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"Altostratus is a middle altitude cloud genus belonging to the stratiform physical category characterized by a generally uniform gray to bluish-green sheet or layer. It is 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 undulatus 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 precedes the approach of a weakened or upper level warm front. Description Altostratus can 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. Occurrence Altostratus tends to form ahead of a warm or occluded front, the altostratus gradually thickening into rain bearing nimbostratus. However it can also occur together with cumulus congestus or cumulonimbus at a cold front. Subtypes :*Species: Altostratus is not divided into species because it is always nebulous or featureless in structure. ::*Opacity-based varieties: Altocumulus translucidus is relatively thin so that the sun or moon is always visible as if seen through frosted glass. It is strictly a middle cloud and is coded CM1 in the SYNOP report. The opacus variety is sufficiently thick to obscure the sun or moon and can extend vertically into the high étage. It is therefore sometimes classified as vertical or multi-level, but is still coded CM2 as a middle cloud. ::*Pattern-based varieties: Radiatus, duplicatus, and undulatus are all occasionally associated with altostratus. :::*Precipitation- based supplementary features: Altostratus opacus can be thick enough to produce both virga or precipitation features. :::*Cloud-based supplementary feature: Mamma may be occasionally seen with altostratus, especially if it is associated with or changing to or from altocumulus. :::*Accessory cloud: Pannus forming in precipitation is sometimes seen with altostratus and is coded CL7\. :::*Genitus mother clouds: Altostratus altocumulogenitus forms due to the fusing of altocumulus elements. Altostratus cumulonimbogenitus results from the spreading of the middle part of a cumulonimbus cloud. :::*Mutatus mother clouds: Cirrostratus can thicken into altostratus cirrostratomutatus, usually ahead of an approaching disturbance. Nimbostratus associated with an organized weather system may lift and change into altostratus nimbostratomutatus, especially if the disturbance is weakening or moving out of a particular area. "Velum" is considered a type of Cumulonimbus cloud so it is not listed. See also *List of cloud types Sources ; Footnotes ; Bibliography External links *Clouds-Online.com Cloud Atlas with many photos and description of the different cloud genus *National Science Digital Library - Altostratus Category:Stratus "
"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 whole being at a lower height, usually below . Weak convective currents create shallow cloud layers because of drier, stable air above preventing continued vertical development. Historically, in English, this type of cloud has been referred to as a twain cloud for being a combination of two types of clouds. Description=Occurrence= Vast areas of subtropical and polar oceans are covered with massive sheets of stratocumulus. These may organize into distinctive patterns which are currently under active study. In subtropics, they cover the edges of the horse latitude climatological highs, and reduce the amount of solar energy absorbed in the ocean. When these drift over land the summer heat or winter cold is reduced. 'Dull weather' is a common expression incorporated with overcast stratocumulus days, which usually occur either in a warm sector between a warm and cold front in a depression, or in an area of high pressure, in the latter case, sometimes persisting over a specific area for several days. If the air over land is moist and hot enough, stratocumulus may develop to various cumulus clouds, or, more commonly, the sheet of stratocumulus may become thick enough to produce some light rain. On drier areas they quickly dissipate over land, resembling cumulus humilis. This often occurs in late morning in areas under anticyclonic weather, the stratocumulus breaking up under the sun's heat and often reforming again by evening as the heat of the sun decreases again. =Precipitation= Most often, stratocumulus produce no precipitation, and when they do, it is generally only light rain or snow. However, these clouds are often seen at either the front or tail end of worse weather, so they may indicate storms to come, in the form of thunderheads or gusty winds. They are also often seen underneath the cirrostratus and altostratus sheets that often precede a warm front, as these higher clouds decrease the sun's heat and therefore convection, causing any cumulus clouds to spread out into stratocumulus clouds. =Comparison altocumulus= Stratocumulus clouds are same in appearance to altocumulus and are often mistaken for such. A simple test to distinguish these is to compare the size of individual masses or rolls: when pointing one's hand in the direction of the cloud, if the cloud is about the size of the thumb, it is altocumulus; if it is the size of one's fist, it is stratocumulus. This often does not apply when stratocumulus is of a broken, fractus form, when it may appear as small as altocumulus. Stratocumulus is also often, though not always, darker in colour than altocumulus. =Optical effects= Stratocumulus clouds are the main type of cloud that can produce