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.

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

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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


Give very short answer:

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

Ans: An atmosphere is a layer or a set of layers of gases surrounding a planet. An atmosphere is more likely to be retained if the gravity it is subject to is high and the temperature of the atmosphere is low. The atmosphere can be divided into layers based on its temperature. These layers are the troposphere, the stratosphere, the mesosphere and the thermosphere.

The composition of the atmosphere in the light of its gaseous mixture is shown in the chart below: 

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: Based on chemical composition of the gases the atmosphere is broadly divided into two major layers:

(i) Homosphere.

(ii) Heterosphere.

The characteristics of these layers are:

(i) Homosphere: Homosphere is attached with the earth’s surface. This layer intended upto the height of 80 km. The majority portion of the atmosphere is included in this layer. The chemical composition of the gases is almost same in it.

(ii) Heterosphere: The chemical composition of the atmosphere gases changes in the layer above the Homosphere which layer is called Heterosphere. In this layer the gases remain separated based on their atomic mass and they form different layers of gases. There are four major gaseous layers in it. These layers are Nitrogen layer, oxygen layer, Helium layer and Hydrogen layer.

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: Atmosphere is also considered as life of a Human beings. So many atmosphere layers are present on the Earth upper side.

Because of having atmosphere, humans can breathe and oxygen is transferred from one place to another. We refer word Atmosphere in various languages. As well as, Atmosphere words means Nature and Environment in short words.

The atmosphere is divided into consisted of various layers which are called as ‘Layers of Atmosphere’. The layers of atmosphere are dependent or formed because of their temperature.

(i) Troposphere: (a) There is comparatively high temperature in its lower part and temperature decreases gradually above it. The rate of decrease in temperature with increase in height in the troposphere is 6.5°C kilometer on the average. It becomes as low as- 60 at the tropopause. So, the weather activities like cloud, rain etc. occurred in this layer.

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

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

(ii) Stratosphere: (a) Atmospheric temperature is found to increase pern the tropopause through the stratosphere. At the lower level of it temperature is about 60°C and it increases there after gradually reaching about 0°C in the upper level. The ozone layer also exists in this stage.

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

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

(iii) Mesosphere: (a) Atmospheric layer that lies above the stratosphere and below the thermosphere.

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

(iv) Thermosphere: (a) In this layer temperature rises as high as about 1650° C The density of air becomes minimum here. This layer lies within the heterosphere zone.

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

(c) This layer reflects radio waves sent from the earth’s surface.

(v) Exosphere: (a) This layer extends from 400 km upto a height of 10000 km from the earths surface.

(b) The topmost and the last layer of the atmosphere that lies above the thermosphere.

(c) This layer temperature rises to about 5550°C.

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: Air expands when it is heated. The warm and expanded air has low density. Air of low density means light air and its pressure also becomes low. Again the warm air has more ability to absorb water vapour. Then it becomes wet and such wet air has low atmospheric pressure. In contrast, the cold air has the less ability to absorb water vapour and such air thus remains comparatively dry. Such dry air has high atmo spheric pressure. So, it can be said that where there is high temperature, there is low pressure of air.

(ii) Altitude from the Earth’s Surface: When one goes up from the sea level the depth of atmosphere decreases as its mass decreases. So, its pressure also decreases. Atmospheric pressure is found to decrease along with the increase in height. Any place located at high elevation has thus low atmospheric pressure and the place located at low elevation has certainly high atmospheric pressure.

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

Ans: On the earth’s surface, there are seven pressure belts. They are the Equatorial Low, the two Subtropical highs, the two Subpolar lows, and the two Polar highs. Except for the Equatorial low, the others form matching pairs in the Northern and Southern Hemispheres. There is a pattern of alternate high and low-pressure belts over the earth. This is due to the spherical shape of the earth- different parts of the earth are heated unequally. The Equatorial region receives a great amount of heat throughout the year. Warm air being light, the air at the Equator rises, creating low pressure. At the poles the cold heavy air causes high pressure to be created/formed.

The major pressure belts of the world are:

(i) Equatorial low pressure belt: (a) This belt extends on both sides of the equator from 0° to 10° N and S latitudes

(b) This belt remains calm and is also known as equatorial coldrum.

