# WBBSE Class 10 Geography Solutions Chapter 2.4 Pressure Belts and Winds

Detailed explanations in West Bengal Board Class 10 Geography Book Solutions Chapter 2.4 Pressure Belts and Winds offer valuable context and analysis.

## WBBSE Class 10 Geography Chapter 2.4 Question Answer – Pressure Belts and Winds

Answer in short : 2 Marks

Question 1.
What do you mean by air pressure?
The pressure exerted by the column of air of the atmosphere lying above a unit area of the earth’s surface is called air pressure.

Question 2.
What is an altimeter?
Altimeter is an instrument for measuring altitude. It also measures air pressure along with altitude. It is commonly used by pilots, mountaineers etc.

Question 3.
What is a Fortin’s Barometer?

Fortin’s Barometer is an instrument most commonly used for measuring atmospheric pressure. It is provided with a main scale and a vernier scale to measure air pressure with most accuracy.

Question 4.
What is an isobar?
Isobars are imaginary lines drawn on weather maps joining places having same air pressure at a particular time of the year, considering the places to be at the mean sea level.

Question 5.
The slope between the physical distance of two places and their pressure differences is known as the pressure gradient.

Question 6.
What are pressure belts ?
The zones of the earth which exhibit similar pressure conditions on an average in circular belts according to the latitudes are called pressure belts. They can be either high pressure belts or low pressure belts.

Question 7.
What are pressure troughs?
Although the entire earth is divided into pressure zones, some areas may exhibit a little different pressure conditions than their surroundings. These areas may be enclosed with certain isobars. Such circular zones of pressure conditions are called pressure troughs.

Question 8.
Mention the factors on which difference of air pressure depends.
The air pressure conditions depend on the following factors :

1. Temperature of the wind.
2. Altitude of the place.
3. Amount of water vapour in the air.
4. Rotation of the earth.
5. Depth or height of the air column above the place.
6. Distribution of land surface and water bodies etc.

Question 9.
What is wind and air current?
The horizontal movement of the air from high pressure regions to low pressure regions is known as wind. The vertical movement of air above the earth’s surface is known as air current.

Question 10.
State Ferrel’s law.
Ferrel’s law states that due to the coriolis force created by the rotation of the earth, the winds are deflected towards the right in the northern hemisphere and towards the left in the southern hemisphere.

Question 11.
What is a Beufort scale?
Beufort scale is a scale designed by Sir Francis Beufort, to measure the energy of wind. It is marked from 0-12. ‘0’ refers to a calm state of wind and ’12’ refers to a highly destructive and ferocious tornado.

Question 12.
What are roaring forties ?
The roaring forties are extremely powerful westerly winds that blow in the southern hemisphere between latitudes 40°-50°. These gale-force, west to east currents are the result of the combination of three variables : warm air movements, the earth’s rotation and near absence of relevant landmasses in southern hemisphere.

Question 13.
What is Loo?
The ‘Loo’ is a strong, dusty, hot and dry summer wind from the west which blows over the Indo-Gangetic plain of North India and Pakistan. It is mainly blows in Summer (May to June). Due to its very high temparature, exposure to it often leads to total heat strokes.

Question 14.
What is Buys Ballot’s law ?
In the Northern Hemisphere, if a person stands with his back to the wind the atmospheric pressure is low to the left, high to the right. This is because, wind travels anticlockwise around low pressure belt in the Northern Hemisphere, and is reversed in the Southern Hemisphere.

Question 15.
What is Geostrophic wind ?
In the condition of Geostrophic equilibrium, a thin wind stream is generating when pressure gradient force and coriolis force come into exact balance. It is a upper layer atmospheric wind.

Question 16.
What is an anemometer?
Anemometer is an instrument used to measure wind speed.

Question 17.
What are planetary winds?
The earth has certain fixed high pressure and low pressure belts that lie paralle] to the equator. This generates winds blowing from high to low pressure regions in fixed directions throughout the earth. These winds that blow regularly in specific directions throughout the year are called planetary winds.

