SEBA Class 9 Geography: Atmosphere – Structure, Pressure Belts & Wind System

Very Short Answer:

1. What is atmosphere? Write a brief with data about its structure.

Answer: The atmosphere is the gaseous envelope of air surrounding the earth from its surface up to about 10,000 km. It is held to the earth by gravitational force. Besides gases, it also contains water vapour and dust particles which make life possible on earth.

In its structure, nitrogen forms 78.08% and oxygen 20.94%. Argon makes up 0.93%, carbon dioxide 0.036%, and other gases like neon, helium, methane, krypton, hydrogen, ozone and xenon together form 0.014%. Water vapour and dust particles are confined mainly to the lower layers. The amount of gases and density gradually decrease with increase in altitude.

2. What are the main layers of the atmosphere based on the chemical composition of the gases? Write briefly about the characteristics of these layers.

Answer: On the basis of chemical composition, the atmosphere is divided into two main layers – the Homosphere and the Heterosphere.

The Homosphere extends up to about 80 km from the earth’s surface. In this layer the proportion of gases remains almost the same. Nearly 99% of the atmospheric mass is confined within 32 km. It includes the troposphere, stratosphere and mesosphere.

The Heterosphere lies above 80 km altitude. In this layer, gases separate according to their atomic weight. It has four divisions – Nitrogen layer (80–200 km), Oxygen layer (up to 1125 km), Helium layer (up to 3540 km) and Hydrogen layer (up to about 10,000 km). These layers together fall under the Thermosphere and Exosphere of the atmosphere.

3. Write with diagram about the layers of the atmosphere based on the variations in altitude and temperature and write the characteristics of each layer in brief.

Answer: On the basis of altitude and temperature, the atmosphere is divided into the following five layers:

1. Troposphere –
This is the lowest layer touching the earth’s surface. Its average height is about 12 km, maximum 16 km at the equator and minimum 8 km at the poles. Here temperature decreases with height at the rate of 6.5°C per km, known as Lapse Rate. Almost all weather phenomena like clouds, rainfall and storms occur in this layer. The upper limit is called the Tropopause.

2. Stratosphere –
This layer lies just above the troposphere and extends up to about 40 km from the tropopause. In this layer, temperature increases with altitude because of the presence of the ozone layer. No weather phenomena occur here, so air remains calm. The upper limit is known as the Stratopause.

3. Mesosphere –
This layer is above the stratosphere and extends for about 30 km. In the mesosphere, temperature decreases with altitude and reaches nearly –100°C at its upper limit called the Mesopause.

4. Thermosphere –
This layer extends from about 80 km to 400 km. Here, temperature again increases with height and may rise up to 1650°C. It contains electrically charged ion particles, so it is also called the Ionosphere. Radio waves sent from the earth are reflected from this layer.

5. Exosphere –
This is the outermost layer of the atmosphere. It extends from about 400 km to nearly 10,000 km. The air is very thin, mainly made of hydrogen and helium. The temperature rises up to about 5550°C in this layer.

4. Discuss with examples the factors responsible for variation in atmospheric pressure.

Answer: The variation in atmospheric pressure mainly depends on two factors – air temperature and altitude from the earth’s surface.

1. Air Temperature –
The distribution of heat is not the same at all places. In the equatorial region, high temperature makes the air light and pressure low. In the polar region, low temperature makes the air heavy and pressure high. Warm air absorbs more moisture, so moist air has lower pressure, while cold and dry air has higher pressure.

2. Altitude from the Earth’s Surface –
Atmospheric pressure decreases with increase in height. As altitude increases, the depth of the atmosphere and its weight decrease, so pressure becomes low. For every 900 feet rise, the air pressure decreases by about 1 inch or 34 millibars.

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

Answer: On the basis of distribution of heat and air, the earth has four major pressure belts. Each of them has its own location and special characteristics.

1. Equatorial Low Pressure Belt –
This belt lies roughly between 10° North and 10° South latitudes. Here the sun’s rays fall vertically throughout the year. The air becomes very hot, light and moist, so pressure remains permanently low. The air rises vertically upward and no horizontal movement is seen. That is why this calm zone is called the Equatorial Doldrums.