(c) The air of this region remains comparatively warmer and lighter and hence its den sity becomes low. As a result of it, there prevails permanent low pressure. The water covered area in this region is more than the land area and water vapour is more in the air. Thus, the atmospheric pressure gets reduced here. In this belt air moves upward after getting warmed. So, in this region there is found. Virtually no wind blowing parallel to the earth’s surface as such air seems to be calm and quite here.

(ii) Sub-Tropical High Pressure Belt: (a) The cold and heavy winds coming from the equatorial and polar regions get accumulated near the tropic of Cancer of the north and the tropic of Capricorn of the South and create high pressure over the area. The wind usually moves downward here and hence blowing of wind is not perceived. Here the speed of the wind is so low that sometimes it becomes difficult for movement of ship in the area. This sub-tropical high pressure belt season-wise extends sometimes towards north and sometimes towards south.

(b) This belt extends in the both the hemisphere between 25 and 35 N and South latitudes. 

(e) This region wind moves downward here and there is no perception of blowing wind. Also know as the “Horse Latitude” this belt.

(iii) Sub-Polar Low Pressure Belt: (a) The air of the Sub-Polar regions gets deflected towards sub-tropical regions and then due to decreased amount of air there develops low pressure in the sub-polar regions. This low pressure belt season-wise extends sometimes towards north and sometimes towards south.

(b) This belt extend in both the hemisphere between 60° and 70° N and South latitudes.

(c) This belt is variations in these low pressure belt towards north and south latitudes during different seasons.

(iv) Polar High Pressure Belt: (a) The Polar region is too cold as it is covered by ice. Its air is cold and contains almost no water vapour. There exists permanent high pressure in the polar region.

(b) This belt extend over the North and South Poles of 90°N and 90’S latitudes.

(c) This belt permanently high pressure belt prevails over the polar region Belt is snow-covered and extremely cold.

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, (ii) Protects the earth, (iii) Provides rain and water, (iv) Protects the earth from extreme heat and cold.

Atmosphere is one of the important elements of natural environment. The atmosphere supplies the essential gases for the living world. The atmo- sphere helps in distribution and transfer of heat of the surface of the earth. It is due to movement of air that the temperature variations between the very warm and cold places become less. 

The oxygen and carbon dioxide are very much useful for lives and it also plays important role to control climate. The atmosphere protects the organisms on the earth from the harmful ultraviolet rays coming form the space. The ozone gas absorbs the ultraviolet rays. The various phenomena essential for organism like rain, wind etc. occured in the atmosphere. The existence of plants and animals on the earth cannot be possible without atmosphere.

Atmosphere is a very important element of the earth’s physical environ- ment. The atmosphere along with land and water, i.e. lithosphere and hydro- sphere has made it possible to create a favourable environment for the origin. growth and sustenance of living creatures on the earth. It is known as life word of Biosphere.

Besides providing gases essential for human, plants and animals on the earth the atmosphere helps in the distribution and circulation of heat and moisture on the earth’s surface. It is due to the maintenance of a well-balanced proportion of gases in the earth’s atmosphere a congenial temperature favorable to human and other living-beings privalis on the earth. Again due to movement of air many places to some extent get rid of exces sive cold and hot conditions. This atmosphere obstructs the sun’s very harm- ful ultraviolet radiations reaching the earth.

The formation and change of weather and climate also take place in the atmosphere. It is due to atmosphere rainfall occurs on the earth and, plants and animals thrive, On the whole, the existence of lives on the earth cannot be imagined without atmosphere. That is why the place of earth is considered topmost and unique among the planets in the solar system.

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

Ans: Wind is caused by differences in the atmospheric pressure. When a difference in atmospheric pressure exists, air moves from the higher to the lower pressure area, resulting in winds of various speeds. On a rotating planet, air will also be deflected by the Coriolis effect, except exactly on the equator.

The factors of origin of wind are: (i) Temperature variations, (ii) Pressure Gradient.

The wind, that is movement of air, basically depends on the temperature difference of the atmosphere and it’s closely associated atmosphere pressure.