crepuscular rays. Thin stratocumulus clouds are also often the cause of corona effects around the Moon at night. All stratocumulus subtypes are coded CL5 except when formed from free convective mother clouds (CL4) or when formed separately from co-existing (CL8). Species Stratocumulus stratiformis opacus in Pennsylvania Stratocumulus lenticularis clouds in Jackson, Wyoming Stratocumulus Stratiformis are extensive flat but slightly lumpy sheets that show only minimal convective activity. Stratocumulus Lenticularis are separate flat elongated seed-shaped clouds. They are typical for polar countries or warmer climate during winter seasons. They also can be formed by winds passing hills or mountains, such as Foehn winds, and in this case they can be very regularly shaped. Stratocumulus Castellanus have stronger convective activity due to the presence of increasingly unstable air. They are distinct from other stratocumulus by puffy tower-like formations atop the cloud layer. They look like cumulus congestus, but can be easily confused: "towers" of cumulus congestus grow above separate clouds, whereas in the case of stratocumulus castellanus, there is always a more or less defined layer of clouds. Stratocumulus castellanus may develop into cumulus congestus (and even further into cumulonimbus) under auspicious conditions. Any showers from stratocumulus castellanus are not usually as heavy as those from cumulus congestus. Opacity-based varieties Stratocumulus Opacus is a dark layer of clouds covering entire sky without any break. However, the cloud sheet is not completely uniform, so that separate cloud bases still can be seen. This is the main precipitating type, however any rain is usually light. If the cloud layer becomes grayer to the point when individual clouds cannot be distinguished, stratocumulus turn into stratus clouds. Stratocumulus Perlucidus is a layer of stratocumulus clouds with small spaces, appearing in irregular pattern, through which clear sky or higher clouds can be seen. Stratocumulus Translucidus consist of separate groups of stratocumulus clouds, with a clear sky (or higher clouds) visible between them. No precipitation in most cases. Pattern-based varieties Stratocumulus undulatus, seen from a plane. Stratocumulus Undulatus clouds appear as nearly parallel waves, rolls or separate elongated clouds, without significant vertical development. Stratocumulus Radiatus clouds appear as the same as stratocumulus undulatus, but stratocumulus undulatus move perpendicular to the wind shear, while stratocumulus radiatus move parallel to the wind shear. Stratocumulus Duplicatus clouds appear as stratocumulus clouds with two or more layers or sheets. Stratocumulus duplicatus is common on species lenticularis or lenticular cloud. Stratocumulus Lacunosus clouds are very uncommon. They only occur when there are localized downdrafts striking through the stratocumuliform cloud. Supplementary feature Stratocumulus Mamma is a type of mammatus cloud. Precipitation-based supplementary features Stratocumulus Virga is a form of precipitation that evaporates in mid-air and doesn't reach the ground. Stratocumulus Praecipitatio is a form of precipitation that reaches the ground as light rain or snow. Mother clouds Stratocumulus Cumulomutatus the specific type of stratocumulus clouds, are flat and elongated. They form in the evening, when updrafts caused by convection decrease making cumulus clouds lose vertical development and spread horizontally. They also can occur under altostratus cloud preceding a warm or occluded front, when cumulus usually lose vertical development as the sun's heat decreases. Like all other forms of stratocumulus apart from castellanus, they are also often found in anticyclones. Stratocumulus Cumulogenitus out of cumulus or cumulonimbus clouds, disrupted by decreasing convection. During formation period, puffy tops of cumulus clouds can protrude from stratocumulus cumulogenitus for a relatively long time until they completely spread in horizontal direction. Stratocumulus cumulogenitus appear as lengthy sheet or as group of separate elongated cloud rolls or waves. Future A recent simulation has suggested that within a century, stratocumulus clouds may disappear, contributing to climate change. See also * Actinoform cloud References External links * Clouds-Online.com Cloud Atlas with many photos and description of the different cloud genus * National Science Digital Library – Stratocumulus * Muriel Martin Online – Stratocumulus Cloud * WW2010 University of Illinois – Stratocumulus Clouds * BBC Weather Centre – Types of Cloud – Stratocumulus and Altocumulus Category:Cumulus Category:Stratus "
