NCERT Class 11 Geography Chapter 11 Water in The Atmosphere

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NCERT Class 11 Geography Chapter 11 Water in The Atmosphere

Also, you can read the NCERT book online in these sections Solutions by Expert Teachers as per Central Board of Secondary Education (CBSE) Book guidelines. CBSE Class 11 Geography Solutions are part of All Subject Solutions. Here we have given NCERT Class 11 Geography Part I: Fundamentals of Physical Geography, Part II: Indian: Physical Environment, Part III: Practical Work in Geography. NCERT Class 11 Geography Chapter 11 Water in The Atmosphere Notes, NCERT Class 11 Geography Textbook Solutions for All Chapters, You can practice these here.

Water in The Atmosphere

Chapter: 11

GEOGRAPY [ PART – I ]

SHORT ANSWER TYPE QUESTIONS

Q.1. What is sublimation?

Ans. Ice on melting becomes liquid or sometimes even the water vapour through the process called sublimation.

Q.2. What is the greatest source of atmosphere? What do you mean by the term smog?

Ans. Smog is the combination of two wards (Smoke + Fog) very thick smokey fog is called smog. It is formed in industrial areas like Black country of Britain. It reduces visibility to 200 m.

Q.3. What is meant by humidity?

Ans. The water vapour present in the air is termed as humidity. It is a measure to express the presence of water vapour in the air.

Q.4. What factors control the evaporation?

Ans. 1. Aridity.

2. Temperature.

3. Movement of air.

Q.5. On the basis of origin describe the types of rain fall.

Ans. On the basis of origin rainfall may be classified into three main types:

(i) Conventional rainfall.

(ii) Orographic rainfall.

(iii) Cyclonic or Frontal rainfall.

Q.6. What are the different types of fog?

Ans. Fogs are of three types:

1. Radiating Fogs.

2. Advection Fogs.

3. Frostal Fogs.

Q.7. Name the temperature at which a given sample of air becomes saturated.

Ans. Dew point.

Q.8. Give a single term for the following:

(i) The process by which water is transformed from liquid to vapour.

Ans. Evaporation.

(ii) The amount of water vapour actually present in per unit volume of air.

Ans. Absolute humidity.

(iii) The amount of water vapour actually present in per unit weight of air.

Ans. Specific humidity.

(iv) Ratio of amount of water actually present in the air at a particular temperature to the amount of water vapour required to saturate the same air at the same temperature.

Ans. Relative humidity.

(v) The air that contains moisture to its full capacity.

Ans. Saturated air.

SHORT ANSWER TYPE QUESTIONS

Q.1. What is haze? How is it caused?

Ans. Haze is caused by smoke and dust particles in individual areas. It is caused to unequal refraction of light in air of different densities. It is caused in areas in areas of low humidity. It reduces the visibility to 2 kms.

Q.2. Describe the distribution of relative humidity.

Ans. Relative humidity is not evenly distributed over the earth’s surface, because it is affected by the intensity of solar radiation, the annual movement of the earth and seasons. Hence, it is distributed in two forms.

(i) Zonal. and 

(ii) Seasonal.

Zonal distribution of relative humidity:

Following are the characteristics of the zonal distribution of relative humidity:

1. It is the highest at the equator and decreases towards poles.

2. It is minimum in subtropical regions.

3. It increases from 30°, north and south towards poles, because of the decrease in temperature.

4. The belt of relative humidity shifts north- and south-wards with the annual movement of the sun.

Seasonal Distribution of Relative Humidity:

1. Between 30°N and 30°S latitudes, the relative humidity is higher in summer than in winter.

2. It is higher in the higher latitudes in winter than in summer.

Q.3. Explain relative humidity.

Ans. Relative humidity is the ratio of water vapour present in the air at a particular temperature to the amount of water vapour required to saturate the same air at the same temperature. It is always expressed in percentage. It is derived by the following simple formula:

Q.4. Difference between Condensation and Evaporation.

Ans. 

