Get notes, summary, questions and answers, MCQs, extras, and PDFs of Chapter 14 “Atmospheric Pressure and Winds”, which is part of ICSE Class 9 Geography (Morning Star/Total) textbook/workbook answers. However, the notes should only be treated as references, and changes should be made according to the needs of the students.
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Summary
Air is a real substance that has weight. This weight presses down on the Earth’s surface, and we call this Air Pressure. Think of it like a column of air pushing down. We measure this pressure, often in millibars, using an instrument called a barometer. The average pressure at sea level is about 1013 millibars. If you go up a mountain, the air pressure gets lower because there’s less air above you.
Changes in temperature make air expand or shrink, which changes its density and pressure. These pressure differences cause the air to move, and this moving air is what we call wind. Winds are important because they carry heat and moisture around the world. On weather maps, lines called isobars connect places that have the same air pressure.
Several things affect air pressure. Besides altitude, temperature is a big factor. Warm air is lighter and creates low pressure, like at the Equator. Cold air is heavier and creates high pressure, like at the Poles. Air with a lot of water vapor (humid air) is actually lighter than dry air, so it also creates lower pressure. The Earth’s rotation also plays a part in moving air and affecting pressure. When air pressure changes quickly over a short distance, we call it a steep pressure gradient, which means strong winds. If it changes slowly, the gradient is gentle, and winds are weak. Sometimes, when people like scuba divers move too quickly from high pressure to low pressure, they can get “the bends,” which is a painful condition caused by gases forming bubbles in the body.
The Earth has several major pressure belts. Near the Equator, there’s a low-pressure belt called the “doldrums.” It’s a very calm area because the warm air mostly rises. The word “doldrums” describes a state of inactivity or low spirits, which fits a place with little wind for sailing ships. Around 30 degrees north and south of the Equator are high-pressure belts called the “horse latitudes.” Long ago, sailing ships could get stuck here for weeks due to lack of wind. If they were carrying horses and running low on fresh water, they sometimes had to throw the horses overboard to save water for the crew. It’s a sad story that explains the name. Further towards the Poles are low-pressure belts, and at the Poles themselves are high-pressure areas. These belts can shift slightly north or south with the seasons because of the Earth’s tilt.
Winds always blow from high-pressure areas to low-pressure areas. The Earth’s rotation causes winds to curve – to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This is called the Coriolis effect. There are main types of winds. Permanent winds, like Trade Winds and Westerlies, blow consistently in certain directions. Trade Winds were named because they helped sailors on their trading journeys by providing a steady route. Periodic winds, like land and sea breezes or monsoons, change direction regularly. Local winds are specific to certain areas. For example, the Chinook wind in the Rocky Mountains is called a “snow eater” because this warm, dry wind can melt snow quickly.
Cyclones are large weather systems with low pressure at their center and winds spiraling inwards. Tropical cyclones, also known as hurricanes or typhoons in different parts of the world, are very powerful and form over warm ocean waters. They have a calm “eye” at the very center. Anticyclones are the opposite, with high pressure at the center and winds spiraling outwards, usually bringing calm, clear weather. High up in the atmosphere, there are fast-flowing air currents called jet streams, which can affect weather patterns and are used by airplanes to fly faster and save fuel.
Textbook solutions (Morning Star/Total)
Multiple-Choice Questions
1. The weight of air on a unit area of the earth:
(a) weight pressure
(b) air pressure
(c) unit pressure
(d) area pressure
Answer: (b) air pressure
2. Horizontal movement : _________ :: Vertical movement: air current
(a) conduction
(b) convection
(c) winds
(d) coriolis
Answer: (c) winds
3. The force per unit area exerted against a surface by the weight of the air above that surface:
(a) Force Pressure
(b) Area Pressure
(c) Air Pressure
(d) Atmospheric Pressure
Answer: (d) Atmospheric Pressure
4. The average atmospheric pressure at sea level:
(a) 1014.5 mb
(b) 1015.2 mb
(c) 1013.25 mb
(d) 1017.75 mb
Answer: (c) 1013.25 mb
5. The decrease in the pressure per unit distance in the direction in which the pressure decreases most rapidly is called
