Class 9 Social Science Geography Chapter 2 Atmosphere

Class 9 Social Science Geography Chapter 2 Atmosphere The answer to each chapter is provided in the list so that you can easily browse throughout different chapters Assam Board Class 9 Social Science Geography Chapter 2 Atmosphere and select needs one.

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Class 9 Social Science Geography Chapter 2 Atmosphere: Structure, Air Pressure and Wind System

Also, you can read the SCERT book online in these sections Solutions by Expert Teachers as per SCERT (CBSE) Book guidelines. These solutions are part of SCERT All Subject Solutions. Here we have given Assam Board SEBA Class 9 Social Science Geography Chapter 2 Atmosphere Solutions for All Subject, You can practice these here…

Atmosphere: Structure, Air Pressure, and Wind System

Chapter: 2



Q.1: What is atmosphere ? Write in brief with data its structure.

Ans: Atmosphere is the veil of odourless,tasteless and colourless gases,dust particles and water vapour surrounding the earth which forms a protective boundary between the outer space and the biosphere. It is composed of different gases in different volumes. Nitrogen and Oxygen together constitute about 99 percent of the total volume of atmosphere gases. The remaining 1 percent is occupied by argon,carbon dioxide and various other gases. The composition of the atmosphere in the light of its gaseous mixture is shown in the chart below: Atmosphere  gaseous composition and volume.

Q.2: What are the main layers of the atmosphere based on chemical composition of the gases ? Write briefly about the characteristics of these layers.

Ans: The main layers of the atmosphere based on chemical composition of the gases are homosphere and heterosphere.

The characteristics of these layers are :

[I] Homosphere :

(i) The lower zone of the atmosphere.

(ii) Extends upto an attitude of 80 km from the earth’s surface.

(iii) 99% of the atmosphere elements is confined to within 32 km of the homosphere.

(iv) There is uniformity in the composition of various gases throughout the homosphere.

(v) Based on attitude,the layer is further divided into troposphere, stratosphere and mesosphere.

[II] Heterosphere:

(i) The upper zone of the atmosphere.

(ii) Extends beyond 80km to about 10,000km.

(iii) Density of air is negligible low in this region whereas there is spectacular rise in temperature.

(iv) Based on the variation in the atomic masses of the gases,certain gases remain unevenly concentrated in this layer and form different sub-layers,which fall under the thermosphere and exosphere. There sub-layers are:

(a) Nitrogen layer(from 80km to 200km)

(b) Oxygen layer (from 200km to 1125km).

(c) Helium layer(from 1125km to 3540km).

(d) Hydrogen layer (beyond 3540km to the extreme boundary of the atmosphere at about 10,000km)

Q.3: Write with diagram about the layers of the atmosphere based on the variations in altitude and temperature and write the characteristics of each layer in brief.

Ans: Based on the variations in altitude and temperature,the layers of the atmosphere are divided into troposphere, stratosphere,mesosphere, thermosphere  and exosphere.

The characteristics of these layers are: 

[I] Troposphere :

(i) Bottom-most layer of the atmosphere that remains close on the earth’s surface.

(ii) Upper limit of this layer is called tropopause where the temperature remains constant.

(iii) Average height of the layer is 12km which extends to a maximum height of 16km at the equator and 8km at the poles.

(iv) Temperature tends to decrease with increas  in altitude. The rate of decrease in temperature is 6.5°C per kilometre. At the tropopause is reduced to -60° C.

(v) Contains three-fourth of total atmospheric gases and abundant water vapour and dust particles which leads to formation of clouds along with rains,storms,cyclones,etc. So all weather phenomena occur in this  layer.

[II] Stratosphere :

(i) Atmospheric layer above the troposphere and below the mesosphere.

(ii) Upper limit of the layer is called stratopause respectively.

(iii) Extends up to 50km from the earth’s surface and its thickness beyond the tropopause is 40km on an average.

(iv) Temperature increases with increase in elevation in this layer due to the presence of molecules of ozone gas which absorb the sun’s radition. The minimum temperature of this layer is -60°(at the tropopause) and the maximum temperature is 0°C (at the stratopause).

(v) Ozone layer present in this layer shields the earth from harmful ultra-violet rays of the sun.

(vi) Aeroplanes and jet aircrafts fly in this layer due to horizontal wind movement and absence of clouds.

[III] Mesosphere :

(i) Atmospheric layer that above the stratosphere and below the thermosphere.

(ii) Lower and upper limits of this layer are called stratopause and mesopause respectively.

(iii) Extends upto 80km from the earth’s surface and its average thickness is 30km.

