NCERT Class 11 Geography Chapter 10 Atmospheric Circulation and Weather Systems

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NCERT Class 11 Geography Chapter 10 Atmospheric Circulation and Weather Systems

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Atmospheric Circulation and Weather Systems

Chapter: 10

GEOGRAPY [ PART – I ]

SHORT ANSWER TYPE QUESTIONS

Q.1. Give one term for the following:

(i) A large body of air with uniform properties of temperature and humidity.

Ans. Airmass.

(ii) The contrast line of air masses of different properties.

Ans. Front.

(iii) A zone of calmness in the vicinity of equator on both sides.

Ans. Doldrums.

(iv) Winds blowing over south and south-east Asia that are characterised by the reversal in wind direction with change in season.

Ans. Monsoon winds.

(v) A hot wind of local importance in Alps.

Ans. Foehn.

(vi) A warm and dry wind moving down the Rockies and commonly called snow- eater.

Ans. Chinooks.

(vii) A very cold and dry wind with high velocity blowing from the Alps towards the Mediterranian sea.

Ans. Mistral.

(viii) An airmass originating over tropical oceans.

Ans. Tropical cyclone.

Q.2. What is millibar?

Ans. The air pressure is measured as the force per unit area. The unit used by meteorologist for this purpose is millibars (mb).

Q.3. Most of the hot deserts of the world lie on the western margins of the continents. Why?

Ans. Deserts are areas of low rainfall. Hot deserts lie in the trade winds belt. These winds give rain on the eastern margines but trade winds got dry as then reach western margins. These are offshore winds so. These are known as trade winds, deserts such as Alacama, Kalahari, Sahara etc.

Q.4. What is Coriolis force and who discovered it first?

Ans. On the rotating earth a point on equator moves fastest and the speed of points decrease towards poles. It is because of this difference in speed that any moving body such as winds and ocean currents get deflected when they move towards the equator or away from the equator. This is the force of deflection. It was first discovered by Coriolis, a French physicist and hence called Coriolis force.

Q.5. What are the three kinds of planetary winds?

Ans. The three kinds of planetary winds are:

1. Trade winds.

2. Westerlies.

3. Polar winds.

Q.6. Name the instrument by which the atmospheric pressure is measured.

Ans. Barometre.

Q.7. What are subtropical highs?

Ans. Subtropical highs are the high pressure areas found along 30°N and 30°S.

Q.8. Name the hot winds which blow in U.S.A. and Canada.

Ans. Chinooks.

Q.9. What is the normal rate of pressure decrease with altitude?

Ans. 1 cm for every ascent of 110 metres.

Q.10. Name the hot winds which blows in Alps.

Ans. Foehn.

Q.11. Name the cold wind which blows from high mountains to the Mediterranean coast of France.

Ans. Mistral.

Q.12. Name the hot wind which blow in north India in summer.

Ans. Loo.

Q.13. What is the standard sea level pressure in millibars?

Ans. 1013.25 mb.

Q.14. What are the forces acting on the horizontal winds near the earth’s surface?

Ans. 1. The pressure gradient force.

2. The frictional force.

3. The Coriolis force.

Q.15. Where do you find maximum and minimum deflection of winds by Coriolis effect?

Ans. At poles and equator, respectively. At equator it is almost nil.

Q.16. What are tertiary winds?

Ans. Tertiary winds are generated by immediate influence of the surrounding terrains. These winds include mountain and valley winds, land and sea breezes and other local winds such as loo, foehn, chinook, mistral etc. Some are hot and some are cold winds. These all are local winds.

Q.17. What are isobars?

Ans. An isobar is an imaginary line drawn through the places which have equal atmospheric pressure reduced to sea level. The spacing of isobars indicates the pressure gradient. The close spacing of isobars expresses the high pressure gradient and wide spacing of isobars, a low pressure gradient.

Q.18. What is an airmass?

Ans. An airmass is a large body of air whose physical properties, especially temperature and moisture control are relatively uniform. Naturally an airmass extends over hundred of kilometres and consists of several layers each having homogeneous conditions.

Q.19. What is the literal meaning of chinook?

Ans. Snow-eater.

Q.20. Why is a stable airmass dry?

Ans. When a warm tropical airmass moves over a cold surface, the lower layers are cooled. A surface inversion of temperature develops. The lower layers become stable under the compression of upper layers. There is no upward rising of air. Consequently, there is no process of condensation of clouds or rainfall. Hence a stable airmass is dry.

