Clouds

RELEVANCE – UPSC GS PRELIMS & GS MAINS – I (Salient features of world’s physical geography)

First go through the text (given after the questions) and then attempt the questions.

QUESTIONS ASKED IN UPSC EXAM

UPSC PRELIMS 2002 

QUES 1 . Sun’s halo is produced by the refraction of light in:

(a) water vapour in Stratus clouds
(b) ice crystals in Cirro-Cumulus clouds
(c) ice crystals in Cirrus clouds
(d) dust particles in Stratus clouds

Answer – c

PRACTICE QUESTIONS FOR PRELIMS

QUES 1 . Consider the following pairs :
Definition – Type of Cloud
1. A type of clouds which is the thunder cloud and
associated with heavy rain  –  Cumulonimbus
2. This cloud is made up of  tiny ice particles – Stratocumulus
3. A type of cloud generally appear as a low, lumpy
layer of clouds that is sometimes cirrus accompanied by weak intensity precipitation – Cirrus
Which of the above pairs is correctly matched?
(a) 1 only                    (b) 1 and 2
(c) 1, 2 and 3                (d) 1 and 3

Answer – a

QUES 2 . Which type of clouds have a sheetlike appearance and are composed of ice crystals?

(a) Stratocumulus Clouds
(b) Cirrostratus Clouds
(c) Nimbostratus Clouds
(d) Altostratus Clouds

Answer – b

Ques 3 . Which type of clouds tend to be in groups and have a light gray color?

(a) High level clouds
(b) Middle level clouds
(c) Low level clouds
(d) All of the above

Answer – b

QUES 4 . Which type of clouds cover the entire sky?

(a) Cirrus Clouds
(b) Altocumulus Clouds
(c) Nibostratus Clouds
(d) Altostratus Clouds

Answer – d

QUES 5 . The two ingredients needed to form clouds aloft are:
(a) Instability and lifting
(b) Lifting and saturated air
(c) Wind shear and Lifting
(d) Air with a high dewpoint and instability

Answer – b

PRACTICE QUESTION FOR MAINS

QUES . By giving the salient features of each type discuss about the various cloud types found in the atmosphere . 

What are clouds?

They are the visible aggregate of minute particles of water and/or ice. They form when water vapor condenses.

They can grow very tall or appear flat as a pancake. They are typically white in color but also appear in different shades of grey or in brilliant yellow, orange or red. They can weigh tens of millions of tons yet float in the atmosphere.

Formation of Clouds

There are two ingredients needed for clouds to become visible- water and nuclei.

Nuclei

In one form or another water is always present in the atmosphere. However, water molecules in the atmosphere are too small to bond together for the formation of cloud droplets. They need a “flatter” surface, an object with a radius of at least one micrometer on which they can form a bond. Those objects are called nuclei.

Nuclei are minute solid and liquid particles found in abundance. They consist of such things as smoke particles from fires or volcanoes, ocean spray or tiny specks of wind-blown soil. These nuclei are hygroscopic meaning they attract water molecules.

Called “cloud condensation nuclei”, these water molecule attracting particles are about 1/100th the size of a cloud droplet upon which water condenses.

Therefore, every cloud droplet has a speck of dirt, dust or salt crystal at its core. But, even with a condensation nuclei, the cloud droplet is essentially made up of pure water.

Temperature’s role

But having water attracting nuclei is not enough for a cloud to form as the air temperature needs to be below the saturation point. Called the dew point temperature, the point of saturation is where evaporation equals condensation.

Therefore, a cloud results when a block of air (called a parcel) containing water vapor has cooled below the point of saturation. Air can reach the point of saturation in a number of ways. The most common way is through lifting of air from the surface up into the atmosphere.

Why clouds appear and disappear?

The atmosphere is in constant motion. As air rises drier air is added (entrained) into the rising parcel so both condensation and evaporation are continually occurring. So cloud droplets are constantly forming and dissipating.

Clouds form and grow when there is more condensation on nuclei than evaporation from nuclei. Conversely, they dissipate if there is more evaporation than condensation. Thus clouds appear and disappear as well as constantly change shape.

The Four Core Types of Clouds
1 . Cirro-form

The Latin word ‘cirro’ means curl of hair. 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.

2 . Cumulo-form

Generally detached clouds, they look like white fluffy cotton balls. 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.

3 . Strato-form

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.

4 . Nimbo-form

‘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.

Ten Basic Clouds

From the World Meteorological Organization’s (WMO) International Cloud Atlas, the official worldwide standard for clouds, the following are  the ten basic cloud types.

I . High Level Clouds

Cirrus (Ci), Cirrocumulus (Cc), and Cirrostratus (Cs) are high level clouds. They are typically thin and white in appearance, but can appear in a magnificent array of colors when the sun is low on the horizon.

1 . Cirrus (Ci)

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.

2 . Cirrocumulus (Cc)

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.

3 . Cirrostratus (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.

II . Mid Level Clouds

Altocumulus (Ac), Altostratus (As), and Nimbostratus (Ns) are mid-level clouds. They are composed primarily of water droplets, however, they can also be composed of ice crystals when temperatures are low enough.

In Latin, alto means ‘high’ yet Altostratus and Altocumulus clouds are classified as mid-level clouds. ‘Alto’ is used to distinguish these “high-level” clouds and their low-level liquid-based counterpart clouds; Stratus and Cumulus.

4 . Altocumulus (Ac)

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 .

5 . Altostratus (As)

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.

6 . Nimbostratus (Ns)

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.

III . Low Level Clouds

Cumulus (Cu), Stratocumulus (Sc), Stratus (St), and Cumulonimbus (Cb) are low clouds composed of water droplets. Cumulonimbus, with its strong vertical updraft, extends well into the the high level of clouds.

7 . 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.

8 . 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.

9 . Stratocumulus (Sc)

Gray or whitish patch, sheet, or layered clouds which almost always have dark tessellations (honeycomb appearance), rounded masses or rolls.

10 . Stratus (St)

A generally gray cloud layer with a uniform base which may, if thick enough, produce drizzle, ice prisms, or snow grains. 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.

If the sky is blue, why are clouds white?

Unlike Rayleigh scattering, where the light waves are much smaller than the gases in the atmosphere, the individual water droplets that make up a cloud are of similar size to the wavelength of sunlight. When the droplets and light waves are of similar size, then a different scattering, called ‘Mie’ scattering, occurs.

Mie scattering does not differentiate individual wave length colors and therefore scatters all wave length colors the same. The result is equally scattered ‘white’ light from the sun and therefore we see white clouds.

Yet, clouds do not always appear white because haze and dust in the atmosphere can cause them to appear yellow, orange or red. And as clouds thicken, sunlight passing through the cloud will diminish or be blocked, giving the cloud a grey color. If there is no direct sunlight striking the cloud, it may reflect the color of the sky and appear bluish