2. Sub-Tropical High Pressure Belt –
This belt is found around 25°–35° North and South latitudes. The cold and heavy air descends in these regions, creating high pressure. Wind movement is very weak, and in the past sailors faced difficulties, so this zone is also called the Horse Latitudes.

3. Sub-Polar Low Pressure Belt –
This belt lies between 60°–70° latitudes in both hemispheres. Here the volume of air decreases due to the earth’s rotation and deflection of winds. As a result, low pressure develops and this belt shifts slightly north or south with the seasons.

4. Polar High Pressure Belt –
This belt exists in the polar regions. Due to lack of sunlight, the regions are extremely cold and covered with snow. The air is dry and heavy, so pressure remains permanently high throughout the year.

6. Write the importance of atmosphere towards creation of a favourable physical environment on the earth.

Answer: The atmosphere is a very important element of the earth’s physical environment. Along with land and water, it has created a favourable condition for the origin, growth and survival of living beings on the earth. It provides gases essential for humans, plants and animals and also helps in the distribution of heat and moisture on the earth’s surface.

The atmosphere keeps the earth’s temperature balanced by maintaining the proportion of gases. It prevents excessive heat and cold, absorbs harmful ultraviolet rays of the sun, and causes weather changes like rainfall, storms and cyclones. Without atmosphere, plants and animals could not exist. Therefore, the atmosphere makes the earth a unique planet in the solar system.

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

Answer: Wind means the movement of air in the atmosphere. The air in motion is called wind. Generally, wind movement takes place parallel to the earth’s surface, which is called horizontal wind. When the air moves vertically upward, it is called vertical wind or current.

The origin of wind mainly depends on temperature difference and atmospheric pressure. The earth’s surface is not heated uniformly by the sun. In the equatorial region, the temperature remains high as the sun’s rays fall vertically, which makes the air light and creates a low-pressure area. Towards the poles, the temperature is low, air becomes heavy and creates a high-pressure area. Air always moves from high-pressure regions to low-pressure regions. This pressure difference, known as the pressure gradient, starts the movement of air.

8. What is the most important reason for the origin of wind? Briefly discuss the factors determining velocity and direction of wind.

Answer: The most important reason for the origin of wind is the difference in atmospheric pressure caused by unequal heating of the earth’s surface. Wind always moves from a high-pressure region to a low-pressure region.

The factors determining velocity and direction of wind are:

1. Pressure Gradient Force –
It is created due to difference in pressure between two places. A steep pressure gradient makes wind blow with high speed, while a gentle gradient reduces its speed and keeps conditions calm.

2. Gravitational Force –
The Earth’s gravity pulls the atmosphere towards itself. With increasing altitude, gravitational pull becomes weaker, pressure falls, and vertical movement occurs. Gravity also makes air move from valleys to mountains.

3. Coriolis Force –
Due to the Earth’s rotation, winds do not move in a straight line. In the northern hemisphere they deflect to the right, while in the southern hemisphere, they deflect to the left.

4. Frictional Force –
Friction opposes the movement of air on the surface. Over mountains, forests, and cities, wind velocity becomes low, while over oceans and smooth surfaces, wind flows faster with less resistance.

9. What do you mean by 'Coriolis Force'? With the help of diagram briefly describe its contribution in determining the direction of wind.

Answer: The Coriolis Force is the outward force generated from the earth’s center due to its rotation on its axis. This phenomenon was first discovered by the French mathematician Gaspard de Coriolis in 1844, and accordingly this force is known as the Coriolis Force.

When wind blows from a high-pressure region to a low-pressure region, the Coriolis Force slightly changes the wind direction. In the northern hemisphere, the wind is deflected towards the right in clockwise direction, and in the southern hemisphere, the wind is deflected towards the left in anticlockwise direction. The Coriolis Force is zero at the equator, gradually increases with latitude, and becomes the highest at the poles. This deflection of wind direction due to the earth’s rotation is expressed in Ferrell’s Law.

10.What is meant by 'Pressure Gradient Force? What is its contribution in air movement?