(a) Pressure Gradient
(b) Distance Pressure
(c) Decreased Pressure
(d) Unit/Distance Gradient
Answer: (a) Pressure Gradient
6. What are also called the Horse Latitude?
(a) Equatorial Low Pressure Belts
(b) Sub-Tropical High Pressure Belts
(c) Circum-polar Low Pressure Belts
(d) Polar High Pressure Areas
Answer: (b) Sub-Tropical High Pressure Belts
7. Which pressure belt is also called Doldrums?
(a) Equatorial Low Pressure Belts
(b) Sub-Tropical High Pressure Belts
(c) Circum-polar Low Pressure Belts
(d) Polar High Pressure Areas
Answer: (a) Equatorial Low Pressure Belts
8. Horizontal movement of air in the atmosphere:
(a) Winds
(b) Currents
(c) Pressure
(d) None of the above.
Answer: (a) Winds
9. The steady winds blowing from the Sub-Tropical High Pressure towards the Equatorial Low Pressure:
(a) High Pressure Winds
(b) Low Pressure Winds
(c) Trade Winds
(d) Steady Winds
Answer: (c) Trade Winds
10. Which amongst the following is NOT a characteristic of Trade Winds?
(a) They have varying velocity and are irregular
(b) They are permanent or prevailing winds
(c) They are called North-East Trades in Northern Hemisphere
(d) They have fixed velocity and are regular
Answer: (a) They have varying velocity and are irregular
11. Alps : _________ :: Rockies : _________
(a) Chinook, Mistral
(b) Polar Easterlies, Foehn
(c) Foehn, Mistral
(d) Foehn, Chinook
Answer: (d) Foehn, Chinook
12. What is the tropical cyclone over land called?
(a) Tropical Cyclone
(b) Temperate Cyclone
(c) Tornado
(d) Mistral
Answer: (c) Tornado
13. The concentrated bands of rapid air movement at the tropopause:
(a) Jet Streams
(b) Tornado
(c) Mistral
(d) Wave Cyclones
Answer: (a) Jet Streams
14. Which of the following is true about Jet streams?
(a) Forecast weather
(b) Reduce flight time
(c) Increase precipitation
(d) All of the above.
Answer: (d) All of the above.
15. Calm region in the centre of the cyclone:
(a) focus
(b) eye
(c) core
(d) All of the above.
Answer: (b) eye
16. Which of the following is NOT a tropical cyclone?
(a) hurricane
(b) typhoons
(c) mistral
(d) willy-willies
Answer: (c) mistral
17. Your uncle was puzzled because his flight from Delhi to London was more than 9½ hours long but his return from London was about 8½ hours. As a student of geography how would you explain this to him.
(a) The pressure gradient supported the return flight.
(b) Flying along a jet stream reduces flight time.
(c) The Coriolis Force deflects air reducing flight time.
(d) None of the above.
Answer: (b) Flying along a jet stream reduces flight time.
Short Answer Questions
1. Name the four main pressure belts of the earth.
Answer: On the earth’s surface, there are in all seven pressure belts. These can be grouped into four main types: the Equatorial Low, the Sub-tropical Highs, the Sub-polar Lows, and the Polar Highs. Except for the Equatorial low, the others form matching pairs in the Northern and Southern Hemispheres.
2. What is the Circum-polar Low Pressure Belt?
Answer: The Circum-polar Low Pressure Belts are located between 60° and 70° in each hemisphere. In the Sub-tropical region, the descending air gets divided into two parts; one part blows towards the Equatorial Low Pressure Belt, and the other part blows towards the Circum-polar Low Pressure Belt. This zone is marked by the ascent of warm Sub-tropical air over cold polar air blowing from poles. Due to earth’s rotation, the winds surrounding the Polar region blow towards the Equator. Centrifugal forces operating in this region create the low pressure belt appropriately called the Circum-polar Low Pressure Belt. This region is marked by violent storms in winter.