When air temperature of a place increases, the air becomes lighter and moves up, and a low pressure is formed at that place. As a result of this air movement takes place from high pressure region characterized by relatively low temperature to this law pressure region. It means that the pressure gradient resulting from the pressure difference starts the movement of air. This air movement always takes place from relatively a high pressure region to a low pressure region, that is in the direction of pressure gradient. The wind velocity, however, on the pressure gradient force.

The warm air of the equatorial region which moves up from the earth’s surface blows towards the high latitude region and again descends in the polar region. Again, that air from the polar region moves towards the equatorial region as cold air through the lower atmosphere.

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: Wind is caused by differences in atmospheric pressure, which are mainly due to temperature differences. When a difference in atmospheric pressure exists, air moves from the higher to the lower pressure area, resulting in winds of various speeds. On a rotating planet, air will also be deflected by the Coriolis effect, except exactly on the equator. 

(i) Pressure Gradient Force: The force generated from the atmospheric pressure difference on the earth’s surface is known as the pressure gradient force, the velocity of wind becomes more if the pressure difference between two places is more or the pressure gradient is more steep. As the air movement always takes place from high at pressure region to low air pressure region, the pressure gradient determines the wind direction also.

(ii) Gravitational Force: The envelope of the atmosphere surrounding the earth itself is being pulled by the earth’s gravitation. We know that as the gravitational force decreases with the increasing altitude, the air pressure falls. In this way a vertical movement of air in the atmosphere from high pressure to low pressure continues. Due to the gravitational force the movement of air from one place to another becomes curved instead of straight line. 

(iii) Centrifugal Force: Due to the earth’s rotation on its axis, an out- ward force is generated from its center. This is called Centrifugal force. This Wind direction witnesses slight deflection due to it. This force deflects the wind towards right in clockwise direction in the northern hemisphere, and wards left in the anticlockwise direction in the southern hemisphere.

(iv) Frictional Force: The frictional force acts negatively in determining wind velocity. If the wind blows considerably above the earth’s surface, the amount of friction become lower and as such wind velocity increases.

This force not only slows down the wind velocity but also changes the wind direction. The impact of frictional force will also be significantly less if wind blows through water or snow-covered areas.

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: 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, Gaspard 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.

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

Ans: Coriolis force is an apparent force caused by the earth’s rotation. The Coriolis force is responsible for deflecting winds towards the right in the northern hemisphere and towards the left in the southern hemisphere. This is also known as ‘Ferrel’s Law’. 

The deflection is more when the wind velocity is high. The Coriolis force acts perpendicular to the pressure gradient force. The pressure gradient force is perpendicular to an isobar. The higher the pressure gradient force, the more is the velocity of the wind and the larger is the deflection in the direction of wind.

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

Ans: The most important though is the Earth’s gravitational force. As gravity compresses the Earth’s atmosphere, it creates air pressure the driving force of wind. The force actually responsible for causing the movement of air though is the pressure gradient force.

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

Ans: The naming of wind is done based on wind direction. In other words, the name of a wind is given according to the direction from which the wind blows. For instance, the wind that blows from west direction is called westerly wind, from north-east in north-easterly wind, etc.

The air mass present in the wind absorbs the physical characteristics like heat, moisture, etc of the region through which the wind blows. That is why the wind blowing over the ocean contains large amounts of water vapour. On the other hand, the wind blowing over the tropical deserts generally remains dry and hot. On the earth’s surface, the direction from which wind comes is  called windward, and the direction to which it blows is called Leeward.

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

Ans: In weather study the knowledge of wind direction and velocity is very important. One can understand the direction of wind in an area by merely observing the movement of clouds, smoke, water waves, etc. But, the correct direction of wind can be determined with the help of an instrument called Wind Vane. Generally the wind direction is expressed in terms of angle in degree with reference to the magnetic north. For example, if the wind in an area blows in magnetic north direction, its direction will be 00. On the other hand, when the wind blows exactly towards east, its direction will be 900.

The velocity of wind is measured with the help of an instrument called Anemometer. Nowadays different types of Anemometer instruments are available. Among them, with the help of an instrument called Anemograph the direction and velocity of wind are automatically recorded. A British Scientist Sir Francis Beaufort developed a 0-12 Number Scale of wind velocity in 1805 to understand the nature and impact of different types of wind. This is known as Beaufort Scale.