Question 18.
Winds that blow from the subtropical hight pressure belts or horse latitudes (30°-35° north and south) to the low pressure belts or doldrums (5°N-5°S) are known as the Trade winds.

Question 19.
Why are trade winds so called ?
Trade winds blow over the region of the earth which has much landforms and large population. In early days, the sailors used to sail their ships with the help of the trade winds for trade and business from one continent to another. Hence the winds are called ‘Trade winds’.

Question 20.
What are westerlies?
The winds that blow from sub-tropical high pressure belts towards the sub-polar low pressure belts are called westerlies. They blow towards the poles generally from south-west in the northern hemisphere and from the north-west in the southem hemisphere. As they blow from the west in both the hemispheres, they are called westerlies.

Question 21.
What are polar winds?
The winds that blow from the high pressure region of poles towards the sub-polar low pressure zones are called polar winds. Since they blow from the east in both the hemispheres, they are also called Polar Easterlies.

Question 22.
What are periodic winds ?
Some winds that blow for a limited period of a day or time to time or for a certain season of the year are called periodic winds. Eg. – sea breeze, land breeze, monsoon winds etc.

Question 23.
What is sea breeze?
During day time, the wind that blows from sea towards the land is known as sea breeze. This happens due to unequal heating of land and sea and difference in pressure conditions thus created.

Question 24.
What is land breeze?
During the night, the wind that blows from land towards the sea is called land breeze. This is because of the unequal rates of radiating heat by land and sea after sunset, hence creating a difference in pressure conditions over the land and sea surfaces.

Question 25.
What are local winds ?
There are certain type of winds which are produced by local temperature differences created due to local topographic landform. These winds are called local winds, and their zone of influence is also very small. Eg. – Sirocco, Loo etc.

Question 26.
What are sudden or irregular winds ?
Intense heating or cooling of land and sea sometimes develop severe low pressure centre or centre of high pressure. It brings about irregular winds of great or mild speed. These are called sudden or irregular winds. They do not have any routine time of occurrence.

Question 27.
What is a cyclone?

Cyclones are stormy whirls of wind. They come suddenly with great force and velocity towards the centre of low pressure. Cyclone develops due to the formation of intense low pressure centre. As it approaches, it spins clockwise in the southern hemisphere and anti-clockwise in the northern hemisphere.

Question 28.
What is a Jet stream ?

Jet stream is a thin stream of air generated in the higher reaches of the Troposphere and below the Stratosphere at about 7.4-14 km. altitude. It blows from west to east. Length of this jet stream can be a few thousand kilometer, but the width is only a few hundred kilometre.

Question 29.
What do you mean by ‘Roaring Forties’?
In the southern hemisphere, sea surface is much more compared to land surface. Hence, the westerlies can blow over this zone unhindered and with great speed near the 40° S latitude. The wind blows with so much speed that it makes a roaring sound. Hence, the region is called ‘Roaring Forties’.

Question 30.
What are Anabatic wind?
Anabatic winds are also called up-slope winds. In mountainous regions, during the day, when the solar radiation heats the mountain slopes, the adjacent cool air gets heated, lightened and rises upward. These are called Anabatic wind.

Question 31.
What are Katabatic winds ?
Katabatic winds are also called down-slope winds. In mountainous regions, during the night, when the slopes cool down due to outgoing solar radiation, the adjacent air also cools, gets heavier and sinks down the slopes into the valleys.

Question 32.
What is Sirocco?
It is a very hot dry wind of the Sahara desert. It blows from Sahara to the Mediterranean region. It is known as Khamsin in Egypt, Harmattan in Guinea coast of West Africa and Sirocco in Sicily.

Question 33.
What is Mistral ?
Mistral is a strong, cold northernly wind experienced on the shores of the northwest Mediterranean region. It blows along the coasts of Spain and France.

Question 34.
What is Bora?
Bora is a cold dry wind that blows along the adriatic coast and in northern Italy during winter.

