Cloud is a mass of minute water droplets or tiny crystals of ice formed by the condensation of the water vapour in free air at considerable elevations.
As the clouds are formed at some height over the surface of the earth, they take various shapes.
Four basic cloud types
According to their height, expanse, density and transparency or opaqueness clouds are grouped under four types : (i) cirrus; (ii) cumulus; (iii) stratus; (iv) nimbus.
Cirrus
The Latin word ‘cirro’ means curl of hair.
Cirrus clouds are formed at high altitudes (8,000 – 12,000m).
They are thin and detatched clouds having a feathery appearance.
They are always white in colour.
It indicates fair weather and gives brilliant sun set.
Composed of ice crystals, cirro-form clouds are whitish and hair-like.
These are the high, wispy clouds to first appear in advance of a low pressure area such as a mid-latitude storm system or a tropical system such as a hurricane.
Cumulus
Generally detached clouds, they look like white fluffy cotton balls or cotton wool.
They show vertical motion or thermal uplift of air taking place in the atmosphere.
They are usually dense in appearance with sharp outlines.
The base of cumulus clouds are generally flat and occurs at the altitude where the moisture in rising air condenses.
They are generally formed at a height of 4,000 – 7,000 m.
They exist in patches and can be seen scattered here and there.
Stratus
From the Latin word for ‘layer’ these clouds are usually broad and fairly wide spread appearing like a blanket.
They result from non-convective rising air and tend to occur along and to the north of warm fronts.
The edges of strato-form clouds are diffuse.
As their name implies, these are layered clouds covering large portions of the sky.
These clouds are generally formed either due to loss of heat or the mixing of air masses with different
temperatures.
Nimbus
‘Nimbus’ is a Latin word for rain.
These are special rainy cloud category which combine the three forms: Cumulo + Cirro + Stratus.
The vast majority of precipitation occurs from nimbo-form clouds and therefore these clouds have the greatest vertical height.
Nimbus clouds are black or dark grey.
They form at middle levels or very near to the surface of the earth.
These are extremely dense and opaque to the rays of the sun.
Sometimes, the clouds are so low that they seem to touch the ground.
Nimbus clouds are shapeless masses of thick vapour.
Types of clouds according to height
A combination of the four basic types can give rise to the following types of clouds:
High clouds – cirrus, cirrostratus, cirrocumulus
Middle clouds – altostratus and altocumulus
Low clouds – stratocumulus and nimbostratus
Clouds with extensive vertical development – cumulus and cumulonimbus.
High clouds
Mainly cirrus (Ci) which are feathery form at 6 km above the ground.
High, thin clouds primarily transmit incoming solar radiation; at the same time, they trap some of the outgoing infrared radiation emitted by the Earth and radiate it back downward, thereby warming the surface of the Earth.
Cirrus (Ci)
Cirrus clouds are formed at high altitudes (8,000 – 12,000m).
They are thin and detatched clouds having a feathery appearance.
They are always white in colour.
It indicates fair weather and gives brilliant sun set.
Detached clouds in the form of white, delicate filaments, mostly white patches or narrow bands.
They may have a fibrous (hair-like) and/or silky sheen appearance.
Cirrus clouds are always composed of ice crystals, and their transparent character depends upon the degree of separation of the crystals.
As a rule, when these clouds cross the sun’s disk they hardly diminish its brightness.
When they are exceptionally thick they may veil its light and obliterate its contour.
Before sunrise and after sunset, cirrus is often colored bright yellow or red.
These clouds are lit up long before other clouds and fade out much later.
At all hours of the day Cirrus near the horizon is often of a yellowish color; this is due to distance and to the great thickness of air traversed by the rays of light.
Cirro Cumulus (Cc)
This appears as white globular masses, forming a mackerel sky.
Thin, white patch, sheet, or layer of clouds without shading.
They are composed of very small elements in the form of more or less regularly arranged grains or ripples.
In general, Cirrocumulus represents a degraded state of cirrus and cirrostratus, both of which may change into it and is an uncommon cloud.
There will be a connection with cirrus or cirrostratus and will show some characteristics of ice crystal clouds.
Cirro Stratus (Cs)
Transparent, whitish veil clouds with a fibrous (hair-like) or smooth appearance.
A sheet of cirrostratus which is very extensive, nearly always ends by covering the whole sky.
During the day, when the sun is sufficiently high above the horizon, the sheet is never thick enough to prevent shadows of objects on the ground.
A milky veil of fog (or thin Stratus) is distinguished from a veil of Cirrostratus of a similar appearance by the halo phenomena which the sun or the moon nearly always produces in a layer of Cirrostratus.
This resembles a thin white sheet.
The sky looks milky and the sun and moon shines through this clouds and form a ‘halo’.
Middle Clouds
Mainly Alto (Alt) clouds at 2 km to 6 km above the ground.
Altocumulus (Alt-Cu)
These are woolly, bumpy clouds arranged in layers appearing like waves in the blue sky.
White and/or gray patch, sheet or layered clouds, generally composed of laminae (plates), rounded masses or rolls.
They may be partly fibrous or diffuse and may or may not be merged.
When the edge or a thin semitransparent patch of altocumulus passes in front of the sun or moon, a corona appears. This colored ring has red on the outside and blue inside .
They indicate fine weather.
Altostratus (Alt-St)
These are denser and have watery look.