"A nimbostratus cloud is a multi-level, gray, often dark, amorphous, nearly uniform cloud that usually produces continuous rain, snow, or sleet but no lightning or thunder.Cloud in the Columbia Electronic EncyclopediaNimbostratus in the American Heritage DictionaryNimbostratus in the Oxford Dictionaries Online Although it is usually a low-based cloud, it actually forms most commonly in the middle level of the troposphere and then spreads vertically into the low and high levels. Nimbostratus usually produces precipitation over a wide area. Nimbo- is from the Latin word nimbus, which denotes precipitation. Downward-growing nimbostratus can have the same vertical extent as most large upward-growing cumulus, but its horizontal extent tends to be even greater. Appearance Nimbostratus has a diffuse cloud base generally found anywhere from near surface in the low levels to about in the middle level of the troposphere. Although usually dark at its base, it often appears illuminated from within to a surface observer. Nimbostratus usually has a thickness of about 2000 to 4000 m. Though found worldwide, nimbostratus occurs more commonly in the middle latitudes. It is coded CM2 on the SYNOP report. Formation Nimbostratus occurs along a warm front or occluded front where the slowly rising warm air mass creates nimbostratus along with shallower stratus clouds producing less rain, these clouds being preceded by higher- level clouds such as cirrostratus and altostratus. Often, when an altostratus cloud thickens and descends into lower altitudes, it will become nimbostratus. Nimbostratus, unlike cumulonimbus, is not associated with thunderstorms, however at an unusually unstable warm front caused as a result of the advancing warm air being hot, humid and unstable, cumulonimbus clouds may be embedded within the usual nimbostratus. Lightning from an embedded cumulonimbus cloud may interact with the nimbostratus but only in the immediate area around it. In this situation with lightning and rain occurring it would be hard to tell which type of cloud was producing the rain from the ground, however cumulonimbus tend to produce larger droplets and more intense downpours. The occurrence of cumulonimbus and nimbostratus together is uncommon, and usually only nimbostratus is found at a warm front. Forecast Nimbostrati often have very few visual features. Nimbostratus is generally a sign of an approaching warm or occluded front producing steady moderate precipitation, as opposed to the shorter period of typically heavier precipitation released by a cold-frontal cumulonimbus cloud. Precipitation may last for several days, depending on the speed of the frontal system. A nimbostratus virga cloud is the same as normal nimbostratus, but the rain or snow falls as virga which doesn't reach the ground. Stratus or stratocumulus usually replace the nimbostratus after the passage of the warm or occluded front. Origin of name Under Luke Howard's first systematized study of clouds, carried out in France in 1802, three general cloud forms were established based on appearance and characteristics of formation: cirriform, cumuliform and stratiform. These were further divided into upper and lower types depending on altitude. In addition to these three main types, Howard added two names to designate multiple cloud types joined together: cumulostratus, a blending of cumulus clouds and stratus layers, and nimbus, a complex blending of cirriform, cumuliform, and stratiform clouds with sufficient vertical development to produce significant precipitation. Later, in the 20th century, an IMC commission for the study of clouds put forward a refined and more restricted definition of the genus nimbus, effectively reclassifying it as a stratiform cloud type. It was then renamed nimbostratus, and published with the new name in the 1932 edition of the International Atlas of Clouds and of States of the Sky. This left cumulonimbus as the only nimboform type as indicated by its root name. Subtypes and derivative types :*Species and varieties: Nimbostratus is very thick, opaque, and featureless, so this genus type is not subdivided into species or varieties. :::*Precipitation-based supplementary features: Nimbostratus is a major precipitation cloud and produces the virga or praecipitatio features. The latter can achieve heavy intensity due to the cloud's vertical depth. :::*Accessory cloud: Nimbostratus pannus is an accessory cloud of nimbostratus that forms as a ragged layer in precipitation below the main cloud deck. Pannus is coded CL7\. :::*Genitus mother clouds: This genus type can form from cumulus and cumulonimbus. :::*Mutatus mother clouds: Nimbostratus can form due to the complete transformation of altocumulus, altostratus and stratocumulus. Relation to other clouds Multi-level nimbostratus is physically related to other stratiform genus-types by way of being non-convective in nature. However, the other sheet-like clouds usually each occupy only one or two levels at the same time. Stratus clouds are low-level and form from near ground level to at all latitudes. In the middle level are the altostratus clouds that form from to in polar areas, in temperate areas, and in tropical areas. Although altostratus forms mostly in the middle level of the troposphere, strong frontal lift can push it into the lower part of the high- level. The main high-level stratiform cloud is cirrostratus which is composed of ice crystals that often produce halo effects around the sun. Cirrostratus forms at altitudes of in high latitudes, in temperate latitudes, and in low, tropical latitudes. Of the non-stratiform clouds, cumulonimbus and cumulus congestus are the most closely related to nimbostratus because of their vertical extent and ability to produce moderate to heavy precipitation. The remaining cumuliform (cumulus) and stratocumuliform (stratocumulus, altocumulus, and cirrocumulus) clouds have the least in common with nimbus. ReferencesExternal links * National Science Digital Library - Nimbostratus * Nimbostratus and Other Low Clouds Category:Stratus "