Answer: The Pressure Gradient Force is the force generated due to the difference in atmospheric pressure between two places on the earth’s surface. The rate of change of pressure over a given distance is called the pressure gradient. If the pressure difference between two places is more, the pressure gradient is steep and the wind velocity becomes high. If the distance is more, wind velocity decreases.

The contribution of the pressure gradient in air movement is very important. Air always moves from a high-pressure region to a low-pressure region, and this pressure gradient decides both the direction and velocity of wind. When the pressure gradient is high, winds are strong, and when it is low, winds are calm.

11.Briefly discuss the contribution of the earth's gravitational force in air movement.

Answer: The earth’s gravitational force plays an important role in air movement. It pulls the whole envelope of the atmosphere towards the earth. The weight or pressure of air also depends on gravitational force. With increase in altitude, the pull of gravity decreases, so air becomes lighter and pressure falls. This creates vertical movement of air from high pressure to low pressure.

Variation in landforms also causes pressure difference, and air moves from valleys (high pressure) to mountains (low pressure). Moreover, due to gravity, air movement does not follow a straight line but becomes curved.

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

Answer: Naming of wind means giving name to the wind according to its direction. The wind is always named from the direction it comes. For example, the wind that comes from the west is called Westerly Wind, and the wind that comes from the northeast is called North-Easterly Wind.

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

Answer: The velocity of wind is determined with the help of an instrument called the Anemometer. Nowadays different types of anemometers are used, and with an Anemograph the direction and velocity of wind are recorded automatically.

The unit of wind velocity is the Knot. One Knot means one nautical mile per hour, which is equal to about 1.854 kilometres per hour or 30.9 metres per minute.

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

Answer: The classification of wind is done on the basis of their spatial extent. Winds are mainly of three types.

(a) Primary Circulation
Primary circulation is the permanent wind system that covers the whole earth. These circulations continue from the sub-tropical and polar high-pressure belts to the equatorial and sub-polar low-pressure belts. The examples of primary circulation are trade winds, westerlies, and polar easterlies.

(b) Secondary Circulation
Secondary circulation occurs due to variation in landforms, differences in the distribution of land and water bodies, and regional or seasonal variation in temperature and pressure. These winds bring about significant weather change and sometimes make the atmosphere unstable. Cyclone, anticyclone, air mass, front and monsoon are the main examples.

(c) Tertiary Circulation (Local Winds)
Tertiary circulation or local winds occur in a limited area depending on local factors like topography and altitude. Their influence is not widespread but they affect the weather condition of a place. Sea breeze, land breeze, mountain wind and valley wind are the main examples.

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

Answer: Primary circulation of wind refers to the permanent movement of air on the earth that is controlled by the global pressure belts. These winds blow regularly from the sub-tropical and polar high pressure belts towards the equatorial and sub-polar low pressure belts in both hemispheres. Because of their continuous nature, they are also called Permanent Winds or Planetary Winds. The main examples of primary circulation are the Trade Winds, Westerlies and Polar Easterlies.

Types and Characteristics:

1. Trade Winds –
These blow from the subtropical high-pressure belt (30° N and 30° S) towards the equatorial low-pressure belt. Due to the Coriolis force, they blow from northeast to southwest in the northern hemisphere and from southeast to northwest in the southern hemisphere. They meet at the equatorial calm zone called the Doldrums.

2. Westerlies –
These winds blow from the subtropical high-pressure belt towards the subpolar low-pressure belt. In the northern hemisphere, they deflect from southwest to northeast, and in the southern hemisphere from northwest to southeast. They are strong, especially in the southern oceans, where they are called Roaring Forties, Furious Fifties , and Screaming Sixties.

3. Polar Easterlies –
These winds blow from the polar high-pressure belt towards the sub-polar low pressure belt. They blow from the east in both hemispheres and often meet the westerlies, forming the Polar Front, which creates cyclonic storms.

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

Answer: The wind circulation that develops in different parts of the earth due to variation in landforms, distribution of land and water bodies, and seasonal or regional differences in temperature and pressure is called Secondary Circulation of wind.

Its contribution to climate:
Secondary circulation plays a very important role in changing the weather and climate of a place. It makes the atmosphere sometimes unstable by bringing sudden changes. The main forms of secondary circulation are cyclone, anticyclone, air mass, front and monsoon.