SHORT ANSWER TYPE QUESTIONS

Q.1. What is the unit used in measuring pressure? Why is the pressure measured at station level reduced to the sea level in preparation of weather maps?

Ans. The unit used in measuring pressure is mb and pascals. It is reduced to the sea level in the preparation of weather maps as the gravity of air at the surface is denser and hence has higher pressure.

Q.2. How do air pressure and winds influence the weather?

Ans. The atmospheric pressure is a very important factor in producing changes in our weather as it is closely linked with other elements of weather and climate in a cause-effect relationship. To a great extent, the pressure depends on temperature and in turn, controls winds and affects rainfall. High pressure in an area stands for clear sky and calm condition, whereas low pressure encourages upward movement in the air and cloudiness. Variation in the pressure causes horizontal movement of air called winds. Winds transport heat and moisture from one region to another and thus help in the occurrence of precipitation and affect both temperature and humidity. To a weatherman, the atmospheric pressure is very important factor as it provides him with a major clue for forecasting weather.

Q.3. Define the pressure gradient.

Ans. It is the rate at which pressure rises or falls over a given distance. Gradient is steep when pressure changes in a short distance, and it is mild when pressure changes at long distances. On a map, the nature of pressure gradient is shown by the spaces between isobars. If isobars are closer together, pressure gradient is steep and vice versa.

Pressure gradient and winds: Steep gradient means a great difference between high and low pressure belts, hence, the winds blow faster from high to low. Mild gradient on the other hand means mild slope and winds are light.

Q.4. While the pressure gradient force is from north to south, i.e. from the subtropical high pressure to the equator in the Northern Hemisphere, why are the winds north easterlies in the tropics?

Ans. Rotation of the earth also affects the wind movement earth’s rotation and the coridis force deflect the direction of winds. This is known as Ferrel’s law of deflection. According to this winds are deflected to their right in the northern hemisphere and to their left in the southern hemisphere.

Q.5. Name the seven pressure belts of the earth.

Ans. The seven pressure belts of the earth are:

1. Equatorial low pressure belt.

2. Subtropical high pressure belt (Northern Hemisphere).

3. Subtropical low pressure belt (Southern Hemisphere).

4. Sub-polar low pressure belt (Northern Hemisphere).

5. Sub-polar low pressure belt (Southern Hemisphere).

6. Polar high (Northern Hemisphere).

7. Polar high (Southern Hemisphere).

Q.6. Explain the land and sea breezes.

Ans. Sea Breezes: In the day, the land is heated faster than water. The air above land is heated more than that at sea. The air after being heated becomes lighter and rises. The pressure on the land is lower than that on the sea. The winds move from high to low pressure areas. Hence the winds on the sea move towards the land and the air above the land rises and moves towards sea at higher altitudes. This produces a circulation of air which is evident from the above figure. Sea breeze is cooler and therefore when it reaches the land it cools the atmosphere and relieves people from the heat.

Land Breezes: The condition is different in night. Land is cooler than the sea in the night. The pressure of air is higher on the land than on the sea. Winds blow from the lands towards the seas and the air of the sea brings lighter air movement towards land at high altitudes and descends down on the land. The sea becomes cooler on account of the cool breezes from the land. The breezes have a limited area of influence and therefore have a local importance.

Q.7. What are the geotrophic winds?

Ans. When isobars are straight and when there is no friction, the gradient force is balanced by the Coriolis force and the resultant winds blow parallel to the isobars. These winds are known as the geotrophic winds.

Q.8. What is atmospheric pressure?

Ans. Like any other thing air too, has weight. Since it has weight, it exerts pressure on the surface of the earth. This pressure is termed as atmospheric pressure. It is greatest at sea level and decreases with the altitude. It has been calculated that at sea level-the pressure of air is about 1 kg per square centimetre. But with the increase of height above sea level, the pressure decreases because the overlying column of air goes on getting shorter. It is expressed in millibar or inches or centimetres. But the most common is the millibar. A pressure of 1000 mb is equal to 75 cm in-height of mercury in mercury column. Standard sea-level pressure is about 1013 mb.

Q.9. Why does pressure decreases with altitude?

Ans. The pressure decrease with the altitude is termed as vertical distribution of pressure. On the average, there is a fall of one centimetre of mercury for every 110 metres of ascent, or 34 millibars per every 300 metres of height. This is because of difference in the compressibility of the air. The lowest layer of the air is more compressed and exerts high pressure. In contrasts, the higher layer is less compressed and exerts low pressure.