3. How does the Coriolis Force vary latitudinally?
Answer: The deflection due to the Coriolis Force is the least at the Equator and greatest at the Poles. At the Equator, there is no deflection, while at the poles, the deflection is maximum.
4. Name the three chief types of wind.
Answer: The three chief types of wind are:
(i) Permanent or Prevailing or Planetary Winds, which blow throughout the year from one latitude to the other in response to the latitudinal differences in air pressure.
(ii) Periodic Winds, which reverse their direction periodically with season.
(iii) Local Winds, which flow in comparatively small areas and have special characteristics.
5. What are periodic winds?
Answer: Periodic winds are winds that reverse their direction periodically with season. They blow at regular intervals or in regular cycles and result from localised differences in pressure and temperature. Land and sea breezes and seasonal winds like monsoons are examples of periodic winds.
6. What are local winds? Name any two local winds.
Answer: Local winds are winds that flow in comparatively small areas and have special characteristics. They are restricted to a certain place only and may be warm or cold depending upon the area from which they blow.
Two local winds are:
(i) Loo: In the plains of northern India and Pakistan, sometimes a very hot and dry wind blows from the west in summer in the afternoons.
(ii) Foehn: A strong warm wind that develops on the leeward side of the Alps.
7. Name two types of variable winds. Why are they so called?
Answer: Two chief types of variable winds are Cyclones and Anticyclones.
They are called variable because they do not blow in any definite direction, and their direction varies with the movement of the pressure system. Their speed also depends on the intensity of the depression, and they last only for a few days.
8. Why are cyclones frequent in summer in the tropical region?
Answer: Cyclones are more frequent in summer in the tropical region because of the movement of the Doldrum belt away from the Equator during this season.
9. Mention any two differences between Tropical Cyclones and Temperate Cyclones.
Answer: Two differences between Tropical Cyclones and Temperate Cyclones are:
(i) Tropical cyclones are produced mainly over the sea, whereas Temperate cyclones are produced both on land and on sea.
(ii) Tropical cyclones generally originate in the tropical region between 8° and 20°N and S, while Temperate cyclones originate in the mid-latitudinal region between 35° latitude and 65° latitude.
10. How are cyclones named differently in different parts of the world?
Answer: In low latitudes, an intense depression with a low pressure centre is known as a Tropical cyclone in the Indian Ocean area. They are known as hurricanes in the Caribbean and the south-east Caribbean region, typhoons in China (and eastern China, Japan, and Philippines islands), and willy-willies in Australia. In the Bay of Bengal and Arabian Sea, they are simply called cyclones.
11. What are two chief characteristics of anticyclones?
Answer: Two chief characteristics of anticyclones are:
(i) An anticyclone is a fine atmospheric condition where there is High Pressure in the core or centre and Low Pressure around it.
(ii) In an anticyclone, winds blow gently outwards from the centre.
Define the Following
(a) Pressure Gradient.
Answer: Pressure gradient is defined as the decrease in pressure per unit distance in the direction in which the pressure decreases most rapidly. In other words, the rate of change of atmospheric pressure between two points on the earth’s surface is called pressure gradient. On the weather chart this is indicated by the spacing of isobars. The gradient is steep if they are close together and gentle if they are far apart. Close spacing of isobars indicates a strong pressure gradient, while wide spacing suggests a weak gradient.
(b) Winds.
Answer: Horizontal movements of air are 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. Air flows from areas of high pressure to areas of low pressure. This horizontal movement of air is called wind; it is nature’s attempt to balance inequalities in air pressure.
(c) Coriolis force.
Answer: The earth rotates on its inclined axis. If it did not, winds would follow the direction of the pressure gradient. But the rotation produces another force other than the pressure force. It is called the ‘Coriolis Effect or ‘Coriolis Force’, which deflects the air. The deflection is the least at the Equator and greatest at the Poles. This tends to turn the flow of air by changing its direction from its original straight path. The wind starts deflecting to its right in the Northern Hemisphere. In the Southern Hemisphere it starts deflecting to its left from its original path.
