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…

Q.11: Briefly discuss the contribution of the earth’s gravitational force in air movement.

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

Q.12: What do you mean by naming of wind ? How is the wind blowing over a place named ?

Ans: Giving a name to a particular movement of air over a place or region is known as naming of wind. Naming of wind depends  on wind direction,i.e. the direction from which wind comes is known as ‘windward’ and the direction to which it blows is called ‘ leeward’.e.g. if is called westerly wind,etc.

Q.13: How is the velocity of wind determined ? What are the units of wind velocity ?

Ans: The velocity of wind is determined with the help of an instrument named anemometer. Nowadays,anemograph,a type of anemometer is used to measure and record the direction and wind velocity automatically.The velocity of wind is expressed in terms of knot. The wind velocity of one knot means one nautical mile per hour which would mean 1.854 kilometer per hour or 30.9 metres per minute.

i.e. 1knot = 1nautical mile per hour

Or 1.854 kilometer per hour Or 30.9 metre per minute

Q.14: How is the classification of wind done ? Briefly discuss with examples.

Ans: On the basis of extent of circulation,the different types of wind are :

(i) Primary wind circulation: The regular and permanent wind that originates as a result of permanent pressure belts over the earth’s surface and which affects the whole earth is known as primary circulation. It is also called permanent or planetary wind. The main primary or planetary winds are:

(a) Trade winds (b) westerlies

(c) Polar winds 

(ii) Secondary wind circulation: The wind that originates in certain areas of the earth as a result of the variations in landforms, distribution of land and water bodies,regional and seasonal variations,variations in temperature-pressure conditions,etc. is known as secondary wind circulation or secondary wind. These winds bring about immense change in the weather of an area and create unstable atmospheric conditions. They are also known as periodic winds. The main winds of secondary circulation are: 

(a) Air mass (b) Cyclones

(c) Front (d) Anticyclones

(e) Monsoon wind

(iii) Tertiary wind circulation or local wind: The winds that originates as a result of certain local conditions such as topographic variation,altitude changes,distinct landforms,presence of water, altitude variations,etc. are known as tertiary wind circulation or local winds. These winds blow in a limited area but change the weather of an area significantly.

It is also known as temporary air movement. The main local winds are:

(a) Sea breeze (b) Land breeze 

(c) Mountain wind (d) Valley wind

(e) Other local winds 

Q.15: What do you mean by primary circulation of wind ? Show distribution of primary circulations in a diagram. Mention its important characteristics.

Ans: Do Your Self.

Distribution of primary wind circulation

The main characteristics of the primary circulation wind are :

(i) Trade wind: The wind that blows from the sub-tropical high pressure belts situation on both sides of the equator towards the equatorial low pressure belt from 

30°N and 30°S to 0° is known as trade wind. There are two types of trade winds :

(a) North-east trade wind: The trade wind that blows from the north- east direction to south-west direction in the northern hemisphere due to the Coriolis force is called south-east trade wind.

(b) South-east trade wind: The trade wind that blows from the south-east direction to north-west direction in the southern hemisphere due to the Coriolis force is called south-east trade wind.

Since the trade wind comes from both hemispheres towards the equator,a division is formed after meeting each other in the equatorial region. This division is known as inter-tropical convergence zone. The main feature of this area is that the movement of the wind is hardly perceived here as the Coriolis force is almost zero here. Because of the calm nature of the wind here,this area is known as ‘calm zone’or ‘doldrums’.

(ii) Westerlies: The permanent wind that blows from the high pressure sub-tropical belts towards the high latitude sub-polar low pressure belts are known as westerlies. Since,this wind blows in the opposite direction of the trade wind and is more intense,it is also called anti-trade wind. The main westerlies are: 

(a) South-west westerlies: The wind that blows from the south-west to east and north-east in the northern hemisphere due to the Coriolis force is known as south west westerlies. 

(b) North-west westerlies: The wind that blows from the north-west to east and south-east in the southern hemisphere due to the Coriolis force is known as north-west westerlies.The trade winds and westerlies create a calm zone of wind circulation,known as horse latitude in the sub-tropical high pressure belt. This zone lies within 30°-35°N and S latitudes.

But the westerlies are very intense in the southern hemisphere between (40°S-50°S latitude), furious fifties(50°S-60°S latitude) and screaming sixties(60°S-50°S latitude).

(iii) Polar winds: The wind that blows from the polar high pressure belts to low pressure sub- polar regions in both hemisphere is known as polar wind. These winds are also called polar easterlies as these winds blow from the east in both the hemispheres. The polar wind and westerlies meet each other leading to the formation of polar front and moderate cyclonic storm due to their contrasting nature.