For example, cyclones bring violent storms, heavy rainfall and unstable weather. Anticyclones are calm and create fair and dry weather but sometimes cause cold waves or hot waves. Air masses transfer heat and humidity from one region to another, balancing atmospheric temperature. Fronts bring sudden changes like rainfall, cloud formation and thunderstorms. Monsoon winds are seasonal winds that cause heavy rainfall in South Asia and South-East Asia, strongly influencing agriculture and climate.

Thus, secondary circulation is one of the most important factors that determines the weather and climate of many regions of the world.

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

Answer: A local wind is the wind that blows only in a small or limited area due to local causes like topography, altitude and temperature difference between land and water. It is also called tertiary circulation or temporary wind. Unlike permanent or secondary winds, their influence is short-lived, but they play a very important role in changing the daily weather of a place.

Examples and influence on weather:

1. Sea Breeze and Land Breeze: In coastal regions, the land gets heated quickly during the day, creating low pressure, so cool and moist air blows from the sea to land. This is called sea breeze, which lowers temperature and sometimes brings rainfall. At night, land cools faster than the sea, creating high pressure, so dry and cool air blows from land to sea. This is called land breeze, which makes the nights cooler and clearer.

2. Mountain and Valley Wind: In hilly areas, the dense air of the valleys heats up during the day and rises upward along the slopes, called valley wind. At night, the mountain peaks cool faster, and cold heavy air descends into the valleys, called mountain wind. These winds affect temperature and reduce fog in foothill regions, especially in winter.

3. Other Local Winds: Many other local winds strongly affect the weather. Chinook in North America and Foehn in Europe melt snow quickly and make plains warmer. Loo in northern India creates extremely hot and dry conditions in summer. Sirocco from the Sahara desert carries dust and hot air, while Mistral in Europe brings cold strong winds in winter.

Though local winds blow only for a short time and in a small region, they are very important in deciding whether the weather of a place becomes cool, hot, dry, moist, or stormy. Hence, they play a big role in daily life and local climate.

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

Answer: A cyclone is a circulation of the atmosphere in which high-velocity wind blows around a low-pressure centre. The pressure at the centre is much lower than the surrounding areas, so winds converge towards it in a circular path. Cyclones are violent in nature and often cause sudden weather changes.

The main types of cyclones are:

1. Tropical Cyclone: Found in tropical regions during summer. Wind speed varies from 120 to 280 km per hour and its diameter can extend up to several hundred kilometres. It moves from the sea towards the coast and causes great destruction. They are known by different names – Typhoon in the Pacific coast, Hurricane in the West Indies, Cyclone in the Indian Ocean, Bardoisila in Assam and Willy-Willy in Australia.

2. Extra-Tropical Cyclone: Formed in the temperate and high-latitude regions (30°–65°). It develops in the frontal zone where warm tropical winds and cold polar winds meet. It covers a large area and brings dense clouds, heavy rainfall, hail, thunderstorm and sudden weather changes.

Cyclones have a very strong influence on the climate of a place. Tropical cyclones cause heavy rainfall, stormy weather, flooding and destruction of life and property in coastal regions. Extra-tropical cyclones make the climate of temperate regions unstable, bringing sudden changes such as rain, hail, or thunderstorms, followed by cold and clear conditions.

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

Answer: An air mass is an immense body of air having homogenous temperature and humidity conditions. When the air in a wide area remains stable for a long time, its lower layer absorbs the temperature and humidity character of the land surface, and in this way a special type of air mass is formed.

Air masses are mainly classified into four types based on their source region and characteristics. These are Tropical Continental air mass (cT), Tropical Maritime air mass (mT), Polar Continental air mass (cP) and Polar Maritime air mass (mP).

Air mass plays an important role in determining the climate of a place. When an air mass moves, it changes the weather of the area through which it passes. It also helps in the transfer of heat from warm regions to cold regions and thereby maintains balance in atmospheric temperature. Sometimes an air mass also experiences change in its own temperature and humidity condition when it moves to a new region. In this way, air masses influence both their own character as well as the climate of different places.

20.What do you mean by 'Front'? How it is formed? Briefly discuss the relationship between climate and front.