Q.10. What is the relationship among heating, temperature and pressure?

Ans. There is a close relationship among heating, temperature and pressure. The hot air raises the temperature and lowers the pressure. As we know that hot air is light and hence it exerts less pressure.

The heated air rises up and its water vapour condenses and falls as rain. Thus, the regions of high temperature and low pressure remain cloudy and get comparatively more rain.

The relationship between temperature and pressure is:

High temperature – Low pressure

Low temperature – High pressure

Q.11. Explain the following:

(a) Roaring Forties.

Ans. Roaring Forties: In Southern Hemisphere, between 40° and 50°, there is mostly sea and very little land. Westerlies, therefore, blow without any hindrance and with great velocity. Their roar can be heard far and wide. Hence, the westerlies are called roaring forties in this part of the ocean. They are also called brave westerlies.

(b) Horse Latitudes.

Ans. Horse Latitudes: The subtropical belts or the regions lying between 30° and 35° are known as horse latitudes. This zone is characterized by weak and variable winds and is calm. Hence, the sailors were compelled to throw away some of the horses over board in order to lighten the ships, so that the ships could move. This part, therefore, came to be called Horse latitudes. But now both the subtropical belts are known by this name.

Q.12. Draw diagrams of mountain and valley breezes and explain.

Ans. Mountain and valley breezes are periodic winds formed due to variation in gravity. They are formed in the mountain slopes.

(a) Mountain Breezes: They occur during nights when the air at upper slopes becomes dense, compressed and heavy due to more cooling and ground radiation. At the same time lower slopes are relatively warmer and have low pressure. Now heavy air descends, it is termed as mountain breeze or gravity winds.

(b) Valley Breezes: They occur during the day time. Due to insolation, valley bottom gets heated, air becomes light and it ascends upslopes. It is termed as valley breeze.

Q.13. Explain mistral and loo winds.

Ans. (a) Mistral: Mistral is a cold and dry wind. It blows from higher lands to the Mediterranean coast of France. It moves through the Rhone valley. It blows with a high velocity. Its speed is more than 60 km per hour. Hence, the gardens and orchards have to be protected from it by thick hedges of cyprus trees. The smaller houses have their doors on the opposite side of the mistral, so that they can save themselves from cold. It is very cold and a dry wind. It reduces the temperature below freezing point.

(b) Loo: Loo winds are hot and dry surface winds. They move in the northern plains of India during summer season especially in the months of May and June. They move from west to east. They cause loss of lives in the plains of Bihar and U.P.

Q.14. Why is the study of airmass important?

Ans. The study of air masses is necessary for the study of climatology.

1. Airmasses are related to atmospheric disturbances.

2. The distribution and location of airmasses is essential for understanding the regional climates.

3. Airmasses transfer heat from lower latitudes to higher latitudes.

4. The atmospheric disturbances develop along fronts of different air masses.

5. Air masses are associated with particular wind belts and they determine the weather characteristics of an area.

Q.15. Explain the relation between altitude of a place and atmospheric pressure.

Ans. There is an inverse relation between pressure and altitude. There is a rapid decrease of pressure with increasing height. The pressure decreases at the rate of 12 mtr for every 300 metres. The lower layer are denser than upper layers. The lower layers are compressed under the weight of the upper layers of the atmosphere. It is estimated that about one half of the atmosphere pressure his up to a height of 5 km. The following table show the standard pressure and temperature at selected levels:

Levels	Pressure	Temperature °C
Sea level	1013.25	15.2
1 km	898.76	8.7
5 km	540.48	-17.3
10 km	265.00	-49.7

Q.16. Distinguish between:

(i) Wind and air current.

Ans. Wind: Air that moves horizontally along the earth’s surface is called wind. The temperature and humidity varies in the wind system.

Air current: The vertical or nearly vertical movement of air is referred to as air current. 

(ii) Planetary and periodic winds.

Ans. Planetary winds: The primary winds are known as Planetary winds. Winds which blow throughout the year from one latitude to another in response to the latitudinal difference in pressure are known as planetary winds. These winds blow over the vast area of the continents and oceans. Westerlies are the examples of planetary winds.

Periodic winds: Winds changing their directions periodically with the change in seasons are called periodic winds. Monsoons are the best example.

(iii) Mistral and Foehn.