Condensation	Evaporation
1. Condensation is the process by which water vapour is charged to water.	1. Evaporation is the process by which water is changed to water vapour.
2. Cooling of air leads to condensation.	2. High temperature favour evaporation.
3. This change takes place from gasious to liquid condition.	3. This change take place from liquid to gaseous condition.

Q.5. Name the three types of precipitation.

Ans. Three types of precipitation are:

1. Convectional.

2. Orographic.

3. Cyclonic rainfall.

Q.6. Why does the amount of water vapour decreases rapidly with altitude?

Ans. There is absence of water on high altitudes. Temperature also goes on decreasing so the rates of evaporation decreases.

Q.7. What is rain shadow? Name one part of the Indian sub-continent that experiences rain shadow.

Ans. The leeward slope where the air descends and is dry is called rain shadow. Descending winds become warm and dry. There is no condensation. Deccan plateau lies in the rain shadow of Western Ghats.

Q.8. How are clouds formed? Classify them.

Ans. Cloud is a mass of minute droplets of water or tiny crystals of ice formed by the condensation of the water vapour in free air at a considerable elevation. As the clouds are formed at some height over the surface of the earth, they take various shapes. The clouds are classified as under according to their height, expanses, density and transparency.

1. Cirrus: These are formed at high altitudes (8,000-12,000m).

2. Cumulus: They are formed at a height of 4,000-7,000m. They look like cotton.

3. Stratus: They are layered clouds covering large portions of the sky.

4. Nimbus: These are black or dark grey rain clouds. They are very near to the earth’s surface.

Q. 9. What are the different types of humidity?

Ans. There are three types of humidity.

1. Absolute Humidity.

2. Specific Humidity. and

3. Relative Humidity.

1. Absolute humidity: The weight of actual amount of water vapour present in a unit volume of air is known as absolute humidity. It is usually expressed as grams per cubic metre of air. As ability of a masses of air to hold water vapour depends on temperature, so it changes from place to place and from time to time. Because of this very reason, it is not a reliable index as the change in temperature and pressure cause change in the volume of air and consequently the absolute humidity.

2. Specific humidity: The weight of water vapour per unit weight of air or the proportion of the mass of water-vapour to the total mass of air is known as specific humidity. As it is measured in units of weight, so it is not affected by changes in temperature and pressure.

3. Relative humidity: It is the ratio of the air’s actual water vapour content to its water vapour capacity at a given temperature.

Or

It is the ratio of the amount of water vapour actually present in air as compared with the maximum that could be contained by the same volume of the air at the given temperature and pressure. Relative humidity is expressed in percentage.

Q.10. What is condensation? How does it take place?

Ans. The process by which the water vapours in the atmosphere change into droplets or ice crystals is called condensation. It takes place when the air is saturated with vapour.

How does it take place? Condensation takes place when the moist air is cooled either by rising up or by coming into contact with cold surface. The moist air is cooled to the level when the water vapour present in it condenses on the dust particles. The temperature of this air falls down upto dew point. This condensation may take place in several ways. Water vapour may condense on solid objects as dew or frost or it may condense on dust particles and hang in the air as mist or fog.

Forms of condensation: Following are the forms of the condensation: Dew, frost, fog, mist, white frost and clouds. The clouds are formed when the temperature falls down below dew points, while dew, fog and clouds are formed when the temperature remains higher than the freezing point.

Q.11. Describe the factors controlling the rate of evaporation.

Ans. Evaporation is the process by which water is transformed from liquid to gaseous form. The factors controlling the amount and rate of evaporation are:

(i) temperature.

(ii) the moisture content or the degree of dryness or aridity of air. and

(iii) movement of air.

1. Increase in temperature increases the water absorbing capacity of the given column of air and vice versa.

2. When and where aridity is high, air has a potential of absorbing and retaining a large amount of moisture. In air with higher humidity, this potential is reduced; hence evaporation is slow and small in amount. Evaporation is greater on the oceans in summer than in winters.

3. The greater the movement of air, the greater is the evaporation. The movement replaces the moisture filled layer by a drier layer which has a larger capacity of absorbing moisture.