(d) Altitude.
Answer: The atmospheric pressure decreases with height or altitude. The atmospheric pressure is highest at sea level. This is because at higher altitudes the air is thinner or less dense than the air at the sea level. The maximum air density is at the earth’s surface; air density decreases with height (because the pull of the earth’s gravity is less). The fewer number of gas molecules at higher altitudes means fewer molecular collisions and a decrease in air pressure.
(e) Monsoons.
Answer: Monsoons are periodic seasonal winds blowing in the regions of South East Asia and Northern Australia. The word ‘monsoon’ is derived from the Arabic word Mausim meaning ‘season’. They develop because of differences in heating conditions of the continent and the oceans. They are divided into two wind systems—the Summer Monsoon and the Winter Monsoon.
Distinguish Between the Following
1. Cyclones and Anticyclones.
Answer: Cyclones and Anticyclones are distinct atmospheric phenomena.
A Cyclone is a portion of the atmosphere in which the pressure is lowest in the centre. The winds blow inward in the opposite direction, i.e., from the south or south-west in the front of the cyclone and from the north or north-west in its rear. When a cyclone approaches, the reading on a barometer falls because of low pressure, but the thermometer rises due to the warm south or south-west winds. The rising moisture-laden air of such a cyclone results in heavy rain in the centre of the depression. Cyclones on account of the Coriolis Force blow in an anti-clockwise direction in the Northern Hemisphere and clockwise in the Southern Hemisphere. Tropical Cyclones, a specific type, are associated with turbulent weather conditions, thick cloud cover, strong winds and rainfall, and thus, cause heavy damage to property and loss of human lives. The central part of a tropical cyclone is a calm region known as the eye of the cyclone, surrounded by a turbulent vortex.
An Anticyclone, on the other hand, is a fine atmospheric condition. In an anticyclone, there is High Pressure in the core or centre and Low Pressure around it. Winds blow gently outwards. These winds are clockwise in the Northern Hemisphere and anticlockwise in the Southern Hemisphere. Since they are centres of high pressure, they are found in regions of descending air currents. Temperate cyclones are associated with anticyclones which precede and succeed a cyclone.
2. Permanent and Periodic Winds.
Answer: Permanent Winds and Periodic Winds differ based on their consistency and the factors influencing their direction.
Permanent Winds, also known as Prevailing or Planetary Winds, are the winds which blow throughout the year from one latitude to the other in response to the latitudinal differences in air pressure. Examples include the Trade Winds, the Westerlies, and the Polar Easterlies. The Trade Winds, for instance, are extremely steady winds blowing from the Sub-Tropical High Pressure area towards the Equatorial Low Pressure belt and are permanent or prevailing winds.
Periodic Winds, in contrast, are winds that reverse their direction periodically with season or blow at regular intervals or in regular cycles. They are winds that result from localised differences in pressure and temperature. Examples of periodic winds include land and sea breezes and the seasonal winds like Monsoons. Land and sea breezes are caused by the differential rate of heating of the land and the sea, causing winds to blow from sea to land during the day (Sea Breezes) and from land to sea at night (Land Breezes). Monsoons are periodic seasonal winds that develop because of differences in heating conditions of the continent and the oceans.
3. Summer and Winter Monsoons.
Answer: Summer Monsoons and Winter Monsoons are two distinct wind systems that are part of the seasonal monsoon winds, differing primarily in their timing, direction, and associated weather conditions.
Summer Monsoons occur when, in summer, the land gets more heated than the sea. Consequently, a centre of low pressure develops on the land. Over the adjoining sea, the air is comparatively cool, and a high pressure develops there. This causes the winds to blow from the sea to the land. This system is the ‘Summer Monsoon.’ For example, in May, June, and July, the plains of India and China are heated by the vertical rays of the sun, leading to intense heat that develops a continental low pressure. During these months, over the Indian Ocean, a high-pressure area develops. So, the winds blow from the Indian Ocean northward and north-westward into Asia. As they blow from the sea to the land, they bring heavy rainfall to South-East Asia. The summer monsoon winds blow south-west; so they are known as the ‘South-West Summer Monsoon.’
