Altostratus cloud

upright=1.6|thumb|right|alt=An altostratus radiatus cloud showing the characteristic parallel lines of cloud.|Altostratus radiatus cloud showing distinctive parallel bands

Altostratus are middle-altitude clouds that develop horizontally and have a flat and uniform texture in the mid levels. As a middle-altitude cloud type, they can be made up of water droplets (including supercooled droplets), ice crystals, or a mixture of the two.

Altostratus clouds usually appear as 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 can produce virga, causing the cloud base to appear hazy. Consistent rainfall and lowering of the cloud base causes altostratus to become nimbostratus.

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 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 clouds, both of which are mid-level clouds, This constitutes roughly one third of the Earth's total cloud cover. 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. 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 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.

Microphysical properties

Altostratus can be composed of water droplets, supercooled water droplets, and ice crystals, 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, 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. 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

thumb|right|upright=2.2|alt=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. These conditions usually happen at the leading edge of a warm front, where cirrostratus clouds thicken and lower until they transition into altostratus clouds.

Altostratus clouds are mid-level clouds

Use in forecasting

Altostratus clouds tend to form ahead of warm fronts or occluded fronts and herald their arrival. As the frontal system approaches, cirrostratus clouds will thicken into altostratus clouds, which then gradually thicken further into nimbostratus clouds.

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 and refraction 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

thumb|left|upright=1.6|alt=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 . 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. or as a striated sheet. 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, often called "scud". Altostratus clouds, because they tend to form from warm fronts, and thus are not usually preceded by other types of clouds. Nimbostratus has no species or varieties. Like altostratus, nimbostratus clouds can be made of ice crystals, supercooled water droplets, or water droplets.

Gallery

File:Altostratus_pannus_nimbostratomutatus.jpg|alt=Altostratus pannus nimbostratomutatus|Altostratus pannus nimbostratomutatus

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Notes

Sources

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