Answer: A front is formed when two different air masses with sharp contrasts in temperature, humidity, pressure and density come closer from opposite directions. They do not mix readily and create a line of discontinuity between them, which is called a front. Weather changes generally take place in such fronts.

The formation of a front requires two special conditions:

1. Between two air masses, one should be colder and heavier than the other.

2. Air movement should be convergent so that two air masses from opposite directions move closer to one another.

The relationship between climate and front is very close because the meeting of contrasting air masses brings sudden changes in the atmosphere. The progress of a front changes air temperature and humidity, and in the frontal region clouds are often formed. These changes influence the weather pattern of the area, and repeated formation of fronts makes the climate of that region unstable.

21.What do you mean by 'Monsoon Wind'? How it is formed? In which areas of the earth its impact is quite distinct?

Answer: The wind that blows periodically depending on the change of season is called Monsoon Wind. The word “Monsoon” has come from the Arabic word Mausim and the Malayan word Monsin. It is a type of surface wind whose direction in summer is completely opposite to that in winter.

Formation:
Monsoon wind is formed due to the difference in heating capacity of land and water. In summer, the land surface becomes much hotter than the ocean, creating low pressure over land and high pressure over water. As a result, moist air from the ocean blows towards the land and brings heavy rainfall. In winter, the condition reverses, as land becomes colder than the ocean, creating high pressure over land and low pressure over water. Thus, dry and cold winds blow from land to sea, giving little rainfall except in some coastal areas.

Areas of distinct impact:
The impact of monsoon is very clear in Asia, especially South Asia, South-East Asia and South-East China. Apart from this, monsoon winds are also found in South Africa, north-east Australia, the Gulf of Mexico and the south-east and south-west coasts of the USA, but their effect is strongest in Asia.

22. Write with reasons:

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

Answer: All weather phenomena occur mainly in the troposphere because this layer contains about three-fourths of the total atmospheric gases and almost all the water vapour and dust particles. The presence of water vapour and dust causes clouds, rainfall, storms and cyclones to take place in this layer.

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

Answer: If the amount of carbon dioxide increases in the atmosphere, the atmospheric temperature will also rise. This is because carbon dioxide has the capacity to absorb the solar radiation reflected from the earth’s surface, creating a warmer condition. The gradual increase in CO₂ due to burning of fossil fuels and deforestation leads to rise in global temperature. This phenomenon is known as Global Warming.

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

Answer: The pressure of dry air is higher than moist air because the capability of moisture absorption of cold air is low. Warm air absorbs more moisture and becomes lighter with low pressure, while dry air with less moisture remains heavy and creates high pressure.

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

Answer: The ozone layer is present in the atmosphere at an altitude of about 40–50 km. It absorbs the sun’s very harmful ultraviolet radiation. By stopping these rays from reaching the earth, it protects plants, animals, and human beings. Thus, the ozone layer saves the entire living world and helps in the survival of life on Earth.

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

Answer: The air pressure is highest at the sea level because the depth of the atmosphere is maximum at this level. As altitude increases, the depth and weight of the air decrease, so the air pressure also becomes low.

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

Answer: The vertical extent of the troposphere varies with latitude. In the equatorial region its height is about 16 km, while in the polar region it is only about 8 km. This variation is due to the difference in temperature between these two regions.

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

Answer: In the equatorial low-pressure belt, the air gets heated and moves upward. Under such conditions, no horizontal movement of air parallel to the earth’s surface is visible. That is why this region is also known as the Equatorial Doldrums.

23. Give a short answer to the following questions:

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

Answer: The vertical extent of the atmosphere is 10,000 km.

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

Answer: The chemical composition of gases remains almost the same up to an altitude of about 80 km from the Earth’s surface. This lower layer of the atmosphere is called the Homosphere.
(c) What is the name of the boundary line between the homosphere and the heterosphere?

Answer: The boundary line between the homosphere and the heterosphere is called the Mesopause.
(d) What is Ferrell's Law?

Answer: The force generated due to the Earth's rotation deflects the wind towards right in clockwise direction in the northern hemisphere, and towards left in the anticlockwise direction in the southern hemisphere. This phenomenon is known as Ferrell's Law.