Winter Monsoons occur during the winter season, when the conditions are just the reverse of those of summers. A high pressure develops over a big landmass, such as that stretching from Central Asia up to the north-west Indian plain. At the same time, a low-pressure zone develops in the Indian Ocean. As the winds blow from the land to the sea, they bring cold dry weather and are incapable of producing rain. However, when these winds blow over seas and pass over the adjoining land, they bring some rainfall. For instance, the Southern Coromandel Coast (Tamil Nadu) in India, the Vietnamese Coast, and the west coast of Japan get rain from winter monsoons. The winter monsoon winds blow north-east; so the monsoon is known as the ‘North-East Winter Monsoons.’
Structured Questions
1. (a) What is meant by the term ‘Atmospheric Pressure’?
Answer: Atmospheric Pressure refers to the force per unit area exerted against a surface by the weight of the air above that surface.
(b) Explain briefly the factors that affect Atmospheric Pressure.
Answer: The factors that affect atmospheric pressure are:
- Altitude: The atmospheric pressure decreases with height or altitude. The atmospheric pressure is highest at sea level. This is because at higher altitudes the air is thinner or less dense than the air at the sea level. The maximum air density is at the earth’s surface; air density decreases with height because the pull of the earth’s gravity is less. The fewer number of gas molecules at higher altitudes means fewer molecular collisions and a decrease in air pressure.
- Temperature: Atmospheric pressure decreases with increase in temperature. This is because when the temperature rises, air expands. The molecules of air move far apart, become less dense, and hence exert less pressure. On the contrary, with decrease in temperature the air gets compressed and the space between molecules decreases, becomes more dense, and exerts more pressure on the region.
- Water Vapour: Water vapour concentration affects atmospheric pressure because the molecular weight of water (18 g/mol ) is less than the average molecular weight of air (about 29 g/mol ). When water evaporates and enters the atmosphere as a gas, the water vapour molecules take the place of other gas molecules in the air. So, a volume of wet (or humid) air weighs less than an equal volume of dry air. Therefore, humid air is less dense and exerts less pressure than dry air.
- Rotation of the Earth: Due to the rotation of the earth, bulk of the air at the Poles is thrown away towards the Equator. Since the Equatorial region receives great amount of heat throughout the year, the air becomes warm and light and therefore, it rises and creates low pressure. At the Poles, the cold heavy air sinks down and creates high pressure.
(c) Give a geographical reason for each of the following:
(i) The Westerlies in the Southern Hemisphere blow with greater force than those in the Northern Hemisphere.
Answer: The Westerlies are stronger in the southern hemisphere as there is absence of landmass.
(ii) There is a seasonal shifting in pressure belts.
Answer: If the earth had not been inclined towards the sun, the pressure belts would have been as they are. But it is not so, because the earth is inclined 23½° towards the sun. On account of this inclination, differences in heating of the continents, oceans and pressure conditions in January and July vary greatly. When the sun is overhead on the Tropic of Cancer (21 June) the pressure belts shift 5° northward and when it shines vertically overhead on Tropic of Capricorn (22 December), they shift 5° southward from their original position. The shifting of the pressure belts causes seasonal changes in the climate.
(iii) As we go higher, the atmospheric pressure decreases.
Answer: As we go higher, the atmospheric pressure decreases because at higher altitudes the air is thinner or less dense than the air at the sea level. The maximum air density is at the earth’s surface; air density decreases with height because the pull of the earth’s gravity is less. The fewer number of gas molecules at higher altitudes means fewer molecular collisions and a decrease in air pressure.
(d) Draw a well labelled diagram showing the pressure and wind belts of the earth.
Answer: See Fig. 14.4 of Morning Star/Total English or click here.