Question 35.
How many kinds of air pressure are there?
There are two kinds of air pressure :
(i) High pressure : When air pressure is higher than 760 mm of mercury or 986 millibar.
(ii) Low pressure : When air pressure is lower than 760 mm of mercury or 986 millibar.

Question 36.
What do you mean by Standard Atmospheric Pressure?
The pressure exerted by 76 cm of mercury in a bore tube at 0° C temperature on 45° latitude at sea level is known as Standard Atmospheric Pressure.

Question 37.
What kind of pressure is considered as normal air pressure ?
Air pressure lying between 986-1013.25 millibars is considered as normal air pressure.

Question 38.
What happens if the isobars lie very close to each other?
The closely lying isobars indicate a steep pressure gradient. Steep pressure gradients initiate strong and speedy winds. It increases the chances of storms and cyclones.

Question 39.
Is the atmospheric pressure at sea level all over the world same?
It has been tested that atmospheric pressure at sea level does not rise above 788 mm and does not fall below 760 mm. But atmospheric pressure at sea level all over the world is not absolutely same. It varies due to temperature conditions of adjacent areas.

Question 40.
What is the difference between Trough and Ridge?
A huge region of low pressure is known as Trough and a huge region of high pressure is known as Ridge.

Question 41.
What kind of pressure conditions prevail over the earth in January?
In January, low pressure prevails over the southern hemisphere and high pressure prevails over the northem hemisphere.

Question 42.
What kind of pressure conditions prevail over the earth in July?
In July, high pressure prevails over the southern hemisphere and low pressure prevails over the southern hemisphere.

Question 43.
What is the Normal lapse rate of air pressure ?
With every 110 m rise in altitude, the atmospheric pressure decreases by 1 cm or 10 mm of mercury in the Barometer, or by 1.34 millibar. This is the normal lapse rate of air pressure.

Question 44.
What is ‘Knot’?
‘Knot’ is the unit of measuring winds speed. Knot means the distance in nautical miles, travelled in one hour.

Question 45.
What is an anti-cyclone?

Air pressure increases with decrease in air temperature. When this condition gets intensified, the isobars lie in circular patterns with a high pressure in the centre and low pressure towards the periphery. Thus, winds start blowing outwards from the centre in clockwise direction in the northern hemisphere and in anti-clockwise direction in the southern hemisphere.

Question 46.
Name two local winds in India.
Two local winds in India are Loo and Andhi.

Answer in brief : 3 Marks

Question 1.
Mention the characteristics of air pressure.
The characteristics of air pressure are :

1. Air pressure decreases with increase in altitude.
2. Hot air is lighter and exerts less pressure.
3. Cold air is heavier and exerts more pressure.
4. Water vapour makes the concentration of gaseous molecules in the air lighter, and hence exerts less pressure. Thus, dry air exerts more pressure than moist air.
5. Sudden change in pressure conditions result in severe storms.
6. Air exerts pressure on all directions.
7. Air pressure is measured in millibar.

Question 2.
Compare between high pressure and low pressure.

 High Pressure Low Pressure i. When air exerts much pressure, it is called high pressure. i. When air exerts less pressure, it is called low pressure. ii. Length of mercury column in the glass tube of the barometer is more than 760 mm or 1013.25 millibar. ii. Length of mercury column in the glass tube of the barometer is less than 760 mm or 1013.25 millibar. iii. The high pressure zones of the earth are sub tropical high pressure zones near the Tropics of Cancer and Capricorn, and polar hight pressure zones near the North pole and the South pole. iii. The low pressure zones of the earth are Equatorial low pressure zone, sub polar low pressure zones near the Arctic and Antarctic circles.

Question 3.
Why do high pressure zones lie near the polar areas?
High pressure belts lie around the polar regions because,

1. The sun gives the most inclined rays to this region. Heat is very less, hence the air is very cold and heavy.
2. There is very less water vapour in the air as the rate of evaporation is very low.
3. Some air from the adjacent sub-polar low pressure belts rise and descend towards the poles.