Q.16: What is secondary circulation of wind ? Discuss its contribution in determining the climate of a place with examples.

Ans: Do Your Self.

(ii)The contribution of secondary circulation of wind determining the climate of a place are :

(i) Air mass: Refer to answer of question no.19.

(ii) Front: Refer to answer of question no.20.

(iii) Cyclones: Refer to answer of question no.18.

(iv) Anticyclones: Refer to answer of question no.24 (i).

(v) Monsoon winds: Refer to answer of question no.21.

Q.17: What is local wind ? Briefly discuss with examples how local wind determines the weather condition of a place.

Ans:Do Your Self.

(iii) The role of local wind in determining the weather conditions of a place are :

(i) Sea breeze : The wind that blows during the day from the sea towards the land having low pressure is known as sea breeze. During the day,the land gets heated faster than sea water and as a result there exists low pressure over the land and high pressure over the sea which gets heated much slower. Since this wind comes from the sea,it is cool and full of moisture. The coastal regions of countries are greatly influenced by this wind making such regions ideal for habitation as well as for tourism.

(ii) Land breeze : The wind that blows during the night from the land towards the sea as there exists low pressure over the sea is known as land breeze. During the night,the land masses cool off faster than water bodies and so there exists high pressure over the land and low pressure over the sea as water bodies release the heat absorbed during the day more slowly than land. Therefore,wind blows from the land towards the sea. Since such wind comes from land,they are dry and cold.

(iii) Mountain wind and valley wind : During the day,the dense air of the valleys absorbs heat more quickly than the air over the mountains. As a result,the air over the valleys gets heated up forcing it to rise up along mountain slopes. This wind is known as valley wind. During the night,mountain tops and slopes become cold compared to that of the valleys and as a result,wind blows from the mountains tops towards the valleys,known as mountain wind. These do not allow formation of fog even during winter in the foothill areas of mountains.

(iv) Other local winds: Many local wind originates due to certain local temperature variations. They are known by different names in different countries. The common local winds are Chinook,Foehn,Siroco,Mistral, Loo,etc. Chinook is mostly found in the rocky mountainous region of the U.S.A.while Feohn is noticed in the Alpine regions of Europe .Mistral is a dense and cold local wind of Europe which blows from the southern slopes of the Alps towards the coastal region of the Mediterranean Sea in winter. Sirocco is a warm and dry local wind of North African Sahara region while Loo is a dry local wind of North-western India. Such winds bring about great changes in local weather conditions.

Q.18: What is a cyclone ? What are its types ? Mention briefly how it influences the climate of a place.

Ans: Cyclone is the state of atmospheric circulation in which high velocity wind assumes a cyclic path around a low pressure centre. The most noticeable feature of cyclone is the movement of the wind from the centre to the outer periphery marked by violent cyclical movement of the air often causing terrible havoc. 

The types of cyclones and the way it influences the climate of a place are: 

(i) Tropical cyclones: Tropical cyclone occurs when high velocity wind from a high pressure region take a cyclic path of violent nature all around a low pressure centre in the tropical region.Due to earth’s rotation,these cyclones move in an anticlockwise direction in the northern hemisphere and in a clockwise direction in the southern hemisphere. Generally,the wind velocity of tropical cyclones ranges from 120km to 280km per hour and its diameter ranges from few kilometer to thousands of kilometer. During summer,these cyclones mostly begin in the sea and then move over to the land surface causing great havoc and destruction. These tropical cyclones cause extensive damage in South-eastern part of North America,eastern part of Japan and east coastal regions of India. They are known as Typhoon in the coastal areas of the Pacific in the east,Hurricane in the West Indies region,Cyclone in the Indian Ocean,Bardoisila in Assam and Willy-Willy in North-eastern coast of Australia. 

(ii) mid-latitude/extra-tropical cyclones/temperate cyclones/wave cyclones: Extra-tropical cyclones are found in regions between 30° and 65° latitude in both the hemispheres,i.e. the temperate and high latitude regions. These type of cyclones are mostly formed as a result of meeting of diverse types of wind. Extra-tropical cyclone is the polar front formed with unstable atmospheric conditions due to which temperate regions often witness weather change. The atmospheric pressure difference between the centre and outer part of the cyclone ranges from 10 to 35 milibar. Though cyclones have a circular and elliptical shape,sometimes extra-tropical cyclones are V-shaped. Most of the temerate cyclones have a diameter ranging from 300km to 1500km and one of these cyclones covers nearly 1.6 milion sq km area. Generally,the cyclones of the temperate region move from the south-west to north-east region. This type of cyclone is associated with dense clouds,heavy rainfall,hail and thunder storm. But due to mild air movement,the weather soon becomes clear.

