NCERT EXERCISES
1. Multiple choice questions.
(i) Which one of the following is
the most important constituent of the atmosphere for human beings?
(a) Water vapour (b) Nitrogen (c) Dust particle (d) Oxygen
Ans. (d) Oxygen
(ii) Which one of the following
process is responsible for transforming liquid into vapour?
(a) Condensation (b) Transpiration (c) Evaporation (d)
Precipitation
Ans. (c) Evaporation
(iii) The air that contains
moisture to its full capacity:
(a) Relative humidity (b) Specific
humidity
(c) Absolute humidity (d) Saturated air
Ans. (d) Saturated air
Note - Saturated air has a
relative humidity of 100%.
(iv) Which one of the following is
the highest cloud in the sky?
(a) Cirrus (b) Stratus (c) Nimbus (d) Cumulus
Ans. (a) Cirrus
2. Answer the following questions in
about 30 words.
(i) Name the three types of
precipitation.
Ans. Precipitation - The process of continuous
condensation in free air helps the condensed particles to grow in size. When
the resistance of the air fails to hold them against the force of gravity, they
fall on to the earth’s surface. So after the condensation of water vapour, the
release of moisture is known as precipitation. This may take place
in liquid or solid form.
1. The precipitation in the
form of water is called rainfall,
2. When the temperature is
lower than the 0°C, precipitation takes place in the form of fine
flakes of snow and is called snowfall.
Besides rain and snow,
other forms of precipitation are sleet and hail.
3. When a layer of air with
the temperature above freezing point overlies a subfreezing layer near the
ground, precipitation takes place in the form of sleet. Sleet is
frozen raindrops and refrozen melted snow-water.
4. Sometimes, drops of rain
after being released by the clouds become solidified into small rounded solid
pieces of ice and which reach the surface of the earth are called hailstones.
These are formed by the rainwater passing through the colder layers. Hailstones
have several concentric layers of ice one over the other.
(ii) Explain relative humidity.
Ans. The ability of the air to
hold water vapour depends entirely on its temperature. The percentage of
moisture present in the atmosphere as compared to its full capacity at a given
temperature is known as the relative humidity. With the change of
air temperature, the capacity to retain moisture increases or decreases and the
relative humidity is also affected. It is greater over the oceans and least
over the continents.
(iii) Why does the amount of water
vapour decreases rapidly with altitude?
Ans.
(a)
The amount of water vapour in the atmosphere is added or withdrawn
due to evaporation and condensation respectively.
(b)
Evaporation is a process by which
water is transformed from liquid to gaseous state.
(c)
Heat is the main cause for evaporation.
(d)
The temperature at which the water starts evaporating is referred
to as the latent heat of vapourisation.
(e)
Increase in temperature increases water absorption and retention
capacity of the given parcel of air. Similarly, if the moisture content is low,
air has a potentiality of absorbing and retaining moisture. Movement of air
replaces the saturated layer with the unsaturated layer. Hence, the greater the
movement of air, the greater is the evaporation.
(f)
As altitude increase the movement of air decreases and as a result
the amount of water vapour decreases rapidly with altitude.
(iv) How are clouds formed?
Classify them.
Ans. 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.
According to their height,
expanse, density and transparency or opaqueness clouds are grouped under four
types:
(i) Cirrus (ii) Cumulus (iii) Stratus
(iv) Nimbus.
1. Cirrus - 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.
2. Cumulus - Cumulus clouds look like cotton
wool. 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. They have a flat base.
3. Stratus - 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.
4. Nimbus - Nimbus clouds are black or
dark gray. 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.
3. Answer the following questions in
about 150 words.
(i) Discuss the salient features
of the world distribution of precipitation.
Ans.
1. Different places on the
earth’s surface receive different amounts of rainfall in a year and that too in
different seasons.
2. In general, as we proceed
from the equator towards the poles, rainfall goes on decreasing steadily.
3. The coastal areas of the
world receive greater amounts of rainfall than the interior of the continents.
4. The rainfall is more over
the oceans than on the landmasses of the world because of being great sources
of water.
5. Between the latitudes 35° and
40° N and S of the equator, the rain is heavier on the eastern
coasts and goes on decreasing towards the west.