Question 4.
Why is the equatorial region a low pressure belt ?
The equatorial region has a low pressure belt because:

1. The sun gives its maximum energy to this region by rays perpendicular to the surface. Hence, heat is maximum. The air gets heated, lightened and rises up.
2. Presence of huge water bodies and much heat leads to high rates of evaporation. The air is filled with water vapour and becomes light.
3. The rotation of the earth tends to deflect air due to coriolis force.
4. Presence of vast forests adds up to water vapour in the air due to transpiration.

Question 5.
What are horse latitudes?
The sub-tropical belt of calm lying between 25°-35° N is known as Horse latitudes. In early days when trade was conducted only through the oceans, the ships used to slow down and could not sail through this region due to lack of wind. Then in order to reduce weight and preserve drinking water, the horses being carried in the ships were thrown into the ocean. Thus the region was named horse latitudes.

Question 6.
What do you mean by ‘Shifting of Pressure belts’?
The rotation of the earth, angle of incidence of solar rays, presence of land surface or water surface, altitude and the seasonal drift of the sun towards the north and towards the south cause the pressure belt to shift by 5°-10° northwards and southwards respectively. This temporary movement of the pressure belts makes them move for about 5°-10° from their average positions. This phenomena is known as ‘shifting of pressure belts.’

Question 7.
Differentiate between wind system and wind current.

 Wind system Wind current i. Wind moves horizontally parallel to the earth’s surface. i. Wind moves vertically perpendicular to the earth’s surface. ii. Occurs due to difference in pressure between two places. ii. Occurs due to difference in temperature. iii. Influence on human life is remarkable. iii. Influence on human life is negligible.

Question 8.

 North-eastern Trade winds South-eastern Trade winds i. Blows between 5°-25° N latitudes. i. Blows between 5°-25° S latitudes. ii. The wind blows from the region around the Tropic of Cancer towards the Equator. ii. The wind blows from the region around the Tropic of Capricorn towards the Equator. iii. Wind gets deflected towards the right. iii. Wind gets deflected towards the left. iv. Wind speed is comparatively low due to presence of more land surface in the northern hemisphere. (about 16 km per hour) iv. Wind speed is high due to presence of vast oceanic surface and less land surface in the southern hemisphere. (about 22 km per hour)

Question 9.
List the characteristics of Westerlies.
The characteristics of Westerlies are :

1. Westerlies bring rainfall to western margins of continents.
2. The continents under the influence of westerlies have temperate grasslands on their eastern sides.
3. Westerlies bring about more rainfall during the winter.
4. In the southern hemisphere, the westerlies blow with great speed due to vast ocean surfaces unhindered by land. Hence, the latitudes are named – Roaring Forties, Furious Fifties and Screaming Sixties.

Question 10.
What is an Anticyclone?

Winds generated in a region of high pressure surrounded by low pressure all around is called an Anticyclone. In this system, the winds blow spirally outwards from the centre in clockwise direction in the northern hemisphere and anticlockwise direction in the southern hemisphere. Anticyclones are generally associated with light wind and clear sky. Rainfall is light or in drizzles. They prevail commonly in winter when cold air remains near the ground for many years.

Question 11.
Differentiate between land and sea breezes and monsoon winds.

 Land and Sea breezes Monsoon winds i. Blows near coastal areas. i. Generates near oceans and blows towards land for a long distance in tropical region. ii. Land and Sea breeze blows alternately for few hours. Land breeze blows during the night and sea breeze blows during the day. ii. Blows for a few months in a particular season. During summer the wind blows from sea to land, during winter they blow from land to sea.

Question 12.
Differentiate between cyclones and anti-cyclones.