Q.19: What is an mass ?  How is it classified ? What is its contribution in determining the climate of a place ?

Ans: Air mass means a huge mass of air having homogeneous temperature and humidity. Generally,it extends over a wide area and is responsible for climate and weather conditions that prevail over that area.

On the basis of the characteristics and source of region of the air mass,it can be categorised into the following four groups:

(i) Tropical continental air mass(cT)

(ii) Tropical maritime air mass (mT)

(iii) Polar continental air mass (cP)

(iv) Polar martime air mass (mP) Air mass greatly influences the climate conditions of the place over which it passes. When air mass passes through an area,it influences the air of that area. One of the features of air mass is that its characteristics are passed on to the surrounding air or the air over which it passes over. Vertical circulation of temperature and pressure is a notable feature of air over air mass. Cold air mass generally influences the air below, making it cold while hot air mass makes the bottom air hot too. It not only transfers heat to the surrounding layers of air but also passes on humidity,density, pressure conditions,etc. Thus,air mass,plays a major role in the distribution of heat in the atmosphere. Because of the dynamic movement of air mass,the heat from warm region moves to cold region in the atmosphere and thereby an equilibrium in atmospheric temperature is maintained. 

Therefore,air mass greatly influences the climate and weather conditions of a place.

Q.20: What do you mean by ‘front’ ? How is it formed ? Briefly discuss the relationship between climate and front.

Ans: When two different air masses with sharp contrast in temperature, pressure,etc.come closer from opposite directions,they do not mix easily and form a line of discontinuity. This line of discontinuity of air mass is known as front. Front is a type of secondary wind circulation or periodic wind. On the basis of nature and characteristics,fronts are divided into the following four types:

(i) Cold front: A warm air mass is shifted by a cold air mass.

(ii) Warm front: A cold air mass is shifted by a warm air mass which moves over the cold air mass.

(iii) Stationary front: Remains in static condition and does not help in shifting of air mass.

(iv) Occluded front: Formed as a result of intermixing of both cold and warm fronts and has mixed characteristics.

Formation of front:The following two conditions are essential for the formation of a front: 

(i) There should be two air masses of which one should be colder and heavier than the other.

(ii) Air movement should be convergent so that the two air masses from opposite directions move closer to one another.

Relationship between climate and front:

The coming closer of two airmasses with contrasting characteristics,brings about a change in the climate in the frontal region. The warm front moves over the cold front and maintains equilibrium of air temperature. The mixing of both types of front due to reversal of pressure gradient direction helps transfer of temperature,diffusion of humidity, pressure and other physical characteristics of the atmosphere. The progress of the front also brings about climate changes in the surrounding regions.

Q.21: What do you mean by ‘monsoon wind’ ? How is it formed ? In which areas of the earth its impact is quite distinct ?

Ans: The word ‘monsoon’ has been derived from the Arabic word ‘mausim’ and the Malayan word ‘monsin’ which means ‘seasons’. Thus,monsoon wind is a type of wind which reverses its direction with change of seasons. In summer,it blows from sea to land,and during winter it blows from land to sea. It blows at places situated near the sea within the tropical and sub-tropical regions,where the differences of temperature between summer and winter is high.

Formation of monsoon wind:

(i) Summer monsoon : During summer in the northern hemisphere,the sun’s rays fall vertically in the areas situated near the Tropic of Cancer,due to which a powerful low pressure area develops in these regions. On the other hand,high pressure appears over the sea situation close to the south and soth-east of these regions,and as a result wind blows from sea to land. As this wind blows from sea to land,it is moist and brings a lot of rain to the areas over which it blows. It is due to this reason that the south-west and south-east monsoon wind which blow from the Arabian sea and the Bay of Bengal bring a lot of rain in the foothills regions of Assam southern part of Meghalaya and western slopes of the westernghats.

(ii) Winter monsoon: In winter,the direction of the wind is changed. Because of the existence of high temperature over the water bodies,there exists low pressure over the water and high pressure over the land. Therefore,dry wind blows from the land to the water and high pressure over the land. Therefore,dry wind blows from the land to the sea. However,when this wind passes over the water bodies in their southward journey,

It absorbs certain amount of moisture bringing rain to the lands over which it blows in its later journey. That is why,coastal regions of Tamil And get winter rain from north-east monsoon winds that come from the Bay of Bengal.The main areas of monsoon wind are South-Asia,Africa,South-east and South-west coasts of the U.S.A.,Gulf region of Mexico,North-east Australia,etc. However,the greatest impact of monsoon is seen in Asia particularly in South-Asia and South-east Asia.

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