2. (a) Briefly explain the three chief types of winds.
Answer: The three chief types of winds are:
- Permanent or Prevailing or Planetary Winds: These are the winds which blow throughout the year from one latitude to the other in response to the latitudinal differences in air pressure.
- Periodic Winds: Certain winds reverse their direction periodically with season. These winds blow at regular intervals or in regular cycles and result from localised differences in pressure and temperature.
- Local Winds: There are certain winds in different parts of the world which flow in comparatively small area and have special characteristics. They are restricted to a certain place only and may be warm or cold depending upon the area from which they blow.
(b) Describe some of the important types of local winds.
Answer: Some of the important types of local winds are:
- Loo: In the plains of northern India and Pakistan, sometimes a very hot and dry wind blows from the west in summer in the afternoons. It is known as loo. Its temperature invariably ranges between 45°C and 50°C. It may cause sunstroke to people.
- Foehn and Chinook: Foehn is a strong warm wind that develops on the leeward side of the Alps. Due to regional pressure gradient, air is forced to cross the barrier. As the air ascends the southern slopes of the Alps, it expands and cools, causing rain and snowfall on the higher slopes. However, on descending the northern slopes, the wind experiences an increase in pressure and temperature, becoming compressed and warmed. Most of its moisture is lost and it reaches the valley bottom as a dry, hot wind, called the Foehn. The temperature of the wind is from 15°C to 20°C, useful for melting snow and hastening grape ripening. A similar kind of wind in the USA and Canada, known as Chinook, moves down the west slopes of the Rockies. The word chinook literally means ‘snow eater’ and is beneficial to ranches east of the Rockies as it keeps the grasslands clear from snow.
- Mistral: During winter, areas adjacent to highlands may experience a local cold wind which originates over the snowcapped mountains or highlands and blows down the valley. The most famous is the mistral that blows from the Alps over France towards the Mediterranean Sea. Even though the skies are clear, the mistral brings down the temperature below freezing point.
(c) Give a geographical reason for each of the following:
(i) The winds are directed to the right of their flow in the Northern Hemisphere.
Answer: The winds are directed to the right of their flow in the Northern Hemisphere due to the Coriolis Effect. The earth rotates on its inclined axis. This rotation produces a force called the ‘Coriolis Effect’ or ‘Coriolis Force’, which deflects the air. The wind starts deflecting to its right in the Northern Hemisphere.
(ii) Temperature and pressure are inversely related to one another.
Answer: Temperature and pressure are inversely related because atmospheric pressure decreases with increase in temperature. This is because when the temperature rises, air expands, its molecules move far apart becoming less dense, and hence exert less pressure. On the contrary, with a decrease in temperature, the air gets compressed, the space between molecules decreases, it becomes more dense, and exerts more pressure on the region.
(iii) Humid air is lighter than dry air.
Answer: Humid air is lighter than dry air because the molecular weight of water (18 g/mol ) is less than the average molecular weight of air (about 29 g/mol ). When water evaporates and enters the atmosphere as a gas, the water vapour molecules take the place of other gas molecules in the air. So, a volume of wet (or humid) air weighs less than an equal volume of dry air. Therefore, humid air is less dense.
(d) What is Coriolis Effect? How does it affect the planetary winds?
Answer: The Coriolis Effect, or Coriolis Force, is a force produced by the earth’s rotation on its inclined axis, which deflects the air.
It affects planetary winds by tending to turn the flow of air, changing its direction from its original straight path. The wind starts deflecting to its right in the Northern Hemisphere. In the Southern Hemisphere, it starts deflecting to its left from its original path. Thus, a wind blowing from north becomes north-easterly in the Northern Hemisphere, and a wind blowing from south becomes south-easterly in the Southern Hemisphere, for example, the North East Trade winds. The deflection is the least at the Equator and greatest at the Poles.
3. (a) Explain the weather conditions associated with tropical and temperate cyclones.
Answer: The weather conditions associated with a tropical cyclone are:
- When there is a tropical cyclone the air is still, but the temperature and the humidity are high. There is sudden drop in air pressure.