Q.12. Dew is not formed in cloudy nights, windy nights and shady places, Why?

Ans. The dew is formed when the objects radiate heat thoroughly so that the moist air coming into contact with them may be sufficiently cooled and the water vapour is condensed into water. The clouds act as blanket for the surface of earth. They check the radiation of heat so it does not radiate heat thoroughly. Hence, the dew is not formed in the cloudy night and in shady places.

Similarly, the dew is not formed on the windy night, because the winds blow fast and the moist air does not come along enough into contact with cold objects to get cooled. Hence the condensation does not take place and no dew can be formed.

Q.13. Write short notes on.

(i) Orographic precipitation.

Ans. This is the most widespread form of rainfall. It occurs when mountains stand in the part of moisture laden winds and force them to rise. As the air rises, it expands and cools and the rain falls. The resultant rainfall may often be heavy especially where a high continuous mountain barrier lies close to a coast and the rain bearing winds meet it at right angles. Heavier rainfall occurs on the windward side; Cherrapunji in Meghalaya lies on the windward side of the Khasi hills and has an average of over 1,000 cm of rain. Similarly, the Konkan region on the windward side of the Western Ghats receives over 250 cm of rain. As the air crosses over to the other side (leeward side), it loses most of the moisture and becomes warm and dry in descending and their is less rain. The leeward side of the mountain is called as such a Rain Shadow area. The Deccan region is said to lie in the rain shadow of the Western Ghats.

(ii) Condensation.

Ans. Condensation is the process by which atmospheric water vapour changes into water or ice crystals. It is just reverse of the process of evaporation. When the temperature of saturated air falls below dew point the air cannot hold the extra amount of humidity. This extra amount of humidity changes into water droplets depending upon the temperature at which condensation takes place.

Forms of condensation: Condensation can be classified on the basis of temperature at which the dew point is reached.

1. Forms of condensation when the dew point is higher than the freezing point are: White frost, snow and some clouds.

2. Form of condensation when dew point is lower than the freezing point are: dew, fog, and clouds.

(iii) High Clouds.

Ans. High Clouds: These clouds are found above the altitude of 6,000 metres and include cirrus, cirrostratus and cirrocumulus. Cirrus clouds are fibre clouds with a delicate silky appearances.

Cirrostratus: A thin whitish short of cloud covering the whole sky and giving it milky appearance is called cirrostratus.

Cirrocumulus: These clouds appear as small white flukes or small globulous masses, usually without shadows.

(iv) Rain Shadow.

Ans. Rain Shadow: The rainless area on the leeward side of a mountain is called the rain shadow. The moisture ladden winds give heavy rain on the windward side of the mountain but leeward side remains dry because the descending air becomes warm and gives no rain.

Examples: (i) The plateau of the Deccan is in the rain shadow of the Western Ghats.

(ii) The plateau of Tibet is in the rain shadow of the Himalayas.

LONG ANSWER TYPE QUESTIONS

Q.1. Discuss the salient features of the world distribution of precipitation.

Ans. The salient features of the world distribution of precipitation are as under:

1. The rainfall decreases as we go from equator towards poles.

2. The coastal areas of the world receive greater amount of rainfall than the interior continents.

3. The rainfall is more over the oceans than on the land masses of the world because of the consequent vapourisation.

4. The rain is heavier on the eastern coasts between the 30°C to 40° latitudes in both the hemispheres and decreases towards west.

5. Between 45° and 65° of the equator in both hemispheres, the rainfall is first received on western coasts due to westerlies and it goes on decreasing towards east.

6. Wherever mountains run parallel to the coast the rain is greater on the coastal plain and on windward side and decreases towards the leeward side.

Q. 2. What are forms of condensation? Describe the process of dew and frost formation.

Ans. Transformation of water vapour into water is called condensation. Condensation is caused by loss of heat. When moist air is cooled, it may reach a level when its capacity to hold water vapour ceases.

