Altostratus translucidus over Hong Kong, China on May 13, 2012.
|Variety||alto and altostratus clouds alto cirrus|
|Appearance||Sheet or layer, can usually see the sun through it|
|Precipitation cloud?||Rain possible in thickened clouds. Classification is changed to nimbostratus if rain becomes persistent.|
Altostratus is a middle altitude cloud genus belonging to the stratiform physical category characterized by a generally uniform gray to bluish-gray sheet or layer, lighter in color than nimbostratus and darker than high cirrostratus. The sun can be seen through thin altostratus, but thicker layers can be quite opaque.
Altostratus is formed by the lifting of a large mostly stable air mass that causes invisible water vapor to condense into cloud. It can produce light precipitation, often in the form of virga. If the precipitation increases in persistence and intensity, the altostratus cloud may thicken into nimbostratus.
Altostratus most often takes the form of a featureless sheet of cloud but can be wavy (undulatus) as a result of wind shear through the cloud. It can also be fragmented (fibratus) with clear sky visible, which often signals the approach of a weakened or upper level warm front.
Altostratus may be composed of ice crystals. In some ice crystal altostratus, very thin, rapidly-disappearing horizontal sheets of water droplets appear at random. 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 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.
Altostratus is caused by a large stable air mass that is lifted until water vapor is condensed, usually by an incoming frontal system, and can be found over widespread areas. Their altitude is from 2,400 to 6,100 metres (7,900 to 20,000 ft). They are primarily composed of water droplets. They can rise at velocities up to 0.05 to 0.1 metres per second (0.16 to 0.33 ft/s). They can also be the result of the thickening of a cirrostratus cloud. Altostratus is rarely seen alone in the sky, as there is usually some accompanying stratocumulus or stratus fractus underneath it, this being due to the increased moisture beneath the altostratus layer or the process of cumulus clouds dying as the altostratus reduces surface heating by the Sun. More rarely, under unstable or very warm conditions at a warm or occluded front some cumuliform clouds may remain large enough under the altostratus to give rise to showers, and this can be a sign that cumulonimbi may be embedded within the nimbostratus that usually follows the altostratus, meaning that the usual frontal rain may be accompanied by occasional torrential downpours and thunderstorms. The altostratus and cumulus congestus combination is more common ahead of an advancing cold front.
- Palmer, Chad (16 October 2005). "Stratus Clouds". Weather. USA Today. Retrieved 20 April 2011.
- "The cloud classification system - Altostratus". Archived from the original on 26 April 2009. Retrieved 26 May 2009.
- "Alto Clouds". Web Weather for Kids. University Corporation for Atmospheric Research. Retrieved 20 April 2011.
- Platt 1977, p. 344
- Field 1999, p. 1929
- Field 1999, p. 1933
- IUPAC 1997
- "Common Cloud Names, Shapes, and Altitudes" (PDF). Georgia Tech. p. 8. Archived from the original on 19 July 2011. Retrieved 10 July 2011.
- Field, Paul R. (June 1998). "Aircraft Observations of Ice Crystal Evolution in an Altostratus Cloud". Journal of Applied Meteorology (American Meteorological Society) 56: 1925 1941. Bibcode:1999JAtS...56.1925F. doi:10.1175/1520-0469(1999)056<1925:AOOICE>2.0.CO;2. ISSN 1520-0469. Retrieved 20 April 2011.
- "Altostratus Cloud (in Atmospheric Chemistry)" (PDF). IUPAC Compendium of Chemical Terminology (2 ed.). International Union of Pure and Applied Chemistry (IUPAC). 1997. Retrieved 20 April 2011.
- Platt, C. M. R. (April 1977). "Lidar Observation of a Mixed-Phase Altostratus Cloud". Journal of Applied Meteorology (American Meteorological Society) 16 (4): 339 345. Bibcode:1977JApMe..16..339P. doi:10.1175/1520-0450(1977)016<0339:LOOAMP>2.0.CO;2. ISSN 0021-8952. Retrieved 20 April 2011.
|Wikimedia Commons has media related to: Altostratus clouds|
- Clouds-Online.com Cloud Atlas with many photos and description of the different cloud genus
- National Science Digital Library - Altostratus