Gray or bluish cloud sheets or layers of striated or fibrous clouds that totally or partially covers the sky.
They are thin enough to regularly reveal the sun as if seen through ground glass.
Altostratus clouds do not produce a halo phenomenon nor are the shadows of objects on the ground visible.
Low Clouds
Mainly Stratus or sheet clouds below 2 km height.
Low, thick clouds primarily reflect solar radiation and cool the surface of the Earth.
Stratocumulus (St-Cu)
Gray or whitish patch, sheet, or layered clouds which almost always have dark tessellations (honeycomb appearance), rounded masses or rolls.
This is rough and bumpy clouds with wavy structure.
Stratus (St)
This is very low cloud, uniformly grey and thick, appears like highland fog.
It brings dull weather and light drizzle.
It reduces the visibility and is a hindrance to air transportation.
When the sun is visible through this cloud, its outline is clearly discernible.
Often when a layer of Stratus breaks up and dissipates blue sky is seen.
Sometimes appearing as ragged sheets Stratus clouds do not produce a halo phenomenon except, occasionally at very low temperatures.
Nimbostratus (Ni-St)
This is dark dull cloud, clearly layered, as it brings rain, snow and sleet and it is called as rainy cloud.
Resulting from thickening Altostratus, this is a dark gray cloud layer diffused by falling rain or snow.
It is thick enough throughout to blot out the sun.
Also, low, ragged clouds frequently occur beneath this cloud which sometimes merges with its base.
The cloud base lowers as precipitation continues.
Because of the lowering base it is often erroneously called a low-level cloud.
Both Altostratus and Nimbostratus can extend into the high level of clouds.
Clouds with extensive vertical development
Cumulonimbus (Cb)
The thunderstorm cloud, this is a heavy and dense cloud in the form of a mountain or huge tower.
The upper portion is usually smoothed, fibrous or striated and nearly always flattened in the shape of an anvil or vast plume.
Under the base of this cloud which is often very dark, there are often low ragged clouds that may or may not merge with the base.
They produce precipitation, which sometimes is in the form of virga.
Cumulonimbus clouds also produce hail and tornadoes.
Cumulus (Cu)
Detached, generally dense clouds and with sharp outlines that develop vertically in the form of rising mounds, domes or towers with bulging upper parts often resembling a cauliflower.
The sunlit parts of these clouds are mostly brilliant white while their bases are relatively dark and horizontal.
Over land cumulus develops on days of clear skies, and is due to diurnal convection .
It appears in the morning, grows, and then more or less dissolves again toward evening.
PRACTICE QUESTIONS
QUES . Which one of the following statements is correct? UPSC 2004
(a) Cirrus clouds are composed of ice crystals.
(b) Cirrus clouds exhibit a flat base and have the appearance of rising domes.
(c) Cumulus clouds are white and thin and form delicate patches and give a fibrous and feathery appearance.
(d) Cumulus clouds are classified as high clouds.
(a) Cirrus clouds look fibrous and appear like wisps in the blue sky, and composed of ice crystals. Cumulus clouds are with a round top and horizontal base, and these are classified as clouds with no definite height.
QUES . Consider the following statements: UPSC 2022
1. High clouds primarily reflect solar radiation and cool the surface of the Earth.
2. Low clouds have a high absorption of infrared radiation emanating from the Earth’s surface and thus cause warming effect.
Which of the statements given above is/are correct?
(a) 1only
(b) 2 only
(c) Both 1 and 2
(d) Neither 1 nor 2
(d)
QUES . Sun’s halo is produced by the refraction of light in: UPSC 2002
(a) Water vapour, in Stratus clouds
(b) Ice crystals in Cirro-Cumulus clouds
(c) Ice crystals in Cirro stratus clouds
(d) Dust particles in Stratus clouds
(c)
QUES . Match List-I with List-II and select the correct answer using the code given below the Lists : CDS 2018
List-I : List-II
(Cloud) : (Characteristic)
A. Cirrus : 1 . Rain-giving
B. Stratus : 2 . Feathery appearance
C. Nimbus : 3 . Vertically growing
D. Cumulus : 4 . Horizontally spreading
Code :
(a) A B C D
3 1 4 2
(b) A B C D
2 1 4 3
(c) A B C D
2 4 1 3
(d) A B C D
3 4 1 2
(c)
QUES . Which one of the following clouds is a rain-bearing cloud? NDA 2021
(a) Cumulus cloud
(b) Stratus cloud
(c) Nimbus cloud
(d) Cirrus cloud
(c)
QUES . Match List I with List II and select the correct answer using the code given below the Lists :
List I : List II
(Cloud Form) : (Characteristic)
A. Cirrus : 1 . Storm clouds
B. Cumulus : 2 . High- fog “Low Ceiling””
C. Nimbostratus : 3 . Fair-weather clouds
D. Stratus : 4 . Entirely of ice particles
(a) A – 2, B – 1, C – 3, D – 4
(b) A – 2, B – 3, C – 1, D – 4
(c) A – 4, B – 3, C – 1, D – 2
(d) A – 4, B – 1, C – 3, D – 2
(c)
QUES . Which one of sets are the low clouds ?
(a) Altostratus, Altocumulus and Nimbostratus
(b) Cirrus, Cirrostratus and Cirrocumulus
(c) Stratus, Nimbus and Cumulus
(d) Cirrus, Cirrostratus and Altostratus
(c)