Ans. The Mistral is the name given to the strong, northerly or north-westerly wind experienced on the shores of the north-west Mediterranean. It is most prevalent during the winter. The wind is strong and may sometimes have a speed of over 100 km per hour. It is also very cold and harmful to plant life.

The Foehn is the name given to the hot dry wind which blows down the leeward slopes of mountains.

Foehn winds often blow with great velocity and cause much discomfort. They cause snow to disappear very quickly and thus make pasture available for animals sooner than would otherwise be the case. Similar winds blowing eastwards across the Prairies of North America from the Rockies are known as Chinook Winds.

(iv) Tropical and Polar Air Masses.

Ans. Tropical Air Masses: Tropical air masses are of two types-tropical continental and tropical maritime. Tropical continental is basically hot and dry. These air masses develop over Sahara area. Hot and dry winds invade Mediterranean Europe, Northern Mexico and Southern California in the form of scorching Sirocco.

Tropical maritime is considerably wider than tropical continental because of greater size. These air masses overlying the tropic seas are moist and saturated and are causes of heavy precipitation.

Polar Air Masses: Polar air masses are located over warmer high latitude oceans of North Pacific, North Atlantic and the entire sweep of Southern Hemisphere surrounding Antarctica. The air overlying these areas providing warmer currents is a highly efficient evaporator of readily available moisture. So polar maritime is less cold and more moist. North America and North Europe experience this type of airmass.

(v) Airmass and Wind.

Ans. 

Airmass	Wind
1. An airmass is a thick and extensive part of the atmosphere.	1. Wind is a horizontal movement of the air parallel to earth’s surface. It is a thin layer.
2. An airmass has uniform temperature and humidity.	2. The temperature and humidity varies in a wind system.
3. An airmass has many layers.	3. Its temperature decreases with altitude. It does not have distinct layers.
4. Its origin is related to source region such as polar or tropical and continental or maritime.	4. It is generated due to variation in the pressure system. It moves from high pressure belt to low pressure belts.

(vi) Temperature and Tropical Cyclones.

Ans. 

Temperate Cyclones	Tropical Cyclones
1. These cyclones are concentrated in the middle latitudes between 35° and 65° in both hemispheres.	1. These cyclones are notorious for their velocities and for carrying widespread destruction.
2. They are generally extensive having a vertical thickness ranging from 9 to 11 km and a diameter of about 1000 km.	2. They develop over oceans, particularly in tropical regions.
3. It is just like a spearhead having the shape of an upper mid ‘V’.	3. Their major climatic significance is in causing widespread rainfall.
4. The approaching temperate cyclones are noticed by the appearance of dark clouds in the background of white clouds.	4. Most of the tropical cyclones develop in a belt of 8° to 15° north and south latitudes.

(vii) Distinguish between Cyclones and Anti cyclones.

Ans. 

Cyclone	Anti cyclone
1. A cyclone is a centre of low pressure surrounded by high pressure on all sides.	1. Anti cyclone is a centre of high pressure surrounded by low pressure on all sides.
2. A cyclone has a converging system of winds which tend to blow towards the centre.	2. An anti cyclone has a diverging system of winds radiating from the centre.
3. Winds blow anti clockwise in the northern hemisphere and clockwise in the southen hemisphere.	3. Winds blow in a clockwise direction in the northern hemisphere and anti-clockwise in the southern hemisphere.
4. Cyclones are two types temperate and tropical cyclone.	4. Anti cyclones are of two types cold and warm.

Q.17. What are the isobars? Why are these reduced to sea level?

Ans. Isobar: Isobar is combination of two words (Iso = equal bar = pressure) so isobar is line of equal pressure.

Isobar is an imaginary line joining the places of equal pressure reduced to sea level. Pressure is reduced to sea level to eliminate the effects of altitude. If a place is 300 m high and actual pressure is 900 mtr its pressure is reduced to sea level would be 900 + 34 = 934 mtr because the pressure decreases at the rate of 34 mtr for every 300 metres. Isobars generally run in the east-west direction.

LONG ANSWER TYPE QUESTIONS

Q.1. Describe the global pattern of the horizontal distribution of pressure.

Ans. The Horizontal Distribution of Air Pressure in January: A study of the weather map of January reveals the flowing facts of the horizontal distribution of air pressure of this month:

1. Equatorial low pressure belt is situated south of the equator due to the south-ward apparent movement of the sun. Hence, the main low pressure cells are situated in Australia, South America and Africa.