Another important point to know is that the velocity of wind is ex- pressed in Knot. The wind velocity of 1 Knot means 1 nautical mile per hour, that is, 1.854 Kilometre per hour or 30.9 meter per minute. Knot is the basic unit of velocity of wind.

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

Ans: The way movement of wind is a complex process, the same way it is very difficult to find out exactly the number of different types of wind on the whole earth. However, mainly based on the spatial extent the winds can be broadly divided into three groups–

(i) Primary Circulation: The air movement covering the whole earth based on the distribution of permanent pressure belts on the earth’s surface is called Primary Wind Circulation. The primary circulation pattern which prepares necessary environments for other lower order circulation patterns. Sub-tropical and polar high pressure belts to the equatorial and sub-polar low pressure belts in both the hemispheres. Such winds, westerlies and polar easterly winds fall under this primary wind system.

(ii) Secondary Circulation: The wind circulation resulted in different parts of the earth due to variation in the nature of landforms and differences in the distribution of land and water bodies, and regional and seasonal variation in temperature pressure are generally known as Secondary Circulations. Among the secondary circulations, cyclone, anticyclone, air mass, front, monsoon, etc. are the most important. These winds bring about significant weather change in an area and sometimes make the atmospheric condition very unstable.

(iii) Tertiary Circulation or Local winds: There are some winds which are very much visible in different parts of the earth at local level. Basically, the wind circulation that takes place in a limited area depending on some purely local factors, viz. topographic variation and altitude variation, is known as local Wind or Tertiary Circulation. It is also called temporary air movement. Although the impact of this local wind is quite notable in determining the weather of an area, its zone of influence is very limited. Among the local winds, sea breeze, land breeze, mountain wind, valley wind, etc are worth mentioning.

Q.15: What is a cyclone? What are its types? Mention briefly how it influences the climate of a place.

Ans: The state of atmospheric circulation in which high velocity wind takes a cyclic path around a low pressure centre is called cyclone. 

The cyclone mainly is of two types: 

(i) Tropical cyclone and (2) Extra Tropical cyclone.

The impact of cyclone is significant in the climate of a place. As a very high amount of energy is released from the well developed tropical cyclones, such cyclones are quite violent, destructive and harmful in nature. Typhoon, Hurricane, cyclone, Bardoisila etc. are such type of Tropical cyclone. 

The fronts are created as a result of the meeting of the cold polar wind and tropical warm wind. It is because of the extra Tropical cyclone the temperate region often witnesses change in weather with unstable atoms fall. When the air of higher layer of such cyclones becomes very cold, it spheric condition. These cyclone often cause dense clouds and heavy rain often results in hail and thunderstorm. But after sometime the sky becomes clear without any rainfall and clouds. Of course, due to mild air movement it remains somewhat cold.

Q.16: What is an Air mass? How is it classified? What is its contribution in determining the climate of a place?

Ans: Air mass is a body of air with horizontal uniform levels of temperature, humidity and pressure. They are classified according to the temperature and moisture characteristics of their source regions.

it can be mainly classified into four divisions.

(a) Tropical continental Air mass (CT).

(b) Tropical Maritime Air mass (MT).

(c) Polar continental Air mass (CP).

(d) Polar Maritime Air mass (MP).

These air masses can be further divided depending on their temperature and degree of dynamism.

Weather changes take place if there arises a special kind of air mass. The progress of a front brings about changes in air temperature and humidity, and results in the formation of clouds in the frontal region. 

An immense body of air having homogenous temperature and humidity Conditions is called air mass. Actually, when the air in a wide area remains Mtable for considerably long period of time, its lower layer is influenced by the underlying land surface, and in the process this air absorbs the prevailing temperature and humidity character of the surface layer. That is why a considerable variation in air temperature and humidity is observed vertically up from the land surface. Thus whenever an air mass having some special character is formed, it starts diverging. This diverging air mass determines the weather of the area through which it passes Naturally, the air mass is formed in the zone of divergence itself.