- When the front of the vortex arrives, there are strong winds and thick clouds, then the winds become violent with great speed. Dense clouds and heavy rain reduce visibility.
- The eye of the cyclone heralds a calm condition.
- When the rear of the vortex arrives, there are violent winds, thick clouds and heavy rain. The wind blows in the opposite direction.
The weather conditions associated with a temperate cyclone include a fall in temperature, a fall in the mercury level, wind shifts, and a halo around the sun and the moon upon its approach. A light drizzle follows which turns into a heavy downpour. In a temperate cyclone, wind speed is low and all the sectors of the cyclone have different temperatures. The rainfall is light and continues for many days.
(b) What are the Jet Streams? What is the significance of the Jet Streams?
Answer: Jet streams refer to the concentrated bands of rapid air movement found at the tropopause and the stratosphere, located at 10-15 kilometres above the surface of the Earth. They are formed near boundaries of adjacent air masses with significant differences in temperature, such as the Polar region and the warmer air to the south.
The importance of Jet Streams includes:
- Most weather systems do not just sit over an area; they are instead moved forward with the jet stream. The position and strength of the jet stream then helps meteorologists forecast future weather events.
- The airline industry consistently uses the jet stream for its flights. By flying commercial aircraft within the jet stream the flight time gets reduced. The reduced flight time and aid of the strong winds lead to reduction in fuel consumption.
- Precipitation usually increases in California because the polar jet stream moves farther south and brings more storms with it. In addition, precipitation often increases in Europe because the jet stream is stronger in the Northern Atlantic. Whatever the position of the jet stream, it has a significant impact on the world’s weather patterns and severe weather events like floods and droughts.
(c) Give a geographical reason for each of the following:
(i) Doldrums is a low pressure belt.
Answer: Doldrums, also known as the Equatorial Low Pressure Belt, is a low pressure belt because due to the vertical rays of the sun in this region, there is intense heating. The air therefore, expands and rises as a convection current, causing a low pressure to develop here.
(ii) Equatorial regions have low atmospheric pressure throughout the year.
Answer: Equatorial regions have low atmospheric pressure throughout the year because the Equatorial region receives a great amount of heat throughout the year. Warm air being light, the air at the Equator rises, creating a low pressure. This is due to the vertical rays of the sun causing intense heating, which makes the air expand and rise as a convection current.
(iii) Low atmospheric pressure prevails over the Circum-polar region.
Answer: Low atmospheric pressure prevails over the Circum-polar region because this zone is marked by the ascent of warm Sub-tropical air over cold polar air blowing from poles. Due to earth’s rotation, the winds surrounding the Polar region blow towards the Equator. Centrifugal forces operating in this region create the low pressure belt. Also, in the Sub-polar region around latitudes 60° to 65° North and South of the Equator, the rotation of the earth pushes up the bulk of the air towards the Equator, creating a low pressure belt in this region.
(d) Draw a well labelled diagram showing a cyclone in the Northern Hemisphere.
Answer: See Fig. 14.11 of Morning Star/Total English or click here.
Thinking Skills
1. State two situations when you physically felt the effect of the atmospheric pressure. Give reasons to support your answer.
Answer: Two situations when I physically felt the effect of atmospheric pressure:
- While travelling to a hill station: As we drove up to a higher altitude, I felt my ears pop. This happened because atmospheric pressure decreases with height. At higher altitudes, the air becomes thinner, and the pressure outside the body drops compared to the pressure inside the ears, causing a popping sensation as the pressure equalises.
- While flying in an aeroplane: During take-off and landing, I experienced a sensation of pressure in my ears and sometimes a bit of discomfort. This occurs because the aircraft rapidly changes altitude, causing quick changes in atmospheric pressure. Although the cabin is pressurised, these changes still affect the body, especially the ears, due to the difference in pressure inside and outside.