2. The subtropical highs are located in the oceans in the Southern Hemisphere because the water is colder than the land in this month. It is summer there.

3. In Northern Hemisphere, high pressure cells are not properly developed in sub-tropical areas but appear as a long high pressure ridge.

4. There are two low pressure cells in the subtropical areas of Northern Hemisphere which locate over the Northern Atlantic and Northern Pacific Ocean. They are known as Icelandic.

5. In eastern central Asia, there is a fully developed and broad based high Alentian low pressure cell which has all the characteristics favourable for intense heating. No such cell with such a high pressure is found in North America.

The Horizontal distribution of Air pressure in July: A study of the weather map of July reveals the following facts of horizontal distribution of Air pressure:

1. The equatorial low pressure belt has shifted towards north of the equator due to the northward movement of the sun.

2. The whole pressure system has shifted to the north following the corresponding shift to the sun.

3. The subtropical high pressure belt of Southern Hemisphere extends continuously more in July than in January.

4. The subtropical high pressure belt of Northern Hemisphere exists over the North Atlantic and North Pacific Oceans.

5. The subpolar low is deep and continuous in Southern Hemisphere but it is only a faint oceanic low in Northern Hemisphere.

Q.2. Discuss the factors on which the atmospheric pressure depends.

Ans. Atmospheric pressure varies from place to place and from time to time. This variation depends on the following factors:

1. Altitude: At sea level, the air layers are dense, so it exerts greater pressure. Hence, the pressure is higher at sea level and it decreases with the altitude because the air becomes thinner and exerts lesser pressure. The loss of pressure is one centimetre for every ascent of 110 metres.

2. Temperature: The temperature also affects the pressure. The hot air expands and is lighter. Hence, it exerts less pressure. On the other hand, cold air contracts and is heavier, so exerts high pressure.

Thus, a low pressure belt lies near the equator because of high temperature and a high pressure belt lies on the poles because of low temperature. The following relation holds between temperature and pressure:

Formula: High Temperature – Low Pressure.

Low Temperature – Height Pressure.

3. Water vapour: The moist air is lighter and exerts lesser pressure than that of dry air because the water vapour is lighter than air.

4. Rotation of the earth: The rotation of the earth also affects the pressure distribution on the earth’s surface. A low pressure belt near the 60°N and 60°S is developed by the rotation of the earth. Due to the rotation of the earth, the air near 60°N and 60°S moves towards the sub-tropical regions. Thus, the low pressure belt develops near 60°N and 60°S and high pressure belt near 30° and 30°S.

Q.3. Discuss the factors affecting the speed and direction of wind.

Ans. The direction and speed of the winds are controlled by the combination of the following factors:

1. Pressure gradient force: The difference in the atmospheric pressure produces a force. The pressure gradient is strong where the isobars are close to each other and is weak where the isobars are apart. This is the driving force and wind moves from high pressure to low pressure.

2. Frictional force: The frictional force is present near the surface. The irregularities of the earth’s surface causes friction and obstructs the winds and influence their speed and direction. Over the sea surface the friction is minimal.

3. Coriolis force: The rotation of the earth about its axis affects the direction of the wind. This force is called the Coriolis force. Due to the Coriolis effect or force the winds are deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

4. Centripetal acceleration: Due to the inward acceleration of air towards the centre of rotation on the rotating earth, it is possible for the air to maintain a curved path about a local axis of high or low pressure. It is known as centripetal acceleration.

Q.4. Draw a simplified diagram to show the general circulation of the atmosphere over the globe. What are the possible reasons for the formation of subtropical high pressure over 30° N and S latitudes?

Ans. 

The possible reasons for the formation of subtropical high pressure over 30°N and S latitudes are as under.

1. The air at the Inter Tropical Convergence Zone (ITCZ) rises because of convection caused by high insolation and a low pressure is created. To this low pressure the winds from tropics-the easterlies, converge. The converged air rises upto the height of 14 km and moves towards the North and South Poles. This causes accumulation of air at about 30°N and S. Part of the accumulated air sinks to the ground and forms a subtropical high.

2. The cooling of air when it reaches 30°N and S latitudes is also another reason for the sinking down below near the land surface. The air flows towards the equator as the easterlies. The easterlies from the side of the equator converge in the ITCZ. In the middle latitude the circulation is that of sinking cold air that comes from the poles and the rising warm air that blows from the subtropical high. Thus it forms the subtropical high pressure over 30°N and S latitudes.