We should know that apart from wwm and cold condition of air, the vertical distribution of temperature abo indicates the stability and instability conditions of the air mass liven though the temperature of air mass changes when it moves after leaving the source region, one can identify the source region of that air mass by studying its horizontal distribution of temperature and humidity. It is only because of the movements of dynamic air mass, conduction of heat takes place from warm region to cold region and thereby an equilibrium in atmosphere temperature is maintained. When a divergent area but also experience change in temperature and humidity condition of its own.

Q.17: 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, humidity, pressure, density, etc come closer from opposite directions, they do not mix readily and form an air surface discontinuity or line of discontinuity between them. This is called Front. Weather changes take place in fronts.

The formation of a front requires two special conditions of the two air masses (1) between two air masses, one should be colder and heavier than the other, and (2) air movement should be convergent so that two air masses from opposite directions moves closer to one another. 

The relationship between climate and front are:

The pressure gradient takes a reverse direction in the frontal region. As result of it air eliminates from there. The progress of a front brings about changes in air temperature and humidity, and results in the formation of clouds in the formation of clouds in the frontal region.

As a result of the meeting of two characteristically contrasting air masses the and comparatively warm lighter air comes over the cold heavier air maintains a state of equilibrium. Air shift occurs in the frontal region due the reversal of the direction of pressure gradient. Moreover, a front brings about changes in air temperature and humidity, and results in the formation of clouds in the frontal region.

Q.18: What do you mean by ‘monsoon wind’? How is it formed? In which areas of the earth its impact is quite distinct?

Ans: A monsoon is a seasonal change in the direction of the prevailing, or strongest, winds of a region. Monsoon winds are caused when the air over land gets heated and rises, causing winds to blow from the ocean towards land. 

The air over the land gets heated and rises. This causes the winds to flow from the oceans towards the land, which gives rise to Monsoon winds.

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.

Q.19: Write with reasons:

(a) Why do all weather phenomena occur mainly in troposphere?

Ans: The lowermost layer of the atmosphere touching the earth’s surface in the troposphere l. The troposphere alone contains about three fourth of total atmosphere gases and almost entire water vapor and dust particle. The temperature in the lower part of this layer is more and it gradually decreases with increase in height. So the all weather phenomena like clouds, rains, cyclones, storms, etc occur in the troposphere.

(b) What would be the consequences of the increase in the amount of carbon dioxide?

Ans: The Earth’s atmosphere is divided into a number of layers. Assigning from the surface to a height of 8 kilometres at the poles and 18 kilometre at the equator is the troposphere, named after the Greek word for mixing because in this way the atmosphere is circulated by winds. conditions in the troposphere vary from region to region and from time to time; the state of the atmosphere in any given place at any given time is called weather. 

As a result nature tries to restore energy balance in that layer by various means, such as convection, and the effects of those processes we call weather. And hence all the weather phenomena take place in the troposphere.

(c) What is the reason behind the prevalence of high pressure of dry air than moist air? 

Ans: Pressure changes mainly due to expansion and contraction of the air. In the atmosphere, the dry air exerts more pressure than most air. As dry air consists mostly of nitrogen and oxygen and other gases which exerts high pressure on the other hand in moist air-water vapour contain increases, the weight of which is less than nitrogen and oxygen. So moist air is lighter than dry air.

(d) How does the ozone layer help the living beings?

Ans: Ozone is a gas in the atmosphere that protects everything living on the Earth from harmful ultraviolet (UV) rays from the Sun; without the atmosphere’s ozone, living organisms could not exist Ozone layer present in the stratosphere at an altitude of about 40-50 km is a vary useful layer for the living world. The ozone molecules present in the ozone layer have the ability to absorb the harmful ultraviolet rays of the sun.

(e) Why is the air pressure highest at the sea level?

Ans: Atmospheric pressure changes at different altitudes. Pressure is greatest at sea level and decreases with height. Air is heaviest at sea level because the air molecules are compressed by the weight of the air above them. Air becomes lighter farther away from Earth’s surface as the air molecules become separated by more space.

(f) What is the reason behind variation in vertical extent of the troposphere in the polar region and equatorial region?

Ans: The reason behind variation in vertical extent of the troposphere in the polar region and equatorial region is that the maximum vertical height of troposphere in the equatorial region is about 16 km which it is 10 km in the polar region Because of its the inclined position of the earth towards the poles.

