Actinoform cloud
Encyclopedia
An actinoform or actiniform cloud
is a collection of marine low clouds that takes a distinct shape. They are named after the Greek
word for "ray" due to their radial structure. Actinoform clouds can spread out over 300 kilometres (186.4 mi) across and thus cannot be easily seen with the naked eye. In addition, actinoform clouds can form "trains" that are up to six times the length of the original cloud field, yet they maintain their own, distinct identity. In a satellite image, they look like distinct leaf-like or spokes-on-a-wheel patterns that stand out from the rest of the low-lying cloud field. However, why they have this shape or how they are formed is not known, but recent evidence suggests that the interaction of both radiation and precipitation may help to organize them on the mesoscale
. The individual convective cells that collectively make up an actinoform cloud are quite shallow, with heights generally less than 2 km, and would be classified as stratocumulus clouds by an observer on the ground. Given the extensive mesoscale organization of these clouds, it is appropriate to describe them as mesoscale stratocumulus cloud systems (MSCS) in analogy with their deeper counterpart, the Mesoscale convective system
.
Actinoform clouds were discovered in the 1960s soon after the launch of the TIROS V
weather satellite
. But until the late 1990s, scientists had dismissed them as merely a relatively uncommon transitional form between open- and closed-cell formations of stratocumulus cloud
s. In fact, the term "actinoform" is not included in the 2000 edition of the Glossary of Meteorology, which is considered to be the comprehensive reference work
for meteorologist
s. However, these clouds are much more prevalent and complex than was originally thought.
Careful study of MISR
images of the western coast of Peru
revealed that actinoform-like clouds showed up roughly a quarter of the time as distinct formations within the more common, stratocumulus clouds in that region. Closer examination showed that actinoform clouds occur worldwide in nearly every region where marine stratus or stratocumulus clouds are common, particularly off the western coasts of continents—especially off Peru, Namibia
in Africa, Western Australia
, and Southern California
. Such cloud systems are persistent year-round off the coast, yet in certain seasons they blow ashore and create the "June Gloom
" effect on land. These cloud systems rarely form near the equator
.
The effect of actinoform clouds on weather
systems and climate
patterns is still being analyzed. Meteorology professor Bjorn Stevens of the University of California, Los Angeles
theorizes that they, in conjunction with open-cell stratocumulus clouds, are associated with drizzle
formation. Observations from field studies in both the Northeast and Southeast Pacific seem to indicate that when marine stratus and stratocumulus clouds exist alone, in the absence of these open cells, the cloud formations are associated with light, or no, drizzle. When the open cells are present—and likewise actinoform clouds—there seems to be a corresponding increase in drizzle. Thus, data from recent field studies suggest that the presence of "pockets of open cells" (POCs) and actinoform clouds is related to the onset of precipitation
. In fact, he discovered that the shape of the cloud field reorganizes itself when the clouds begin to rain.
These clouds have also been known to carry hail
.
Cloud
A cloud is a visible mass of liquid droplets or frozen crystals made of water and/or various chemicals suspended in the atmosphere above the surface of a planetary body. They are also known as aerosols. Clouds in Earth's atmosphere are studied in the cloud physics branch of meteorology...
is a collection of marine low clouds that takes a distinct shape. They are named after the Greek
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...
word for "ray" due to their radial structure. Actinoform clouds can spread out over 300 kilometres (186.4 mi) across and thus cannot be easily seen with the naked eye. In addition, actinoform clouds can form "trains" that are up to six times the length of the original cloud field, yet they maintain their own, distinct identity. In a satellite image, they look like distinct leaf-like or spokes-on-a-wheel patterns that stand out from the rest of the low-lying cloud field. However, why they have this shape or how they are formed is not known, but recent evidence suggests that the interaction of both radiation and precipitation may help to organize them on the mesoscale
Mesoscale meteorology
Mesoscale meteorology is the study of weather systems smaller than synoptic scale systems but larger than microscale and storm-scale cumulus systems. Horizontal dimensions generally range from around 5 kilometers to several hundred kilometers...
. The individual convective cells that collectively make up an actinoform cloud are quite shallow, with heights generally less than 2 km, and would be classified as stratocumulus clouds by an observer on the ground. Given the extensive mesoscale organization of these clouds, it is appropriate to describe them as mesoscale stratocumulus cloud systems (MSCS) in analogy with their deeper counterpart, the Mesoscale convective system
Mesoscale Convective System
A mesoscale convective system is a complex of thunderstorms that becomes organized on a scale larger than the individual thunderstorms but smaller than extratropical cyclones, and normally persists for several hours or more...
.
Actinoform clouds were discovered in the 1960s soon after the launch of the TIROS V
TIROS
TIROS, or Television Infrared Observation Satellite, is a series of early weather satellites launched by NASA, beginning with TIROS-1 in 1960. TIROS was the first satellite that was capable of remote sensing of the Earth. This initial remote-sensing effort was significant because it enabled Earth...
weather satellite
Weather satellite
The weather satellite is a type of satellite that is primarily used to monitor the weather and climate of the Earth. Satellites can be either polar orbiting, seeing the same swath of the Earth every 12 hours, or geostationary, hovering over the same spot on Earth by orbiting over the equator while...