Following are the forms of condensation:

1. Dew.

2. Frost.

3. Fog.

4. Rain.

5. Snowfall.

6. Hail.

1. Dew: When the moisture is deposited in the form of water droplets on cooler surfaces of solid objects such as stones, grass blades and plant leaves, it is known as dew. The ideal conditions for its formation are clear sky and calm air, high relative humidity and cold and long nights. It is also necessary that the dew point is above freezing point.

2. Frost: When the dew point is below freezing point the water vapour will accumulate on the earth’s surface in the form of small white ice particles. The formation of the ice particles over a large area is called frost. The frost is formed when the temperature of the air falls rapidly so that the water vapour present in the air is directly turned into solid particles without turning into liquid state.

3. Fog: When the air near the earth’s surface is cooled throughout condensation takes place in it. Small drops of water are formed and they cling to the dust particles which float in the air and reduce the visibility. This is called mist, if it is very thick then it is known as fog.

4. Rain: When moisture laden air rises up condensation takes place and cloudes are formed. If the clouds are further cooled there is more condensation and drops of water that are formed are big in size. These big drops are too heavy to remain hanging in the air and fall down towards the earth’s surface. The falling of these drops of water from the clouds is known as rain.

5. Snowfall: If condensation takes place at a temperature below freezing points 0°C the water vapour condense into small crystals of ice instead of drops of water. The small crystal then units to form flakes of ice called snow. The coming down of snow flakes towards the earth is called snowfall.

6. Hail: Same times rain drops on their way to the earth surface are carried by strong air currents upward into colour regions. This causes the rain drops to freeze. After freezing they collect more water which too freeze. Thus thing grow in size and became heavy. Therefore they fall to the earth’s surface as hailstorm.

Q.3. How are clouds formed? Describe the three types of clouds based on their average height.

Ans. A cloud is a mass of minute droplets of water or tiny crystals of ice formed by the condensation of the water vapour in free air at considerable elevations. These droplets are so small that they do not fall down. They drift with winds and vertical currents.

Clouds are formed mainly by the adiabatic cooling of air below its dew point. This cooling process is most effectively created by upward movement of light and warm moist air, which in turn reduces pressure, expands and reaches its dew point. With further cooling below dew point, condensation takes place in air forming clouds.

On the basis of formation, clouds are classified into two main classes:

(i) Cumuliform or Heap Clouds: They are of great vertical height.

(ii) Stratiform or Layer Clouds: They have layered structure.

On the basis of the height, clouds are classified into three classes.

They are:

(i) Low Clouds: They are formed upto 2000 metres. Stratocumulus, nimbostratus, cumulus, cumulonimbus and stratus are well known low clouds.

(ii) Medium Clouds: They are formed upto 6000 metres. Alcocumulus and altostratus clouds are the well known medium clouds.

(iii) High Clouds (5-14 km) : Stratus clouds are layered clouds and with uniform layers. These clouds are formed mainly during winter and cause drizzle.

Q.4. Give a brief description of Dew and Fog.

Ans. Dew: The mixture deposited in the form of water drops on the objects near the surface of the earth is called dew. It so happens because, at night everything radiates heat which it had absorbed throughout day. Grass, flowers and leaves of plants radiate heat more quickly than other things and hence they become very cold. When moist air touches them, it is also cooled and it cannot hold all the water vapour it had in it. Some of this water vapour is condensed and it settles down on leaves, flowers and grass in the form of water drops. These drops are known as dew.

Conditions favourable for the formation of dew: Following are the ideal conditions for the formation of dew are:

(i) Clear sky.

(ii) Calmness i.e., wind should not blow.

(iii) High relative humidity.

(iv) Rich vegetation cover.

(v) Long nights.

Fog: At night when moist air comes into contact with cold air, it is cooled and some of the water vapours condense round the floating particles of dust in the atmosphere. This is called fog. Fog is formed when the air is cooled below its dew point.

Types of Fog: The air is cooled by radiation, conduction and mixing of warm and cold air masses. Hence, the three types of fog are formed. They are known as under:

(i) Radiation Fog.