(g) Why is horizontal movement of wind parallel to the earth’s surface not felt in the equatorial low pressure belt?

Ans: The force generated due to earth’s rotation deflects the wind towards right in clockwise direction in the northern hemisphere and towards left in the anticlockwise direction in the southern hemisphere.

Along the equator lies a belt of low pressure known as the “equatorial low or doldrums”. Low air pressure in equatorial regions is due to the fact that hot air ascends there with a gradual decrease in temperature causing thinness of air on the surface.

Q.20: Give very short answer to the following questions:

(a) What is the vertical extent of the atmosphere? 

Ans: 10,000km. (approximately).

(b) Up to what altitude from the earth’s surface does chemical composition of gases remain almost same?

Ans: Up to 80 km altitude from the earth’s surface does chemical composition of gases.

(c) What is the name of the boundary line between homosphere and heterosphere?

Ans: Menopause.

(d) What is Ferrel’s Law?

Ans: Ferrel’s law means wind is deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere due to the coriolis force.

(e) What is Beaufort scale? Write briefly about the utility of this scale with example.

Ans: The Beaufort scale is used to measure the speed of the wind. It is based on observations rather than actual measurements. It is widely used to measure wind speed today. There are 12 levels, including 0 for “ no wind”  in the wind force scale introduced by Beaufort.

Utility: This Scale helps to understands the nature of the wind and it impact of different types of wind. For example, the wind velocity of 1 knot means 1 nautical mile per hour that is 1.854 kilometre per hour on 30.9 meter per minute.

(f) Write in brief about lapse rate?

Ans: The Lapse Rate is the rate at which temperature changes with height in the Atmosphere. Lapse rate nomenclature is inversely related to the change itself: if the lapse rate is positive, the temperature decreases with height; conversely if negative, the temperature increases with height.

Q.21: Write short note:

(a) Air temperature, earth’s surface height and air pressure relationship.

Ans: There is a close relationship among the pressure, temperature and the height of earth’s surface. If the temperature of air increases then its pressure decreases again if temperature decreases its pressure increases. Again pressure is a variable entry of the atmosphere. Atmospheric pressure increases or decreases according to the changes of seasons. Temperature varies according to the diverse height of the surface.

The two vital factors that control and influence the atmosphere pressure of a place are temperature and elevation of the place. When the temperature is high, the air expends and becomes high leading to lower density, which brings about low pressure.

(b) Wind system.

Ans: The formation of the wind system begins with the sun’s radiation, which is absorbed differently on the earth’s surface. The earth’s surface is heated differently because of scenarios like cloud cover, mountains, valleys, water bodies, vegetation and desert lands.

As a result of this uneven heating, there are bound to be earth surfaces that vary a lot in temperature. Air on surfaces with higher temperatures will then begin to rise because it is lighter (less dense). As the air rises, it creates low atmospheric pressure. Air on surfaces with cooler temperatures sink (do not rise). The sinking creates higher atmospheric pressure. This behaviour or warm gases or liquids moving upward and being replaced by cooler particles is called Convection. The energy moving during convection is called convectional current.

(c) Trade wind.

Ans: The trade winds are winds that reliably blow east to west just north and south of the equator. The winds help ships travel west, and they can also steer storms such as hurricanes.

The trade winds have been used by sailors for centuries. Sailors traveling from Europe or Africa used the trade winds to travel to North or South America. Just like airplanes can use the wind boost from the jet stream to shorten a journey flying east, sailors can use the trade winds to shorten a sea journey when sailing west.

(d) Horse latitude.

Ans: The horse latitudes are located at about 30 degrees north and south of the equator. It is common in this region of the subtropics for winds to diverge and either flow toward the poles (known as the prevailing westerlies) or toward the equator (known as the trade winds). These diverging winds are the result of an area of high pressure, which is characterized by calm winds, sunny skies, and little or no precipitation.

According to legend, the term comes from ships sailing to the New World that would often become stalled for days or even weeks when they encountered areas of high pressure and calm winds. Many of these ships carried horses to the Americas as part of their cargo. Unable to sail and resupply due to lack of wind, crews often ran out of drinking water. To conserve scarce water, sailors on these ships would sometimes throw the horses they were transporting overboard. Thus, the phrase ‘horse latitudes’ was born.