 Cyclone Anti-cyclone i. Low pressure lies in the centre of a cyclone. i. High pressure lies in the centre of an anti-cyclone. ii. Winds blow towards the centre. ii. Winds blow outwards from the centre. iii. Winds are warm and move upward. iii. Winds are cold and move downward. iv. Winds blow anti-clockwise in the northern hemisphere and clockwise in the southern hemisphere. iv. Winds blow clockwise in the northerm hemisphere and anti-clockwise in the southern hemisphere. v. Wind velocity is very high. v. Wind velocity is low. vi. Cause storms and rainfall. vi Usually cause no rainfall. vii. Exists for a short time, but is destructive. vii. Exists for a long time but is not destructive.

Question 13.
Compare the atmospheric pressure conditions of January and July.

 Pressure conditions in January Pressure conditions in July i. High pressure lies in northern hemisphere and low pressure lies in southern hemisphere. i. High pressure lies in southern hemisphere and low pressure lies in northern hemisphere. ii. The pressure belts shift southwards with the southward movement of the sun. ii. The pressure belts shift northward with the northward movement of the sun. iii. During this time the wind blows from land towards the sea in the northern hemisphere. iii. During this time, the wind blows from land towards the sea in the southern hemisphere.

Question 14.
Discuss the effects of air pressure.
The effects of air pressure are –

1. Difference in air pressure of different parts of the earth’s surface is the main cause of wind.
2. Heavy storms and often rainfall occur in regions of very less pressure.
3. Snowfall occurs in high pressure regions.
4. Air pressure influences wind, heat and presence and holding capacity of water vapour by the air.
5. Higher altitudes have low pressure. Hence, mountaineers have to carry oxygen cylinders for breathing.
6. In higher altitudes, due to sudden decrease in atmospheric pressure, bleeding may start from nose, ears, eyes etc.

Question 15.
Discuss the characteristics of winds.
The horizontal movement of air along the earth’s surface from a region of high pressure to a region of low pressure is known as wind. The characteristics of winds are –

1. Winds blow from high pressure to low pressure regions.
2. Winds (especially planetary winds) are named according to the directions from where they blow.
3. Wind speed depends on the pressure gradient between two places.
4. Wind direction is marked by a Wind Vane.
5. Wind speed is measured by Anemometer in ‘Knots’.
6. The force of the wind is calculated on the Beufort Scale.

Question 16.
What is ‘Coriolis Force’?
‘Coriolis Force’ is generated due to the rotation of the earth. The earth being a sphere, speed of rotation goes on reducing from the equator towards the poles. The moving elements on the earth’s surface (wind, ocean currents) thus cannot flow directly in the north-south direction, and get slightly deflected. This was first discovered by the French physicist and mathematician G.G. Coriolis in 1835. Thus the force that causes deflection of moving bodies on the earth’s surface due to rotation is called the ‘Coriolis Force’. In the northern hemisphere, the winds get deflected towards the right and in the southern hemisphere, the winds get deflected towards the left.

Question 17.
Differentiate between the South-West Westerlies and the North-West Westerlies.

 South-West Westerlies North-West Westerlies i. They blow from the Tropic of Cancer towards the Arctic Circle in the northern hemisphere. i. They blow from the Tropic of Capricorn towards the Antarctic Circle in the southern hemisphere. ii. They blow between $$23^{1 / 2}$$° N $$66^{1 / 2}$$°N. ii. They blow between $$23^{1 / 2}$$°S $$66^{1 / 2}$$°S. iii. Wind speed is lesser than the NW westerlies. iii. Wind speed is greater than the SW westerlies. iv. Horse Latitudes lie within this zone. iv. Roaring 40s lie within this zone.

Question 18.
Why do westerlies cause more rainfall on the western coasts of the continents?
The causes of rainfall on the western coasts of the continents by Westerlies are:
i. Westerlies blow from the west towards the east. Hence, before they enter the continent from the western side, they blow over oceans and collect enough moisture.
ii. The continents under the influence of westerlies have mountains and high lands along the western margins. This helps to stop the moisture laden winds and cause rainfall.

