0.7 W/m². The IPCC AR6 notes the importance of mid-level clouds in extratropical feedbacks, with improved representation in recent models. Altostratus plays a role in extratropical systems, influencing development; CMIP6 models better capture their feedbacks.

Contribution to Precipitation

Altostratus clouds may produce light precipitation, such as drizzle or snow, often in the form of virga that evaporates before reaching the ground. Precipitation arises through collision-coalescence in lower portions and the Bergeron process in upper levels, but rates are typically low, less than 1 mm per hour. Precipitation from altostratus is limited by dry air layers below, leading to frequent virga. In mid-latitudes, altostratus contributes minimally to total rainfall, mainly during stable frontal systems. Globally, their role in annual precipitation in temperate zones is small, with output increasing as they thicken into nimbostratus during fronts. Broader studies indicate potential increases in mid-level cloud-associated precipitation due to atmospheric moistening from climate warming.

Associated Phenomena

Optical Effects

Altostratus clouds, particularly the translucidus variety, allow the sun or to appear as a bright spot with a watery or fuzzy outline due to the diffuse transmission of through their relatively thin layers of droplets or crystals. This effect arises from the and partial absorption of visible , creating a ground-glass-like that vaguely reveals the celestial body without sharp contours. In thinner portions of altostratus, —colored rings surrounding the sun or with angular typically 5–10 degrees—can form through by small, nearly uniform water droplets (often <25 μm in ). The mechanism follows , where shorter () appear innermost and longer ones () outermost, with the ring size inversely proportional to droplet via θ ≈ (λ / D), where θ is the angular radius, λ the , and D the . These phenomena are most vivid in layers with narrow size distributions but are less common in uniform altostratus compared to thinner altocumulus. Advanced optical effects like (swirling patches of spectral colors) or parhelia (sundogs) are possible but infrequent in altostratus, as the clouds' relatively uniform particle sizes and mixed-phase composition limit the spatial variability needed for pronounced interference or patterns. Unlike cirrostratus, altostratus rarely produces strong halos, as confirmed by the absence of typical ice-crystal signatures such as 22° rings. The optical thickness of altostratus significantly reduces light transmission and contrast, leading to diminished of underlying features. In the opacus variety, this results in color desaturation, where the takes on a uniform gray tone due to multiple that randomizes directions and suppresses chromatic effects.

Other Meteorological Features

Altostratus clouds exhibit minimal low-level due to their stratiform structure and stable atmospheric layering, though some moderate may occur at mid-levels within the cloud layer. These clouds often form in association with steady winds in frontal zones, where the gradual ascent of warm air over cooler masses promotes their development without intense shear. In contexts, this relative stability contributes to predictable flight conditions, but pilots must remain vigilant for occasional embedded during transitions to thicker forms. Supplementary phenomena linked to altostratus include , where falls from the cloud but evaporates before reaching the ground, often appearing as hanging streaks beneath the . Rare instances of thunder can occur during the thickening stages of altostratus, particularly when embedded convective elements develop prior to full transition to nimbostratus, as reported in pilot observations of within these layers. Additionally, the extensive can lead to acoustic effects, such as muffled propagation of distant sounds, resulting from the dampening of sound waves by the dense overhead layer as a partial barrier. Observational tools like reveal the slow evolution of altostratus, capturing gradual changes in over hours as the cloud layer thickens uniformly. For , altostratus typically reduces surface to 5-10 km due to the uniform gray veil diffusing sunlight, though this is generally better than under lower stratus decks and allows for operations with caution.

Relations to Other Cloud Types

Comparison with Cirrostratus

Altostratus clouds form at mid-level altitudes, typically between 2 and 7 kilometers (6,500 to 23,000 feet), whereas cirrostratus clouds occupy higher levels, ranging from 5 to 13 kilometers (20,000 to 40,000 feet). Altostratus primarily consist of a mixture of water droplets and ice crystals, with water droplets often dominant in their lower portions, while cirrostratus are composed exclusively of ice crystals. In appearance, altostratus present as thicker, gray or blue-gray sheets that cover much of the sky and obscure the sun or , preventing the casting of shadows on the ground, in contrast to the thinner, whiter, and more veil-like cirrostratus, which allow the sun or moon's disc to remain visible and cast shadows. A common transition occurs when cirrostratus clouds thicken and descend through cooling and moisture addition, evolving into altostratus as they lower into warmer air layers. Both cloud types can produce optical halos around the sun or due to , though these effects are more pronounced and frequent in cirrostratus and often fainter or absent in the denser altostratus. Meteorologically, both altostratus and cirrostratus often appear ahead of warm or occluded fronts, sharing origins in large-scale lifting of moist air. However, cirrostratus typically signals an approaching front 24 hours or more before , serving as an early indicator, while altostratus indicates the front is nearer, often within 6 to 12 hours, and may thicken into nimbostratus to produce or . Unlike cirrostratus, which do not produce virga or precipitation, altostratus can exhibit virga—trails of falling precipitation that evaporate before reaching the ground—though this high-altitude persistence is less characteristic of altostratus compared to cirrostratus in transitional phases.