(e) Roaring forties.

Ans: The region of the southern hemisphere especially lying between 40-50 latitudes is covered by ocean. So, the westerlies are very intense in this region. That is why the navigators call the latitude belts as forties as Roaring Forties.

(f) Anticyclone.

Ans: Movement of air from a high pressure centre towards the low pressure surroundings. 

When after formation of a high pressure belt covering a wide area, a divergent wind from its centre takes a cyclic path, it is called Anticyclone. The Meteorologist Sir Francis Galton for the first time used the term ‘Anticyclone’ in 1861. Actually, the anticyclone is an atmospheric condition which is completely opposite of cyclone. As compared the cyclone the contribution of anticyclone towards determining the weather of an area is very insignificant. The anticyclones are formed in the sub-tropical and high latitude regions of both the hemisphere. 

Basically, the anticyclones are of two types:

(1) Sub-Tropical Warm Core Anticyclone and (2) High-Latitude Cold Core Anticyclone.

(g) Jet stream.

Ans: Jet streams are a narrow belt of high altitude(above 12,000m) westerly winds in the troposphere. Their speed varies from about 110Km/h in summer to about 184Km/h in winter.

Jet stream is a very high velocity wind (640 km per hour) blowing towards east in the tropical high altitude regions, i.e. in the tropopause. They bring about immense changes in the weather and climate of a place. Westerlies are also a type of jet stream.

(h) Isobar.

Ans: Isobars are atoms (nuclides) of different chemical elements which differs in the chemical property but has the same physical property. So, we can say that isobars are those elements which have a different atomic number but the same mass number.

Q.22: Write the differences:

(a) Tropical cyclone and extra-tropical cyclone.

Ans: The extratropical cyclone differs from the tropical cyclone in number of ways.

The extratropical cyclones have a clear frontal system which is not present in the tropical cyclones.

Extratropical cyclones cover a larger area and can originate over the land and sea. Whereas the tropical cyclones originate only over the seas and on reaching the land they dissipate.

The extratropical cyclone affects a much larger area as compared to the tropical cyclone. The wind velocity in a tropical cyclone is much higher and it is more destructive.

The extratropical cyclones move from west to east but tropical cyclones, move from east to west.

(b) Sea breeze and breeze.

Ans: The differences between sea breeze and land breeze are:

Land Breeze:

The land breeze is the breeze in which wind blows from the land to sea.

It occurs at the night time or early morning.

It occurs in the winter and autumn season.

Sea Breeze:

The sea breeze is the breeze in which wind blows from the sea to land. 

It occurs at the daytime.

It occurs in the summer and spring seasons.

(c) Horizontal wind and vertical wind.

Ans: The differences between horizontal wind and vertical wind are: 

Horizontal Axis Wind:

(i) A horizontal axis wind turbine is the one whose axis of rotation is horizontal.

(ii) HAWT is the abbreviation used for horizontal axis wind turbine.

(iii) For the horizontal axis wind turbine, the axis of rotation of turbine is parallel to the wind stream.

(iv) In HAWT, the gearbox is installed at the top of the turbine tower.

(v) The design and installation of a horizontal axis wind turbine is complex.

Vertical Axis Wind:

(i) A wind turbine is called a vertical axis wind turbine if its axis of rotation is vertical.

(ii) In VAWT, the gearbox is installed at the bottom of the turbine.

(iii) VAWT is the abbreviation used to denote the vertical axis wind turbine.

(iv) The design and installation of a vertical axis wind turbine is comparatively simple.

(v) For the vertical axis wind turbine, the axis of rotation of the turbine is perpendicular to the wind stream.

(d) Wind and air mass.

Ans: An air mass can be described by properties such as its density, temperature, pressure, and movement. Winds are currents of air that move across the earth’s surface and develop when two adjacent air masses have different densities. The movement of air masses above the earth’s surface helps to create wind, and help to move the air masses that predict our weather. The wind helps to shape the atmosphere and affects weather patterns from the upper atmosphere to the ocean surface.

(e) Cold front and warm front.

