An actinoform or actiniform describes a collection of marine low

cloud In meteorology, a cloud is an aerosol consisting of a visible mass of miniature liquid droplets, frozen crystals, or other particles suspended in the atmosphere of a planetary body or similar space. Water or various other chemicals may ...

s that takes a distinct shape. They are named after the

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word for "ray" due to their radial structure. Actinoform clouds can spread out over across and thus cannot be easily seen with the

naked eye Naked eye, also called bare eye or unaided eye, is the practice of engaging in visual perception unaided by a magnifying, light-collecting optical instrument, such as a telescope or microscope, or eye protection. Vision corrected to norma ...

. 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.

Description

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 , 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 marine stratocumulus cloud systems (MSCS) in analogy with their deeper counterpart, the

Mesoscale convective system A mesoscale convective system (MCS) 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. A mesoscale con ...

History and climatology

Actinoform clouds were discovered in the 1960s soon after the launch of the TIROS V

weather satellite A weather satellite or meteorological satellite is a type of Earth observation satellite that is primarily used to monitor the weather and climate of the Earth. Satellites can be polar orbiting (covering the entire Earth asynchronously), or ...

. 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 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 ...

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 A reference work is a work, such as a paper, book or periodical (or their electronic equivalents), to which one can refer for information. The information is intended to be found quickly when needed. Such works are usually ''referred'' to ...

for

meteorologist A meteorologist is a scientist who studies and works in the field of meteorology aiming to understand or predict Earth's atmospheric phenomena including the weather. Those who study meteorological phenomena are meteorologists in research, while t ...

s. However, these clouds are much more prevalent and complex than was originally thought. Careful study of

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images of the western coast of

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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,

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in Africa,

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, and

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. 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 equator is a circle of latitude, about in circumference, that divides Earth into the Northern and Southern hemispheres. It is an imaginary line located at 0 degrees latitude, halfway between the North and South poles. The term can also ...

Meteorological effects

The effect of actinoform clouds on

weather Weather is the state of the atmosphere, describing for example the degree to which it is hot or cold, wet or dry, calm or stormy, clear or cloudy. On Earth, most weather phenomena occur in the lowest layer of the planet's atmosphere, the ...

systems and

climate Climate is the long-term weather pattern in an area, typically averaged over 30 years. More rigorously, it is the mean and variability of meteorological variables over a time spanning from months to millions of years. Some of the meteorologi ...

patterns is still being analyzed. Meteorology professor Bjorn Stevens of the

University of California, Los Angeles The University of California, Los Angeles (UCLA) is a public land-grant research university in Los Angeles, California. UCLA's academic roots were established in 1881 as a teachers college then known as the southern branch of the Californ ...

theorizes that they, in conjunction with open-cell stratocumulus clouds, are associated with

drizzle Drizzle is a light precipitation consisting of liquid water drops smaller than those of rain – generally smaller than in diameter. Drizzle is normally produced by low stratiform clouds and stratocumulus clouds. Precipitation rates from dri ...

formation. Observations from field studies in both the northeastern and southeastern 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 meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. ...

. In fact, professor Stevens discovered that the shape of the cloud field reorganizes itself when the clouds begin to rain.

Gallery

Image:Actinoform cloud.jpg, Actinoform cloud over the eastern Pacific Ocean. Image:Actinoform OpenCell Clouds MODIS 30sep05.jpg, Here, an actinoform cloud can be seen in the center, while an open-cell formation of stratocumulus clouds is at the center top. Image:TIROS 5 radial clouds monthly review (April 1965).jpg, One of the earliest images of an actinoform cloud. Taken by TIROS 5 satellite in 1965. Image:Open- and Closed-Cell Clouds over the Pacific Ocean.jpg, Open and closed-cell clouds over the

Pacific Ocean The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the conti ...

References

* * * *

External links

Actiniform cloud system in the Southeast Pacific
video at YouTube {{Cloud types Cloud types fr:Stratocumulus maritime#Type actinoforme