(ii) Advection Fog.

(iii) Frontal Fog.

(i) Radiation Fog: It is very common in the winter season. The following conditions are necessary for its formation:

1. Clear sky on the night before the fog occurs.

2. The air movement should be very light or no wind should be there.

3. There should be a surface inversion of temperature.

4. Air should have been under a cloud cover with rain falling throughout the day before the fog occurs.

5. There should be pools of air, cooled to an excessive degree, collected in depression or valleys.

(ii) Advection Fog: It is formed through the transportation of warm moist air over cold surface. It occurs mostly on the sea coasts and shores of large inland water bodies. It is common on land in winter season and on the oceans in the summer season.

(iii) Frontal Fog: It is formed along the front which separates the warm air mass from the cold air mass. Cooling is caused by the forced ascent of air due to convergence and by saturation of the cold surface layer of air by rainfall from the overrunning warm air aloft. It is very common in the temperate latitudes as the fronts are frequent there.

Q.5. Explain adiabatic temperature change.

Ans. As we ascend up the atmosphere the temperature falls. The rate at which the temperature falls with height is called normal lapse rate. Its value is about 2°C per 300 metres or 3.6°F per 100 ft.

If the change in a certain volume of air takes place in such a way that neither any heat enters it from outside nor any amount of heat escapes from it, a change in temperature takes place. For example, the temperature falls when the volume of the air increases and the temperature increases when the volume decreases. Such a change of volume of air is called adiabatic change. This change in the atmosphere takes place when a mass of air rises vertically and rapidly. The volume of the air increases and its lapse rate becomes adiabatic.

If an unsaturated mass of air rises and also cools, the cooling is called adiabatic and the lapse rate in this case is called dry adiabatic lapse rate. Its value is roughly 3°C per 300 metres (5.4°F per 100 ft). It means that if a person ascends in dry air mass adiabatically, the fall of temperature would be 3°C per 300 metres. This value does not change like ordinary lapse rate and remains constant.

If a certain saturated mass of air rises and cools adiabatically, a part of its water vapour is condensed. In this process of condensation the latent heat is released and reduces the rate of cooling. Hence, the saturated or wet adiabatic lapse rate decreases. Its value is about 1.5°C per 300 metres or 2.7°F per 1000 ft.

Q.6. Distinguish between:

(i) Specific heat and latent heat.

Ans. Specific Heat: Water has a number of unique properties like high specific heat and highest surface tension. The specific heat is the amount of energy needed to change the water into water vapour.

Latent Heat: The heat stored in the water vapour is called latent heat. Due to the high surface tension the water molecules are able to attract adjoining water molecules to form the droplets fog, dew and mist.

(ii) Evaporation and Evapotranspiration.

Ans. Evaporation: It is the process by which water is transformed from liquid to gaseous form. It occurs whenever energy is transported to an evaporated surface and temperature rises. The molecules become more mobile and overcome the force binding then to the water surface and break away, resulting in evaporation.

Evapotranspiration: It is the amount of moisture transferred to the atmosphere by evaporation of liquid and solid water plus transpiration from living tissues, principally from plants. The terms potential evapotranspiration refers to idealised condition in which there could be enough rainfall to provide sufficient moisture for all possible evapotranspiration in an area.

(iii) Absolute Humidity and Relative Humidity.

Ans.

Absolute Humidity	Relative Humidity
1. It is the amount of actual water vapour present per unit volume of air.	1. This is a ratio expressed as a percentage between the maximum quantity of water vapour that the atmosphere can hold at a given temperature and the actual quantity of water vapour present in the air at that temperature.
2. The absolute humidity is measured in terms of grams per cubic metres of air.	2. Relative humidity is expressed as a percentage.
3. The absolute humidity does not change with the decrease or increase of temperature.	3. Relative humidity changes with the decrease or increase in temperature.

(iv) Dew and Frost.