Question 19.
Why do the eastern parts of the continents under the influence of westerlies receive less rainfall ?
The eastern parts of the continents under the influence of Westerlies receive less rainfall because –
i. The Westerly winds become more and more dry as they move from west to east over the continents and cannot collect moisture due to lack of water bodies.
ii. They shed maximum water at the western margins of the continents due to collision with highlands and mountains.

Question 20.
Differentiate between Trade winds and Westerlies.

 Trade winds Westerlies i. Blow from the Tropic of Cancer and Capricorn towards the Equator. i. Blow from the Tropic of Cancer to the Arctic Circle and Tropic of Capricorn to the Antarctic Circle. ii. Blow from the east towards the west. ii. Blow from the west towards the east. iii. Wind speed is 16 km per hour in the northern hemisphere and 22 km per hour in the southern hemisphere. iii. Wind speed is low in the northern hemisphere due to presence of landmass. Wind speed is high in the southern hemisphere due to presence of oceans. iv. Helped in sailing of ships in early days. iv. Cause danger to ships sailing in the southern hemisphere due to great wind speed. v. Cause rainfall on the eastern side of continents. v. Cause rainfall on the western side of continents. vi. Cause uniform rainfall in the equatorial region throughout the year. vi. Cause more rainfall in the winter compared to the summer.

Question 21.
Differentiate between Sea Breeze and Land Breeze.

 Sea Breeze Land Breeze i. Blows during day time from the sea towards the land. i. Blows during night from the land towards the sea. ii. Wind speed is greater than land breeze. ii. Wind speed is lesser than sea breeze. iii. Wind speed is greatest during noon. iii. Wind speed is greatest during early morning. iv. Winds cover a larger area while blowing. iv. Winds cover a lesser area while blowing.

Question 22.
What is a Cyclone?

If a small area suddenly gets heated up, the air present over that area also get heated, lightened and rise upwards. This creates a low pressure trough in the area compared to the surrounding areas. Thus, winds gush in with great speed from all sides towards the centre of low pressure. This is known as a Cyclone. The cyclonic winds move in circular patterns. They blow in the anti-clockwise direction in the northern hemisphere and clockwise direction in the southern hemisphere.

Answer in details : 5 Marks

Question 1.
Discuss the factors affecting atmosphere pressure.
The factors that affect atmospheric pressure are :
i. Temperature : Air pressure is inversely related to temperature. Higher the temperature, lower is the air pressure. As air gets heated, it gets lighter and rises upwards, thus exerts less pressure. Conversely, cold air contracts, becomes heavier and descends, thus exerts more pressure.

ii. Altitude : Atmospheric pressure is inversely related to altitude. The lower layers of the air are denser than upper layers. Moreover, the lower layers are compressed
under the weight of the upper layers of air and exert more pressure. As we go higher up, pressure decreases by 10 mm per 110 m rise in altitude.

iii. Water vapour : Air containing water vapour is lighter than dry air. Thus moist air exerts less pressure than dry air.

iv. Rotation of the earth : Rotation of the earth results in swing of air outwards due to centrifugal force. Thus regions of the earth rotating with the highest speed have low pressure zone.

v. Distribution of land and water : Pressure conditions on land and water on the same latitude are not the same. During summer, the land gets heated more than the water, thus a low pressure region is created on the land. The opposite condition occurs in winter.

Question 2.
Discuss the Pressure belts of the world in brief.
The world is divided into seven distinct pressure belts. They are :
(i) The Equatorial low pressure belt
(ii) and (iii) The sub-tropical high pressure belts (North and South)
(iv) and (v) The sub-polar low pressure belts (North and South)
(vi) and (vii) The Polar high pressure belts (North and South)

(i) Equatorial low pressure belt : This belt lies between 5°N and 5°S of the Equator. The earth receives maximum heat here. Hence the air is always hot and light. The air also contains a large amount of water vapour. This light air rises vertically giving, rise to a low pressure belt. The region is generally windless and calm. Hence it is also known as doldrums.