(iv) Temperature falls with altitude . At the bottom layer,the temperature is around 0°C which falls to -100°C at the upper level of this layer.

[IV] Thermosphere:

(i) Atmospheric layer that exists above the mesosphere and below the exosphere.

(ii) Extends from 80km at the mesopause to about 400km from the earth’s surface.

(iii) It is the hottest layer of the atmosphere and the density of air goes down to zero. 

(iv) The temperature increases with increase in height and reaches around 1650°C 

(v) This layer is also called ionosphere at it contains electrically charged ions.

(vi) The layer reflects radio waves sent from the earth’s surface.

[V] Exosphere: 

(i) Topmost and the last layer of the atmosphere that lies above the thermosphere.

(ii) Extends from 400km upto a height of 10000km from the earth’s surface.

(iii) Air is thin due to the presence of only hydrogen and helium gases in this layer.

(iv) Temperature rises to about 5550℃.

Q.4: Discuss with the factors responsible for variation in atmospheric pressure.

Ans: The factors responsible for variation in atmosphere pressure are :

[I] Air temperature: Temperature is one of the most important factors affecting atmospheric pressure.Temperature varies greatly in different parts of the earth’s surface,thus making a place-cold,warm, temperature,etc. Pressure varies inversely to temperature.

(i) High temperature-low pressure:

(a) Due to high temperature,the air near the earth’s surface gets heated and expends. This expanded hot air has low density and exerts low pressure.

(b) Warm air absorbs more amount of water vapour. The air laden with moisture and water vapour becomes wet and has a low atmospheric pressure.

e.g. the hot equatorial region has low pressure.

(ii) Low temperature – high pressure:

In the colder regions,where temperature is low,the air remains,cold,heavy and dense. Absence of water vapour in this region makes the air comparatively dry, which creates high atmospheric pressure over the entire region. e.g. the cold polar regions have high pressure.

[II] Height or altitude from the earth’s surface:

(i) High altitude -low pressure:

Pressure various inversely to altitude. As we go up from the sea level,the amount of gases present in the atmosphere reduces due to which the mass of air becomes less and it exerts a low pressure. Thus,there exists low atmospheric pressure over the places of high elevation e.g. Darjeeling,Shillong,etc. have low pressure.

(ii) Low altitude – high pressure:

As the mass of air is higher near the earth’s surface,high pressure prevails near the places at lower elevation. The rate of decrease in atmospheric pressure is 1inch(34 milibar) per 900ft height. e.g. high pressure prevails in the plains.

Q.5: With the help of diagram discuss the characteristics of the major pressure belts of the world.

Ans: The major pressure belts of the world are:

[I] Equatorial low pressure belt:

(i) Extends on both sides of the equator from 0℃ to 10° N and S latitudes.

(ii) The vertical sun-rays falling in this zone keep the air warm and hot throughout the year which rises upward,thus creating low pressure in this region.

(iii) As there is abundance of water area compared to land area in this region, maximum amount of water 

vapour gets absorbed in the air,thus making the air wet,which creates low atmospheric pressure over the region.

(iv) This region remains calm and is also known as equatorial doldrums,as there is absence of horizontally blowing wind over this region.

[II] Sub-tropical high pressure belts :

(i) Extend in both the hemispheres between 25° and 35°N and S latitudes,i.e. the regions around the Tropic of Cancer and the Tropic of Capricorn.

(ii) The warm air rising upward at the equator gradually cools down and descends towards this region,which results in high pressure over this belt.

(iii) Due to the axial rotation of the earth,the cold air from the sub-polar high pressure belts moves gradually to this area and gets accumulated with the air from the equatorial belt. Thus,dense and cool air results in high pressure over the entire area.

(iv) Wind moves downward here and hence there is no perception of blowing wind.

(v) Also known as the ‘Horse Latitudes’.

[III] Sub-polar low pressure belts: 

(i) Extend in both the hemispheres between 60° and 70°N and S latitudes.

(ii) Air in this region is reduced as it gets deflected towards the sub-tropical high pressure belt due to high rotational movement of the earth in these sub-polar belts which results in low pressure over this region.

(iii) There is variations in these low pressure belts towards north and south latitudes during different seasons.

[IV] polar high pressure belts: 

(i) Extend over the North and South Poles (90°N and 90° S latitudes).

(ii) Region is snow-covered and extremely cold.

(iii) The water is all in the form of ice.

(iv) Permanent high pressure belt prevails over the polar region as the air over both the polar regions contains no water vapour.

Q.6: Write the importance of atmosphere towards creation of a favourable physical environment on the earth.