6. But, between 45° and
65° N and S of equator, due to the westerlies, the rainfall is
first received on the western margins of the continents and it goes on
decreasing towards the east.
7. Wherever mountains run
parallel to the coast, the rain is greater on the coastal plain, on the
windward side and it decreases towards the leeward side.
On the basis of the total amount
of annual precipitation, major precipitation regimes of the world are
identified as follows: -
(a)
The equatorial belt, the windward slopes of the
mountains along the western coasts in the cool temperate zone and the coastal
areas of the monsoon land receive heavy rainfall of over 200 cm per annum.
(b)
Interior continental areas receive moderate rainfall varying
from 100 - 200 cm per annum.
(c)
The coastal areas of the continents receive
moderate amount of rainfall.
(d)
The central parts of the tropical land and the eastern and
interior parts of the temperate lands receive rainfall varying between 50-100
cm per annum.
(e)
Areas lying in the rain shadow zone of the interior of the
continents and high latitudes receive very low rainfall-less than 50 cm per
annum.
(f)
Seasonal distribution of rainfall provides an important aspect to
judge its effectiveness. In some regions rainfall is distributed evenly
throughout the year such as in the equatorial belt and in the western parts of
cool temperate regions.
(ii) What are forms of condensation? Describe the process of dew and frost formation.
Ans. The transformation of
water vapour into water is called condensation. Condensation is caused
by the loss of heat. When moist air is cooled, it may reach a level when its
capacity to hold water vapour ceases. Then, the excess water vapour condenses
into liquid form. If it directly condenses into solid form, it is known as sublimation.
In free air, condensation results
from cooling around very small particles termed as hygroscopic condensation
nuclei. Particles of dust, smoke and salt from the ocean are particularly good
nuclei because they absorb water. Condensation also takes place when the moist
air comes in contact with some colder object and it may also take place when
the temperature is close to the dew point. Condensation, therefore, depends
upon the amount of cooling and the relative humidity of the air.
Condensation is influenced by the volume of air, temperature, pressure and
humidity.
Condensation takes place:
(i)
When the temperature of the air is reduced to dew point with its
volume remaining constant;
(ii)
When both the volume and the temperature are reduced;
(iii)
When moisture is added to the air through evaporation.
(iv)
However, the most favourable condition for condensation is the
decrease in air temperature.
After condensation the water
vapour or the moisture in the atmosphere takes one of the following forms — dew,
frost, fog and clouds.
Forms of condensation can
be classified on the basis of temperature and location. Condensation takes place
when the dew point is lower than the freezing point as well as higher than the
freezing point.
(A) Dew - When the moisture is
deposited in the form of water droplets on cooler surfaces of solid objects
(rather than nuclei in air above the surface) such as stones, grass blades and
plant leaves, it is known as dew. The ideal conditions for its
formation are clear sky, calm air, high relative humidity, and cold and long
nights. For the formation of dew, it is necessary that the dew point is
above the freezing point.
(B)
Frost - Frost forms on cold surfaces when condensation takes place below
freezing point (0°C), i.e. the dew point is at or below the freezing point. The
excess moisture is deposited in the form of minute ice crystals instead of
water droplets. The ideal conditions for the formation of white frost are the
same as those for the formation of dew, except that the air temperature must be
at or below the freezing point.
(C) Fog and Mist - When the temperature of
an air mass containing a large quantity of water vapour falls all of a sudden,
condensation takes place within itself on fine dust particles. So, the fog is a
cloud with its base at or very near to the ground. Because of the fog and mist,
the visibility becomes poor to zero. In urban and industrial centres smoke
provides plenty of nuclei which help the formation of fog and mist. Such a
condition when fog is mixed with smoke, is described as smog. The
only difference between the mist and fog is that mist contains more moisture
than the fog. In mist each nuceli contains a thicker layer of moisture. Mists
are frequent over mountains as the rising warm air up the slopes meets a cold
surface. Fogs are drier than mist and they are prevalent where warm currents
of air come in contact with cold currents. Fogs are mini clouds in which
condensation takes place around nuclei provided by the dust, smoke, and the
salt particles.