2. You must have read in newspapers or seen on TV channels, the news of cyclonic storms like Amphan, Nisarga, Fani, Yaas, Tauktae, Gulab, etc. affecting different parts of the country. Find out the origin and effects of atleast three of these cyclonic storms.
Answer: Origin and effects of three cyclonic storms:
- Cyclone Amphan: Originating in the Bay of Bengal, Amphan developed into a powerful tropical cyclone. It caused extensive damage in eastern India and Bangladesh, including uprooted trees, flooded streets, damaged homes, and disruption of power and communication lines.
- Cyclone Tauktae: This storm formed over the Arabian Sea and affected the western coast of India, especially Gujarat and Maharashtra. It brought strong winds, heavy rainfall, and storm surges, leading to destruction of property, loss of lives, and large-scale evacuations.
- Cyclone Yaas: Also formed in the Bay of Bengal, Cyclone Yaas struck the eastern coast, particularly Odisha and West Bengal. It resulted in coastal flooding, damage to infrastructure, and agricultural losses due to high-speed winds and heavy rainfall.
Project Work
In 2019, the Arabian Sea recorded the formation of four cyclones, Vayu, Hikka, Kyarr and Maha – a phenomenon after more than a century. Formation of cyclonic storms is normal in the Arabian Sea but rare if it develops in October-November. Usually the Bay of Bengal sees cyclones during this period. Prepare a project report tracing cyclonic storms that occurred along the eastern coast as well as the western coast of India over last ten years.
Answer: Do it yourself. Here are the instructions:
- Title your project as: “Cyclonic Storms in India (Last 10 Years) – Eastern and Western Coasts”.
- Start with an introduction in 3-4 lines. Mention that cyclones affect India every year, especially along the coastal states. Say you are studying their pattern over the past ten years.
- Draw or paste a map of India and mark:
- The eastern coast (Bay of Bengal side)
- The western coast (Arabian Sea side)
- States affected by cyclones (like Odisha, Andhra Pradesh, Tamil Nadu, Gujarat, Maharashtra, etc.)
- Create two sections:
- Cyclones on the eastern coast
- Cyclones on the western coast
- Make a table for each coast, with columns:
- Cyclone Name
- Month and Year
- Affected States
- Wind Speed (if available)
- Damage Caused (short points like “Flooding in coastal areas”, “Power cut”, “Evacuation”)
- Some examples to get you started:
- Eastern Coast: Phailin (2013), Hudhud (2014), Fani (2019), Amphan (2020), Yaas (2021)
- Western Coast: Vayu (2019), Kyarr (2019), Maha (2019), Tauktae (2021), Biparjoy (2023)
- Highlight 2019: Point out that the Arabian Sea had four cyclones in one year (Vayu, Hikka, Kyarr, Maha). Say this was very unusual.
- Write a conclusion in 4-5 lines. Say whether more cyclones happened in the east or west, and mention any trend you noticed (like increase in cyclones over the Arabian Sea in recent years).
- Decorate with pictures: Use printed or hand-drawn diagrams of cyclones, weather symbols, or satellite images if possible.
Extras
Additional MCQs
1. Which barometer uses a mercury column to measure atmospheric pressure?
A. Fortin’s barometer
B. Aneroid barometer
C. Wind vane
D. Hygrometer
Answer: A. Fortin’s barometer
40. The formation of jet streams is primarily due to gradients in what?
A. Humidity
B. Temperature
C. Latitude
D. Topography
Answer: B. Temperature
Additional Assertion and Reason
1. Assertion (A): Atmospheric pressure decreases with height or altitude.
Reason (R): At higher altitudes, the air is thinner or less dense than the air at sea level.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R does not explain A.
(c) A is true but R is false
(d) A is false but R is true
Answer: (a) Both A and R are true and R is the correct explanation of A.
15. Assertion (A): Anticyclones are associated with fine atmospheric conditions.
Reason (R): They are centres of high pressure with descending air currents.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R does not explain A.
(c) A is true but R is false
(d) A is false but R is true
Answer: (a) Both A and R are true and R is the correct explanation of A.
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