Q.5. In what ways the general circulation of atmosphere affects the movement of oceanic waters? What is ENSO?

Ans. The general circulation of the atmosphere also affects the oceans. The large scale winds of the atmosphere initiate large and slow moving currents of the ocean. Oceans in turn provide input of energy and water vapour into the air. These interactions take place rapture slowly over a large part of the ocean. The most important is the warming and cooling of pacific ocean. The warm water of the central pacific ocean. Oceans slowly drifts towards South American coast and replaces the cool peruvian current. Such appearances of warm water off the coast of Peru is known as the EL Nino. The EL Nino event is closely associate with the pressure changes in the Central Pacific and Australia. This changes in pressure condition over pacific is known as Southern Oscillation. The combined phenomenon of southern oscillation and EL Nino is known as ENSO. In the years when the ENSO is strong large scale variation in weather occur over the world. The arid West-coast of South America receives heavy rainfall drought occurs in Australia and sometimes in India and floods in China.

Q.6. The monsoon overrides the expected pattern of primary wind system. Explain.

Ans. The word monsoon has been derived from Arabic word mausim meaning season. The monsoon is a seasonal wind but it is so much associated with rain that it invariably means rainy season. In earlier times it was thought that the monsoon winds were land and sea breezes on large scales. Thus the monsoon were considered convectional circulation on a large scale. Among the current theories of origin of monsoon the one put forward, by and large, has the widest acceptance. According to him, the monsoon is a seasonal modification of the general planetary wind system. The monsoon is a south westerly wind and it is northward extension of the equatorial lows. The Asiatic monsoon region is a consequence of the interaction of both planetary and regional factors, both at the surface and in the upper troposphere.

Summer Monsoons: During summer, the tropical high pressure belt and the thermal equator are displaced northwards. In Southern Asia this movement is magnified by heating effects of the vast landmass. The equatorial westerlies embedded in tropical easterlies also move northwards. Their direction becomes from South-west to North-east. From oceans they move towards landmass and blow over Asian continent. These are south-westerly summer monsoons. During the winter, the tropical high pressure belt and the heated equator retreat south-wards because the sun now shines vertically overhead in Southern Hemisphere. In the Northern Hemisphere the normal trade wind is re-established. This is the winter monsoon. The direction of the wind is from north-east to south-west. In the Northern Hemisphere, the summer monsoon lasts from May to September. The winter monsoon blows from October to May. The summer monsoons give heavy rains, but the winter monsoons are dry winds and unless there is a passage of cyclonic storm or it has crossed the sea it may not bring any rain.

The monsoon winds blow over India, Pakistan, Bangladesh, Myanmar, Sri Lanka, Arabian Sea, Bay of Bengal, South East Asia. Northern Australia, China and Japan.

Q.7. Discuss the major types of air masses and how do they influence the area over which they move.

Ans. There are two basic air masses:

1. Polar. and 

2. Tropical, with strong differentiation of temperature.

They are subdivided into continental and maritime in terms of humidity. A continental airmass is relatively humid.

The properties of the air masses are modified by transfer of heat from the bases of airmass and the surface over which it flows. Heating from below expands the airmass thus leading to its rapid cooling. The mechanical and pressure change in the airmass cause change in physical property of the airmass.

The polar maritime air masses are located over warmer high latitude oceans of North Pacific, North Atlantic and oceans fringing Antarctic. The North America and North Europe are experienced by this type of air masses during winter season. The west coast is normally protected from invasion by generally prevailing waterly circulation and the cascade barrier.

Tropical continental air masses develop over greater Sahara area. Hot dry winds invade Mediterranean Europe, Northern Mexico and Southern California in form of scorching Sirocco,

Tropical maritime air masses overlying the tropic seas are moist and saturated and are causes of heavy rainfall along with high temperature conditions.

Q.8. What do you mean by fronts?

Ans. Air masses of different densities do not mix readily and tend to retain their identities as far as temperature and moisture are concerned. The boundary zone of the convergence separating the two air masses are called fronts. When a warmer and lighter airmass moves against a cold and more dense airmass, the former rides up over the later. It is called a warm front. If the cold air mass forces its way under a mass of warm air and pushes the latter upward, the front will be called a cold front.

In the mid-latitudes along the front of the warm and cold air masses temperate cyclones are formed.

Q.9. Discuss the pressure belts.

Ans. Atmospheric pressure on the earth’s surface is not the same everywhere. There exists a pattern of alternate high and low pressure belts over the earth. These pressure belts are the results of unequal heat received from the sun and the rotation of the earth.