Comparison with Altocumulus

Altostratus and altocumulus clouds both occupy the mid-levels of the atmosphere, typically between 2 and 7 kilometers (6,500 and 23,000 feet) above the surface, but they differ markedly in structure and formation processes. Altostratus manifests as a continuous, uniform gray or bluish sheet or layer, often striated or fibrous, that covers much of the sky with a stable, horizontally extensive appearance. In contrast, altocumulus appears as white or gray patches, sheets, or layers composed of discrete, rounded masses, rolls, or laminae, exhibiting a more patchy and weakly convective texture. This distinction arises from altostratus forming in stable air masses lifted by large-scale synoptic systems, while altocumulus develops through localized convection within conditionally unstable layers.
AspectAltostratusAltocumulus
StructureContinuous, layered sheets; minimal vertical developmentDiscrete puffs or rolls; greater vertical development (typically 0.5–2 km thick for elements)
PrecipitationPossible light, continuous or ; may thicken to produce steady Mostly or no at the surface; rare light showers
Associated WeatherIndicates approach of steady warm fronts or cyclonic activitySignals diurnal or elevated , often in fair or pre-thunderstorm conditions
These differences highlight their roles in distinct atmospheric dynamics, though both require adequate mid-level for formation. Altocumulus may occasionally develop from the fragmentation or dissipation of altostratus layers during transitional .

Comparison with Stratus

Altostratus clouds form at mid-level altitudes between approximately 2 and 7 kilometers above the ground, whereas stratus clouds develop at low levels below 2 kilometers. This elevational difference contributes to their distinct roles in systems, with altostratus often linked to dynamic frontal passages that lift moist air masses, leading to transient coverage, while stratus arises in stable, calm conditions such as marine boundary layers where cool, moist air advects over colder surfaces. In terms of composition, altostratus consists of a mixture of droplets and ice crystals, reflecting their mid-level environment where temperatures can drop below freezing, whereas stratus is primarily composed of water droplets due to the warmer conditions at low altitudes. Regarding precipitation, altostratus typically produces light to moderate continuous or , often through or steady fallout, and exhibits higher that dims the sun to a diffuse glow; in contrast, stratus yields only light drizzle or no at all, sometimes transitioning into , with lower optical thickness allowing more uniform without significant dimming. Altostratus can evolve by lowering in subsiding air masses, potentially resembling stratus in appearance during descent, though it retains ties to prior frontal dynamics that stratus lacks entirely. Additionally, altostratus tends to cover expansive synoptic-scale areas associated with fronts, spanning hundreds of kilometers, while stratus forms in more localized patches, often confined to coastal or regions. Both share a sheet-like uniformity in structure.

Comparison with Nimbostratus

Altostratus and nimbostratus clouds represent points on a continuum of stratiform cloud development in the of the atmosphere, with altostratus typically being thinner and non-precipitating compared to the thicker, rain-bearing nimbostratus. Altostratus layers generally range from 1 to more than 5 km in vertical thickness and consist of a mix of water droplets and ice crystals without significant fallout, allowing the or to be discernible through thinner parts of the . In contrast, nimbostratus is markedly thicker, often 2 to 8 km vertically, and produces steady, continuous such as or that diffuses the layer, completely obscuring the sun throughout its extent. This difference in opacity and precipitation arises from nimbostratus's greater accumulation of and particles, resulting in a higher liquid water path than in altostratus. The transition from altostratus to nimbostratus occurs as the initial layer deepens and lowers due to sustained atmospheric lift, often associated with frontal systems where warm or occluded fronts force prolonged ascent. Both types share overlap in their base heights, with altostratus usually forming between 2 and 7 km but extending downward, and nimbostratus bases frequently lowering to below 2 km while remaining mid-level in origin. This shared vertical range facilitates the maturation process, where altostratus's fibrous or uniform structure evolves into nimbostratus's more amorphous, dark gray form without distinct features. In terms of weather implications, altostratus often signals the approach of a system, providing a precursor to with its uniform gray coverage and potential for light or , but without persistent fallout. Nimbostratus, however, marks the active phase, delivering widespread, steady over extended periods and frequently accompanied by low, ragged clouds below the main layer. This progression underscores their roles in synoptic-scale , particularly in common frontal formations where altostratus precedes the intensifying of nimbostratus.

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