. But until the late 1990s, scientists had dismissed them as merely a relatively uncommon transitional form between open- and closed-cell formations of stratocumulus cloud
Stratocumulus cloud
A Stratocumulus cloud belongs to a class of clouds characterized by large dark, rounded masses, usually in groups, lines, or waves, the individual elements being larger than those in altocumuli, and the whole being at a lower altitude, usually below 2,400 m...
s. In fact, the term "actinoform" is not included in the 2000 edition of the Glossary of Meteorology, which is considered to be the comprehensive reference work
Reference work
A reference work is a compendium of information, usually of a specific type, compiled in a book for ease of reference. That is, the information is intended to be quickly found when needed. Reference works are usually referred to for particular pieces of information, rather than read beginning to end...
for meteorologist
Meteorology
Meteorology is the interdisciplinary scientific study of the atmosphere. Studies in the field stretch back millennia, though significant progress in meteorology did not occur until the 18th century. The 19th century saw breakthroughs occur after observing networks developed across several countries...
s. However, these clouds are much more prevalent and complex than was originally thought.
Careful study of MISR
MISR
The Multi-angle Imaging SpectroRadiometer is a scientific instrument on the Terra satellite launched by NASA on December 18, 1999. This device is designed to measure the intensity of solar radiation reflected by the Earth system in various directions and spectral bands; it became operational in...
images of the western coast of Peru
Peru
Peru , officially the Republic of Peru , is a country in western South America. It is bordered on the north by Ecuador and Colombia, on the east by Brazil, on the southeast by Bolivia, on the south by Chile, and on the west by the Pacific Ocean....
revealed that actinoform-like clouds showed up roughly a quarter of the time as distinct formations within the more common, stratocumulus clouds in that region. Closer examination showed that actinoform clouds occur worldwide in nearly every region where marine stratus or stratocumulus clouds are common, particularly off the western coasts of continents—especially off Peru, Namibia
Namibia
Namibia, officially the Republic of Namibia , is a country in southern Africa whose western border is the Atlantic Ocean. It shares land borders with Angola and Zambia to the north, Botswana to the east and South Africa to the south and east. It gained independence from South Africa on 21 March...
in Africa, Western Australia
Western Australia
Western Australia is a state of Australia, occupying the entire western third of the Australian continent. It is bounded by the Indian Ocean to the north and west, the Great Australian Bight and Indian Ocean to the south, the Northern Territory to the north-east and South Australia to the south-east...
, and Southern California
Southern California
Southern California is a megaregion, or megapolitan area, in the southern area of the U.S. state of California. Large urban areas include Greater Los Angeles and Greater San Diego. The urban area stretches along the coast from Ventura through the Southland and Inland Empire to San Diego...
. Such cloud systems are persistent year-round off the coast, yet in certain seasons they blow ashore and create the "June Gloom
June Gloom
June Gloom is a southern California term for a weather pattern that results in cloudy, overcast skies with cool temperatures during the late spring and early summer. June Gloom in southern California is caused by the marine layer effect common to the West Coast, and is enhanced by the Catalina eddy...
" effect on land. These cloud systems rarely form near the equator
Equator
An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass....
.
The effect of actinoform clouds on weather
Weather
Weather is the state of the atmosphere, to the degree that it is hot or cold, wet or dry, calm or stormy, clear or cloudy. Most weather phenomena occur in the troposphere, just below the stratosphere. Weather refers, generally, to day-to-day temperature and precipitation activity, whereas climate...
systems and climate
Climate
Climate encompasses the statistics of temperature, humidity, atmospheric pressure, wind, rainfall, atmospheric particle count and other meteorological elemental measurements in a given region over long periods...
patterns is still being analyzed. Meteorology professor Bjorn Stevens of the University of California, Los Angeles
University of California, Los Angeles
The University of California, Los Angeles is a public research university located in the Westwood neighborhood of Los Angeles, California, USA. It was founded in 1919 as the "Southern Branch" of the University of California and is the second oldest of the ten campuses...
theorizes that they, in conjunction with open-cell stratocumulus clouds, are associated with drizzle
Drizzle
Drizzle is a light rain precipitation consisting of liquid water drops smaller than those of rain, and generally smaller than 0.5 mm in diameter. Drizzle is normally produced by low stratiform clouds and stratocumulus clouds. Precipitation rates due to drizzle are on the order of a millimetre...
formation. Observations from field studies in both the Northeast and Southeast Pacific seem to indicate that when marine stratus and stratocumulus clouds exist alone, in the absence of these open cells, the cloud formations are associated with light, or no, drizzle. When the open cells are present—and likewise actinoform clouds—there seems to be a corresponding increase in drizzle. Thus, data from recent field studies suggest that the presence of "pockets of open cells" (POCs) and actinoform clouds is related to the onset of precipitation
Precipitation (meteorology)
In meteorology, precipitation In meteorology, precipitation In meteorology, precipitation (also known as one of the classes of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric water vapor that falls under gravity. The main forms of precipitation...
. In fact, he discovered that the shape of the cloud field reorganizes itself when the clouds begin to rain.
These clouds have also been known to carry hail
Hail
Hail is a form of solid precipitation. It consists of balls or irregular lumps of ice, each of which is referred to as a hail stone. Hail stones on Earth consist mostly of water ice and measure between and in diameter, with the larger stones coming from severe thunderstorms...
.