(a) The Equatorial Low Pressure Belts : The earth receives maximum heat in the equatorial region with the result that the air is always hot and therefore ascends higher. It is, therefore a belt of rising air, calm and variable winds. It is also referred to as Doldrums.

(b) The Subtropical High Pressure Belts: The two belts of subtropical heighs, sometimes called the Horse Latitudes, lie just outside the Tropics at approximately 30°C to 35°C North and South. They are caused by the pilling up of the air due to the difference in the speed of rotation of the earth which affects the masses of air moving from the equator and the poles. The warm air rising up from the equator blows at high level towards the poles but a part of it having cooled, descends in this region. The air blowing from the circum-polar low pressure belts also descends in this region, thus adding to the already high pressure existing in this region.

(c) The Circum-Polar Low Pressure Belts (or Subpolar Lows) in the vicinity of 60° to 65° north and south are caused largely by the rotation of the earth. Ordinarily it should be a high pressure belt because of very cool air. But the air is rising here and the envelope of air is thin chiefly because rotation swings the bulk of the air towards the equator. These are areas of storminess, particularly in the winter season.

(d) The High Pressure Belts at the Poles: Beyond the Subpolar Lows, pressure appears to rise with increasing latitudes so that at poles, there are high pressure centres. They are probably caused by extreme cold or cold air masses. The polar regions have permanent ice caps.

Q.10. Give an account of prevailing winds.

Ans. There are some winds which blow throughout the year from one latitude to another in response to the latitudinal differences in air pressure. They are known as prevailing winds or planetary winds. Other type of winds are periodic winds which reverse their direction periodically with change of season, and local winds which are confined to small areas and have special characteristics.

Prevailing Winds or Planetary Winds: The major winds’ belts of the world are related to the distribution of major pressure belts of the world. These winds are known as the Planetary winds. As winds blow from high pressure belts to belts of low pressure, the equatorial low pressure and the sub-polar low pressure belts are regions of converging winds. High pressure belts are regions of diverging winds.

These winds are of three types:

(i) Trade winds.

(ii) Westerlies.

(iii) Polar winds.

(i) Trade winds: These are winds which blow from the subtropical high pressure belts (30°N and S) towards the equatorial low pressure belt. These winds blow almost regularly throughout the year. Trade winds are called so after a Latin word “tread” which means track i.e., travel in a constant direction. These are north-east winds in the Northern Hemisphere and south-east winds in the Southern Hemisphere. Trade winds from both the hemispheres converge in the equatorial zone.

They cause heavy rainfall on the eastern margins of the continents in the tropics as they blow from the oceans. On the west coast of the continents trade winds blow from the land to sea and therefore these are dry winds. These regions are mostly deserts.

(ii) Westerlies: These winds blow from the subtropical high pressure towards the sub-polar low pressure belt (from 35°- 40° to 60°- 65° N and S latitudes). These are not as regular as the trade winds. These are called variable westerlies as their direction and velocity vary a great deal. The westerlies are south-westerly winds in the Northern Hemisphere and north-westerly winds in the Southern Hemisphere. In the Southern hemisphere as there is continuous stretch of ocean beyond 40°S, the westerly winds are less variable than in the Northern Hemisphere. The westerly winds’ belt is characterized by cyclones and anticyclones which result in variation in weather from day-to-day. Westerly winds give good rainfall to the western margins of the continents in the belt. The westerlies are well developed between 40° and 65° S latitudes. These latitudes are often called Roaring Forties, Furious Fifties and Shrieking Sixties, which are dreaded terms for the navigators.

(c) Polar Winds: Polar winds blow from polar high pressure to the subpolar low pressure belts. They are north-east winds in the Northern Hemisphere and south-east winds in the Southern Hemisphere. Polar winds are the most irregular among the planetary winds. These winds are generally cold and dry.

Q.11. Briefly describe how the Coriolis effect modifies the movement of air.

Ans. Direction of wind: Had the earth not been rotating about its axis the winds would have blown in the direction of pressure gradient. The earth on account of its rotation produces a force known as Coriolis force named after French physicist Coriolis. This force displaces the winds from the direction of wind gradient. Ferrel formulated a law to deduce the direction of winds in the Northern and Southern Hemisphere. It is known as Ferrel’s law.

According to Ferrel’s law, if we stand with our faces in the direction in which the winds is blowing the wind will turn towards our right hand in the Northern Hemisphere and towards our left hand in the Southern Hemisphere.