Ans: The differences between cold front and warm front are:

Cold fronts often come with thunderstorms or other types of extreme weather. They usually move from west to east. Cold fronts move faster than warm fronts because cold air is denser, meaning there are more molecules of material in cold air than in warm air.

Warm fronts usually show up on the tail end of precipitation and fog. As they overtake cold air masses, warm fronts move slowly, usually from north to south. Because warm fronts aren’t as dense or powerful as cold fronts, they bring more moderate and long-lasting weather patterns. Warm fronts are often associated with high-pressure systems, where warm air is pressed close to the ground. High-pressure systems usually indicate calm, clear weather.

(f) Mountain wind and valley wind.

Ans: The differences between mountain wind and valley wind are:

These winds are opposite from each other. Mountain winds blow from mountain towards valley after sunset, when mountain cools down and valley zone is comparatively warmer. While valley breezes occur when the warm air rises up the sides of the valley, warm air in a mountain breeze will rise up the middle.

(g) Cyclone and anticyclone.

Ans: The differences between cyclone and anticyclone are:


(i) A cyclone is an area of low pressure where air masses meet and rise.

(ii) It indicates bad weather, like rain and clouds.

(iii) Winds in a cyclone blow counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.


(i) An anticyclone is an area of high pressure where air moves apart and sinks.

(ii) It indicates fair weather

(iii) Winds in an anticyclone blow clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere.

(h) Cold wave and hot wave.

Ans: The differences between cold wave and hot wave are:

Cold Wave:

Cold Wave is a speedy fall in temperature over a 24-hour period that requires considerably enhanced protection for agriculture, industry, commerce, and social activities.

When the minimum temperature is 10 degrees Celsius or below and is 4.5 degrees Celsius less than normal for two consecutive days. 

Cold and dry winds blow in northwest India during the months of November and April. 

Hot wave:

Period of prolonged, abnormally high surface temperatures relative to those normally expected. It may span from several days to several weeks.

When the maximum temperature of a region increases by at least 40 degrees Celsius for Plains and at least 30 degrees Celsius for Hilly regions.

According to IMD, For five or more consecutive days during which the daily maximum temperature surpasses the average maximum temperature by 5 degrees Celsius or more.

Hot weather is experienced in certain parts of India during the months of March to July. 

(i) Troposphere and stratosphere.

Ans: The differences between troposphere and stratosphere are:


The troposphere is the atmosphere’s lowest level.

Troposphere is the zone of convection current.

The temperature drops with height.

Height in troposphere fluctuates from 18 km at the equator to the 8 km at the poles.

This layer contains water vapours, clouds, and dust particles.


The stratosphere is a second layer of Earth that exists above the troposphere.

Stratosphere is the non-convective zone of atmosphere.

The temperature rises with height.

Height in stratosphere ranges till 50 Km.

This layer is devoid of water vapours, clouds, and dust particles.

Q.23: Find out the correct answer:

(a) The instrument used for determination of wind velocity is _____.

(i) Anemometer 

(ii) Wind vane

(iii) Beaufort scale 

(iv) Hydrometer

Ans: (i) Anemometer.

(b) The most important reason for air movement is __.

(i) Gravitational force

(ii) pressure difference

(iii) Humidity difference

(iv) Centrifugal force

Ans: (ii) Pressure difference.

(c) Where is ozone layer located?

(i) Troposphere 

(ii) Thermosphere

(iii) Mesosphere 

(iv) Stratosphere

Ans: (iv) Stratosphere.

(d) The name of the cyclone formed in the coastal region of the Pacific Ocean in the east is ___.

(i) Typhoon 

(ii) Hurricane 

(iii) Willy-Willy 

(iv) Cyclone

Ans: (i) Typhoon.

(e) What is amount of oxygen in the atmosphere in terms of volume?

(i) 32.47% 

(ii) 29.01%

(iii) 20.94% 

(iv) 78.08%

Ans: (i) 20.94%.

(f) The unit of wind velocity is ____.

(i) Knot  

(ii) Millibar

(iii) Percentage 

(iv) Degree

Ans: (i) Knot.

(g) Monsoon wind belongs to which of the following class?

(i) Local winds  

(ii) Secondary circulation

(iii) Primary circulation

(iv) Permanent wind

Ans: (ii) Secondary circulation.

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