Ans. The moisture deposited in the form of water drops on the objects near the surface of the earth is called dew. It so happens because at night everything radiates heat which it has absorbed by the day. Grass, flowers and leaves of plants radiate heat more quickly than other things and hence they become very cold. When moist air touches them, it is also cooled and it cannot hold all the water vapour it had in it. Some of this water vapour is condensed and it settles down on leaves, flowers and grass in the form of water drops. These drops are known as dew.

Frost: The frozen dew is called frost. When the dew point is lower than 32°F, or below freezing point; water vapour; condenses as ice. This is called Frost. It consists of particles of frozen moisture formed on the earth’s surface. The ideal conditions for the formation of frost are same as those for the formation of dew. Frost does not fall but condense on cool surfaces from surrounding air. It is harmful to plants as most crops are sensitive to frosts.

(v) Cloud and Fog.

Ans. Cloud: It is a mass of small water drops or the ice crystals formed by the condensation of water vapour in the free air at considerable heights above the earth’s surface. They are mostly formed in the upper regions of the atmosphere.

Fog: When the temperature of an airmass containing a large amount of water vapour falls all of a sudden, condensation takes place within itself on fine dust particles. So fog is a cloud with its base near to the ground.

(vi) Convectional and cyclonic precipitation.

Ans. Convectional precipitation or rainfall: When due to conduction air near the surface at any place becomes warmer than the surrounding air, it begins to rise in the form of convection currents. As it reaches the upper layers of the atmosphere it cools and rainfall occurs.

The convection currents are normally set up in the atmosphere as a result of local heating. The currents rise up at the centre and are drawn up at the sides. The very rapid rising of the warm moist air results in the formation of cumulonimbus clouds which may be several kilometres in depth and may give very heavy rain, but the rainfall does not last long. We speak of it as “thunder showers” rather than thunder rain.

Convectional rainfall is common within the tropics, especially in the equatorial belts where it occurs almost every day in the afternoons. Similarly, the summer rain in the Steppes and Prairies is largely of this type but the amount of rainfall is low as these areas are far away from the sea.

Cyclonic rainfall or precipitation: When hot and hurried airmass meets a cloud, the warm air tries to enter the cold air at the frontal region but the cold air pushes the warm air from below. This moves the warm air on to the cold airmass and warm air begins to ascend. The water vapours of the warm humid airmass and warm airmass are condensed in the ascent of the airmass and rainfall occurs.

This rainfall is due to the convergence of air masses.

There is a low pressure area at the centre of cyclones. The air from the surrounding high pressure area moves encircling towards the centre. This air does not find place to stay at the centre due to the cyclonic rotation and therefore has to rise in the atmosphere. Condensation by ascent causes rainfall.

(vii) Fog and Mist.

Ans. 

Fog	Mist
1. Fog develops when warm damp current of air passes over a cold surface.	1. Mist is the result of condensation of water droplets on particles of smoke and dust present in the air.
2. It is composed of water particles which condense near the surface of the earth.	2. The fog of lesser intensity with rather wet air, is called mist.
3. Visibility in fog is less than one kilometre.	3. Visibility in the mist is more than one kilometre but less than two kilometres.

(viii) Dew and Dew Point.

Ans. 

Dew	Dew point
Dew is a form of condensation. When an unsaturated and cool air comes in the contact with solid cold surface, specially in winter, moisture gets condensed on the cold objects such as roofs, rocks, plants, grasses in term of minute water drops. It is termed as dew. It is common feature during winter in Northern Indian plains.	Dew point is the temperature at which a given sample of air becomes saturated. Dew point leads or promotes condensation or makes air saturated.

(ix) Rainfall and snowfall.

Ans. 

Rainfall	Snowfall
1. When condensed water from the clouds falls through the atmosphere and reaches the earth’s surface in the form of water drops, it is called rainfall.	1. When precipitation takes the form of ice crystals of a delicate feathery structure, it is called snowfall.
2. Rainfall is the most common form of precipitation all over the world.	2. Snowfall takes place only in cold regions and on the high mountains.
3. Rainfall is a type of precipitation.	3. Snowfall is also a type of precipitation.