(ii) and (iii) The Sub-tropical high pressure belts : These belts extend between 30°-35° N and S. These high pressure belts are not thermally induced. They result due to rotation of the earth, sinking, settling down and piling of air from the equator and sub-polar regions. The cool descending air gradually becomes warm and its water holding capacity increases. As a result there is no condensation and occurrence of rainfall. This is one of the reasons for occurrence of hot deserts on the western sides of the continents present in this region.

(iv) and (v) The Sub-polar low pressure belts : These belts lie between 60°-65° N and S. Due to earth’s rotation, the surface air spreads outward, causing creation of low pressure. The belt is more developed in the southern hemisphere compared to the northern hemisphere because of the presence and dominance of water (oceans) in the southern hemisphere in this belt.

(vi) and (vii) the Polar high pressure belts : These belts lie around the poles in both the hemispheres. These regions have permanent ice caps. Due to immense cold and minimum speed of rotation, the air which is cold and dense, descends near the poles, creating high pressure.

Question 3.
Discuss the causes of wind movement over the earth’s surface.
The different causes for wind movement over the earth’s surface are –
i. Difference in temperature : If a place gets heated, the air lying above it also gets heated and lightened and rises up creating a partial vacuum. Thus air rushes from adjacent cooler airs to fill up the vacuum.
ii. Difference in pressure : Difference in temperature between two places cause difference in pressure. Thus winds blow from high pressure to low pressure regions.
iii. Rotation of the earth : While blowing from high pressure to low pressure regions, winds get deflected due to the rotation of the earth.
iv. Nature of landforms : High mountains or plateaus provide obstruction to the wind, hence speed is less. But vaccant and flat places like deserts, plains, etc. experience very speedy winds.
v. Presence of land or water surface: Winds blowing over land surface experience more friction than winds blowing over water surface. Hence wind blowing over land have less speed than wind blowing over water.

Question 4.
Discuss the charateristics of Planetary wind in brief.
There are three types of planetary wind:

2. The Westerlies
3. The Polar winds.

i. The Trade winds : Winds that blow from the Sub-tropical high pressure belts to the Equatorial low pressure belts are called Trade winds. According to Ferrel’s

law, due to Coriolis force, caused by earth’s rotation winds deflect to the right in the northern hemisphere and to the left in the southern hemisphere. Hence, the winds are called north-east trade winds in the northern hemisphere and the south-east trade winds in the southern hemisphere. Velocity of NE trade winds is around 16 km per hour and that of SE trade winds is 22 km per hour (due to less landforms and vast oceans).

ii. The Westerlies : The winds blowing from the sub-tropical high pressure belts to the sub-polar low pressure belts are called Westerlies. As the direction of the Westerlies is opposite to that of trade winds, they are also called Anti-trade winds. In the northern hemisphere they blow from the south-west and in the southern hemisphere they blow from the North-West.

iii. The polar winds : The chilled winds blowing from the polar regions to the subpolar low pressure regions are called the polar winds. Since their direction is similar to that of Trade winds, they are also called Polar Easterlies or Polar Trade winds. In this region, the deflection of winds is so high that they appear to blow almost from the east.

Question 5.
What is the ‘Jet Stream’? Discuss its relation with the monsoon winds.

Jet stream is a thin stream of air generated in the higher reaches of the Troposphere and below the Stratosphere at about 7.5-14 km. altitude. It blows from west to east. The jet stream can be a few thousand kilometer but width is only a few hundred kilometer.

Relation between Jet stream and Monsoon winds : The westerly Jet stream causes high pressure on the Indian subcontinent in winter. Hence the summer, the Tibet plateau gets excessively heated and drives the subtropical westerly Jet stream north of the Himalaya. The tropical easterly jet stream blows through the centre of the Indian sub-continent, causing low pressure conditions. This gradually attracts the monsoon winds. The onset of monsoon depends on the shifting of the sub-tropical westerly jet stream northwards, which allows the easterly jet to set in. A sudden shift of the westerly jet stream may cause a huge ‘Monsoon burst’.

Question 6.
Differentiate between cyclones and anti-cyclones.