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

Answer: The Beaufort Scale is a scale of wind velocity developed by the British scientist Sir Francis Beaufort in 1805. It is divided into numbers from 0 to 12 to understand the strength of wind.

The utility of this scale is that it helps to measure and describe the nature and impact of different types of wind. For example, wind velocity of 1 Knot means one nautical mile per hour, that is about 1.854 km per hour. This makes it easy to compare wind strength in different areas.

(f) Write in brief about Lapse Rate.

Answer: Lapse Rate is the rate at which the air temperature decreases with increase in height in the troposphere. Generally, the temperature decreases at the rate of 6.5°C per kilometre ascent.

24. Write short note:

(a) Homosphere

Answer: The Homosphere is the lower layer of the atmosphere where the chemical composition of gases remains almost the same. It extends up to about 80 km from the earth’s surface. Nearly 99% of the atmospheric mass is confined within 32 km of this layer. The Homosphere is further divided into three parts – Troposphere, Stratosphere and Mesosphere

(b) Heterosphere

Answer: The Heterosphere is the upper layer of the atmosphere lying above 80 km altitude. In this layer, the gases separate according to their atomic weight. It is divided into four layers – Nitrogen layer (80–200 km), Oxygen layer (up to 1125 km), Helium layer (up to 3540 km) and Hydrogen layer (up to about 10,000 km). These layers together fall under the Thermosphere and Exosphere.

(c) Air temperature, earth's surface height and air pressure relationship

Anwer: Air pressure depends on both air temperature and altitude from the earth’s surface. When temperature rises, air expands, density decreases and pressure becomes low. Warm and moist air always has low pressure, while cold and dry air has high pressure. Similarly, air pressure decreases with increase in altitude because the depth and weight of the atmosphere decline. For every 900 feet rise, pressure decreases by about 1 inch or 34 millibars.

(d) Polar High Pressure Belt

Answer: The Polar High-Pressure Belt is found in both the polar regions of the Earth. Due to a lack of direct sunlight, these regions are very cold and remain snow-covered. The air is extremely cold and almost free from water vapor. As a result, atmospheric pressure remains permanently high in both the polar regions.

(e) Wind system

Answer: The movement of air on the earth’s surface is called wind, and its overall pattern is known as the wind system. Wind system plays an important role in transferring heat, moisture and other physical characteristics of the atmosphere from one place to another.

According to their nature and extent, winds are classified into three systems:

1. Primary Circulation: Permanent planetary winds like Trade winds, Westerlies and Polar easterlies that blow continuously due to pressure belts.

2. Secondary Circulation: Winds caused by regional and seasonal changes, like cyclone, anticyclone, monsoon, air mass and fronts.

3. Tertiary Circulation or Local Winds: Temporary winds limited to small areas, such as sea breeze, land breeze, mountain wind, valley wind, Chinook, Loo, Sirocco, Mistral, etc.

Thus, the wind system helps in shaping weather and climate across the world.

(f) Pressure Gradient

Answer: The Pressure Gradient is the rate of change of atmospheric pressure between two places over a certain distance. It is the main cause of wind movement on the earth’s surface. Air always moves from a region of high pressure to a region of low pressure. If the pressure difference is high, the gradient is steep and wind velocity becomes strong. If the pressure difference is low, the gradient is gentle and wind remains calm. The pressure gradient, therefore, determines both the direction and velocity of wind.

(g) Trade wind

Answer: Trade Winds are the surface winds that blow from the sub-tropical high pressure belts on both sides of 30° North and 30° South latitudes towards the equatorial low pressure belt. Due to the Coriolis Force, these winds blow from northeast to southwest in the northern hemisphere and from southeast to northwest in the southern hemisphere. In the equatorial region, the movement of air is weak and calm, so this zone is called the Doldrums.

(h) Horse latitude

Answer: Horse Latitude is the name given to the sub-tropical high-pressure belt located between 25°–35° latitudes in both hemispheres. In this region, cold and heavy air descends and creates high pressure. The wind movement here is very weak, so it often becomes difficult for ships to sail. In the middle ages, European navigators, facing this difficulty, used to throw horses into the sea to reduce the ship’s load. Because of this, the region came to be known as Horse Latitude.