Ans: The importance of the atmosphere in creating a favourable physical environment on the earth are:

(i) Supplies important gases: The gases that living beings including plants,animals and human beings use in their daily life are provided by the atmosphere.Oxygen,the most active gas without which living beings cannot survive on earth is provided by the atmosphere. Similarly,carbon dioxide given out by animals and human beings is essential for the process of photosynthesis in plants. Nitrogen,the most inactive gas is absorbed by the microorganisms which are then absorbed by the plants as nitrogen compounds. Argon,a chemically inactive gas is used in making electric bulbs,welding works,etc.

(ii) Protects the earth: Carbon dioxide,water vapour and other molecules present in the air prevent the terrestrial heat from escaping back into space. This is  known as greenhouse effect. Similarly,the ozone layer found in the upper limits of the stratosphere prevents the harmful ultraviolet rays from reaching the earth.

(iii) Provides rain and water: Without atmosphere we would not have rain or water.These weather phenomena take place in the troposphere as a result of variations in atmospheric pressure and temperature.

(iv) Protects the earth from extreme heat and cold: Atmosphere acts as an umbrella by protecting the earth from extreme heat or cold.

Q.7: What is wind ? Discuss the factors of origin of wind.

Ans: The horizontal movement of air over the surface of the earth is known as wind. In other words,wind is the movement of air that results due to variations in atmospheric pressure caused by temperature differences. Generally,this movement takes place from areas of high pressure to areas of low pressure.

Factors affecting the region of wind are :

(i) Temperature variations.

(ii) Pressure differences.The earth’s surface is not heated equally at all places. The equatorial region gets maximum heat due to perpendicular rays of the sun,while the temperature decrease towards the poles because of inclination of sunrays. When the temperature increase,air becomes light and rises up creating an area of low pressure in the process. The vacuum created by the rising air is soon taken over by the air from the high pressure area which rushes in to  fill the vacuum. This brings about movement of air known as wind.Therefore,wind is mainly caused by pressure gradient resulting from pressure differences which,in turn,is caused by variations in temperature.

Q.8: What is the most important reason for the origin of wind ?  Briefly discuss the factors determining the velocity and direction of wind.

Ans: The most important reason for the orgin of wind is pressure gradient arising from pressure differences caused by variations in temperature.

(i) Pressure gradient force: The rate of atmospheric pressure differences between two places is called pressure gradient. The force generated from the atmospheric pressure differences on the earth’s surface is known as pressure gradient force. The amount of force produced due to pressure gradient depends on the pressure differences between the two places and the distance between them. The velocity of wind would be higher,if the pressure gradient is high or steep and vice versa. Again,the direction in which pressure decreases determines the direction of wind. An increase in pressure gradient increases the velocity of wind and makes the climate conditions of a place unstable and violent and vice versa,e.g. effect of pressure difference from diurnal range of temperature is low while that from latitudinal differences of temperature is high.

(ii) Gravitational force: Air pressure decrease with increasing altitude,i.e.the gravitational pull over the air decrease with increasing altitude. The air which is close to the earth’s surface exerts a higher pressure than that of the air over high altitudes. This creates high pressure over the valleys and low pressure over the mountain areas,resulting in a vertical movement of air from high pressure to low pressure region.Besides the movement of air from one place to another becomes curved rather than a straight line due to gravitational force.

(iii) Centrifugal force: The force generated from the centre of the earth as a result of rotation of the earth on its axis is known as centrifugal force. The wind direction is slightly deflected by this centrifugal force. This force is known as’Coriolis Force’,as this phenomenon was first discovered by a French mathematician,Gasperd de Coriolis. As a result of Coriolis force,wind gets deflected towards right in clockwise direction in the northern hemisphere and towards left in anticlockwise direction in the southern hemisphere. This phenomenon is known as Ferrell’s law. The Coriolis force is zero over the equator and gradually increase towards the poles. The Coriolis force is the highest at the poles.

(iv) Frictional force: Frictional force negatively affects wind velocity. Rugged terrain such as mountains,valleys,thick forest,high-rise buildings,etc.block the movement of wind,thereby reducing its velocity. Similarly,the frictional force is much less,when the wind blows over water or snow-covered 

land. However,the frictional force not only affects the velocity but also affects the wind direction.

With the decrease in the velocity of the wind,the impact of Coriolis force leading to deflection of wind also becomes less and so its impact on the wind direction is also minimum.

Q.9: What do you mean by’coriolis force’s With the help of diagram briefly describe its contribution in determining the direction of wind.

Ans: Refer to answer of question no. 8 (iii) [textual questions and answers]

Q.10: What is meant by ‘pressure gradient force’ ? What is its contribution in air movement.

Ans: Refer to answer of question no.8 (i) [textual questions and answers]

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