The axial rotation of the earth has no effect on the directions of winds in the equatorial areas but as we move towards higher latitudes, the effect of axial rotation is increasingly felt. This effect is maximum at the poles.

Had there been no axial rotation, the winds would have blown at right angles to the isobars but due to this rotation the winds blow almost parallel to the isobars. This condition is found at a height of 1000 miles from the surface of the earth where the friction between the winds and the earth’s surface is reduced to zero. Below this height the friction becomes effective and reduces the force due to the earth’s rotation. Hence the winds in the lower atmosphere strike the isobars at an actual angle of 20°-25°.

Q.12. Give a brief account of trade winds and westerlies.

Ans. Trade winds: Trade winds are tropical winds. They blow from 30° North and South towards equator. They have north-east direction in the Northern Hemisphere and south-east direction in Southern Hemisphere.

Why are they so called? Because, the word trade comes from the word ‘tread’ meaning a ‘track’. Since, these winds do not change their track or always blow in the same direction. Hence, they came to be known as trade winds.

Properties of trade winds: The trade winds have the following climatic properties:

1. The trade winds are dry and stable in their areas of origin. Hence, the poleward parts of these winds are dry.

2. The trade winds are humid and unstable near the equator which causes rainfall there.

3. Near the equator, two trade winds clash with each other and along the line of convergence, they rise up to produce heavy rainfall.

4. These winds are on-shore on the eastern sides of the continents, so they bring rainfall on the eastern sides.

5. They are off-shore on the western sides of the continents so these parts always remain dry.

Because of this very reason, most deserts of the world are located in the western parts of the continents.

Westerlies: Westerlies are also permanent winds. They blow in the temperate latitudes (Between 30° N to 60° N and 30°S to 60°S). They originate in the north parts of the subtropical high pressure belts. They have south-west direction in Northern Hemisphere and north- west in Southern Hemisphere. As these winds blow from the west in both hemispheres, they are termed westerlies.

Westerlies blow at a great velocity. It ranges between 45 to 75 km per hour. They are never calm. They blow with a high velocity and roar between 40°S and 65° S latitudes. So they are called variedly as Roaring forties, Furious fifties or Shrieking sixties.

Climatic effect of westerlies: Westerlies are on-shore winds on the western sides of the continents, hence they bring rainfall in the western parts of the continents. On the other hand, they are off-shore winds on the eastern side of the continents, so these regions are dry lands and mostly deserts are found in these latitudes on the eastern sides of all continents.

Q.13. Why does tropical cyclone originate over the seas? In which part of the tropical cyclone do torrential rains and high velocity winds blow and why?

Ans. The tropical cyclones originate in tropical seas. The conditions for the formation of the tropical cyclones are:

1. Large sea surface with the higher temperature, more than 27°C.

2. Small variation in the vertical wind speed.

3. A pre-existing weak low pressure area.

4. Upper divergence above the sea level system.

Over the Indian Ocean-especially in Bay of Bengal 50% cyclones are formed from a pre- existing shallow low pressure area.

The eye of a cyclone is a region of tropical cyclone where a torrential rain occurs and the high velocity is there. The winds reach maximum velocity in this region as high as 250 km/hour. Around the eye is the eye wall where there is a strong spiralling ascent of air to greater heights in the troposphere.

Q.14. What is ITCZ? Distinguish between Hadley and Farrel Cell.

Ans. The air of the ITCZ rises because of convection caused by high insolation and a low pressure is created. To this low pressure the winds from the tropics the easterlies converge. The converged air also rises along with the convective cell. It reaches the top of the troposphere up to an altitude of 14 km and moves towards the north and south poles. This causes accumulation of air about 30° North and South part of the accumulated air sinks to the grand and forms subtropical high. Another reason for sinking is the cooling of air when it reaches 30° North and South latitudes down below near the land surface the air flows towards the equator as the easterlies. The easterlies from either side of the equator converge in the Inter Tropical Convergence Zone (ITCZ). Such calculation are called cells. The cell in the tropics is called a Hadley Cell.

In the middle latitude the circulation is that the sinking cold air that comes from the poles and the rising warm air that blows from the subtropical high. At the surface the winds are predominently westerlies. This cell is known as the Ferral Cell.

At polar latitudes the cold dense air subrodes near the poles and blows towards middle latitudes as the polar easterlies. This cell is called the Polar Cell.