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Useful notes on the Classification of Clouds

The new International Cloud Atlas describes the 10 main cloud genera (families) which are subdivided into 14 species based on their shape and structure. Besides, 9 general varieties are also described which are based on their transparency and geometrical arrangement. The main cloud genera or families can be listed according to their heights as under:

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A. High (mean heights 5 to 13 km)

(1) Cirrus

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(2) Cirro-cumulus

(3) Cirro-stratus

B. Middle (mean heights 2 to 7 km)

(4) Alto-cumulus

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(5) Alto-stratus

(6) Nimbo-stratus

C. Low (mean heights 0 to 2 km)

(7) Strato-cumulus

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(8) Stratus

(9) Cumulus

(10) Cumulo-nimbus.

Usually alto-stratus, nimbo-stratus, cumulus and cumulo-nimbus clouds extend outside the above noted heights, which are only approximate for the temperate regions. Clouds of a given genus/family are generally lower in the Polar Regions and higher in the tropics.

Definitions of the following genera of clouds as given by the W.M.O. are being given here. Other information’s have also been given for the readers.

(1) Cirrus:

“Detached clouds in the form of white, delicate filaments or white or mostly white patches or narrow bands. These clouds have a fibrous (hair-like) appearance or a silky sheen or both”.

The prefix cirro refers to cloud forms at the same general level with different appearance. All the cirrus or cirro-type clouds are composed of ice crystals, they are all high clouds. The sun or moon shining through these clouds produces a halo. Cirrus clouds have brilliant colours at sunset and sunrise. These clouds do not give precipitation.

(2) Cirro-cumulus:

“Thin, white patch, sheet or layer of cloud without shading, composed of very small elements in the form of grains, ripples, etc. merged or separate, and more or less regularly arranged, most of the elements have an apparent width of less than one degree”.

This type of cloud is not common, and is often connected with cirrus or cirro-stratus. It looks like a patch of small flakes or small globules arranged in small groups or lines. When arranged uniformly, it forms a ‘mackerel sky’.

(3) Cirro-stratus:

“Transparent, whitish cloud veil of fibrous (hair like) or smooth appearance, totally or partly covering the sky, and generally producing halo phenomena”.

This type of cloud is so thin that it gives the sky a milky appearance. At times, it may form a definite sheet. Edge of the sheet is rarely straight, and is often marked by patches of cirrus or cirro-cumulus. It produces halos around the sun or moon. This type always occurs at great heights. Clouds of this type are formed of ice crystals. The sun is obscured so that objects on the ground do not cast shadows.

(4) Alto-cumulus:

“White or gray, or both white and gray, patch, sheet or layer of cloud, generally with shading, composed of laminate, rounded masses, rolls, etc., which are sometimes partly fibrous or diffuse and which may or may not be merged, most of the regularly arranged small elements usually have an apparent width of between one and five degrees”.

Alto-cumulus clouds do not produce halos. They have dark shading on their under-surfaces. There is complete absence of large domes. High globular altocumulus groups are sometimes referred to as ‘sheep clouds’ or ‘ woolpack clouds’. They are generally found in wavy or parallel bands. They are often composed of super-cooled liquid droplets. This type of cloud may occur at various levels simultaneously.

(5) Alto-stratus:

“Grayish or bluish cloud sheet or layer of striated, fibrous or uniform appearance, totally or partly covering the sky, and having parts thin enough to reveal the sun at least vaguely, as through ground glass. Alto-stratus does not show halo phenomena.”

Alto-stratus clouds may cover all or large portions of the sky. The sun may be totally obscured or is visible in hazy outline. Halos are never seen. Under an alto-stratus sheet shadows on the ground are never cast.

The sun or the moon may only appear as a bright spot behind the cloud. Clouds of this type also consist of water droplets, often super-cooled to temperatures well below freezing. Precipitation may fall either as fine drizzle or snow.

(6) Nimbo-stratus:

“Grey cloud layer, often dark, the appearance of which is rendered diffuse by more or less continuously falling rain or snow, which in most cases reaches the ground. It is thick enough throughout to blot out the sun”.

Nimbo-stratus is generally a low cloud form and may be thousands of feet thick. It is a rain, snow, or sleet cloud. It is never accompanied by lightning, thunder, or hail. It can be distinguished from the stratus type in that it is darker. Streaks of rain or snow falling from these clouds but not reaching the ground are called ‘virga’.

(7) Strato-cumulus:

“Grey or whitish, or both gray and whitish, patch, sheet or layer of cloud which almost always has dark parts, composed of tessellations, rounded masses, rolls, etc. which are non-fibrous (except for virga) and which may or may not be merged; most of the regularly arranged small elements have an apparent width of more than five degrees.”

Strato-cumulus is a low cloud layer consisting of large lumpy masses or rolls of dull gray colour with brighter interstices.

(8) Stratus:

“Generally gray cloud layer with a fairly uniform base, which may give drizzle, ice prisms, or snow grains. When the sun is visible through the cloud, its outline is clearly discernible. Stratus does not produce halo phenomena except, possibly, at very low temperatures. Sometimes stratus appears in the form of ragged patches.’

Looked at from above, stratus has a uniform top, which indicates a temperature inversion. Stratus clouds are without any particular form or structure. Sky may be completely covered by this type of cloud. They are frequently broken. It is difficult to differentiate between a high fog and stratus cloud. When stratus clouds are overlain by the higher altostratus, they become thicker and darker.

(9) Cumulus:

“Detached clouds, generally dense and with sharp outlines, developing vertically in the form of rising mounds, domes or towers, of which the bulging upper parts often resemble a cauliflower. The sunlit parts of these clouds are mostly brilliant white; their base is relatively dark and nearly horizontal. Sometimes cumulus is ragged”.

Cumulus clouds represent the tops of strong convective currents. Their flat bases, if extended toward each other, form a nearly perfect plane surface-the dew point level. Irregular patches of cumulus are called fractocumulus.

They may occur during any season. Cumulus is generally found in the day time over land areas. They dissipate at night. They produce only light precipitation. They often represent a transition to cumulo-nimbus, which is the heavier shower cloud. Air that is heated from below or cooled from above produces cumulus clouds.

(10) Cumulo-nimbus:

“Heavy and dense cloud, with a considerable vertical extent, in the form of a mountain or huge towers. At least part of its upper portion is usually smooth, or fibrous, or striated, and nearly always flattened; this part often spreads out in the shape of an anvil or vast plume.

Under the base of this cloud which is often very dark, there are frequently low ragged clouds either merged with it or not, and precipitation sometimes in the form of virga”.

Cumulo-nimbus is a towering cloud sometimes spreading out on top to form an ‘anvil head’. This type of cloud is associated with heavy rainfall, thunder, lightning, hail, and tornadoes. This cloud has a flat top (anvil head) and a flat base.

It appears darker as condensation within it increases, and it obstructs, the sun. It is the great thunderhead, which is the source of the squally, gusty, short-lived thunderstorms. Such thunderstorms are very common during summer afternoons in the middle and low latitudes. This type of cloud is easily recognised by the fall of a real shower and sudden darkening of the sky.

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