(i) Roaring Forties

Answer: The Roaring Forties are the strong westerly winds found in the southern hemisphere between 40° and 50° latitudes. In this region, a large part is covered by ocean, so the westerlies blow with great intensity. Navigators named these latitudes as Roaring Forties because of the violent and continuous nature of the winds.

(j) Anticyclone

Answer: An anticyclone is a high-pressure system in which the wind blows outward from the centre in a cyclic path. The pressure is highest at the centre, so it is also called a high-pressure centre. The term “anticyclone” was first used by Sir Francis Galton in 1861.

Anticyclones are formed in the sub-tropical and high-latitude regions. Due to the Earth’s rotation, the wind moves clockwise around the centre in the northern hemisphere and anticlockwise in the southern hemisphere. They are of two types—sub-tropical warm core anticyclone and high-latitude cold core anticyclone. Generally, anticyclones are associated with fair weather, though sometimes they may cause cold waves or hot waves.

(k) Jet Stream

Answer: Jet Stream is a very high velocity wind that blows towards the east at high altitude in the upper layer of the troposphere in the tropical region. Its speed can reach about 640 kilometres per hour. The Jet Stream plays an important role in bringing significant changes in the weather and climate of any region over which it passes.

(l) Isobar

Answer: An Isobar is a line drawn on a map that joins places having the same barometric pressure at a given time. By studying isobars, we can understand the pressure distribution of a region and also know the direction and velocity of wind. Where isobars are close together, the pressure gradient is steep and wind velocity is high. Where isobars are widely spaced, wind movement is weak and calm.

(m) Tropical Cyclone

Answer: A tropical cyclone is a violent circulation of high velocity wind around a low-pressure centre in the tropical region. It blows anticlockwise in the northern hemisphere and clockwise in the southern hemisphere. Its velocity ranges from 120 km to 280 km per hour, and the diameter may vary from a few to several hundred kilometres.

It is formed during the summer season under the influence of the ocean. After developing over the sea, it moves towards the coast and enters the land. Tropical cyclones are violent and destructive, causing great loss of life and property. They are known as Typhoon in the Pacific coast, Hurricane in the West Indies, Cyclone in the Indian Ocean, Bardoisila in Assam, and Willy-Willy in north-west Australia.

25. Write the differences:

(a) Horizontal wind and Vertical wind

Answer:

(b) Wind and Air mass

Answer:

(c) Tropical cyclone and Extra-tropical cyclone

Answer:

(d) Sea breeze and Land breeze

Answer:

(e) Mountain wind and Valley wind

Answer:

(f) Cyclone and Anticyclone

Answer:

(g) Cold Front and Warm Front

Answer:

(h) Cold wave and Hot Wave

Answer:

(i) Troposphere and Stratosphere

Answer:

26. Find out the correct answer:

(a) Where is the Ozone layer located?
(1) Troposphere
(2) Stratosphere
(3) Mesosphere
(4) Thermosphere

Answer: (2) Stratosphere

(b) What is the amount of oxygen in the atmosphere in terms of volume?
(1) 20.94%
(2) 29.01%
(3) 32.47%
(4) 78.08%

Answer: (1) 20.94%

(c) The most important reason for air movement is
(1) Humidity difference
(2) Pressure difference
(3) Gravitational Force
(4) Centrifugal Force

Answer: (2) Pressure difference

(d) The instrument used for the determination of wind velocity is
(1) Wind vane
(2) Anemometer
(3) Beaufort scale
(4) Hydrometer

Answer: (2) Anemometer.

(e) The unit of wind velocity is
(1) Knot
(2) Milibar
(3) Percentage
(4) Degree

Answer: (1) Knot.

(f) Monsoon wind belongs to which of the following class?
(1) Local winds
(2) Primary circulation
(3) Secondary circulation
(4) Permanent wind

Answer: (3) Secondary circulation.

(g) The name of the cyclone formed in the coastal region of the Pacific Ocean in the east is
(1) Cyclone
(2) Hurricane
(3) Willy-Willy
(4) Typhoon

Answer: (4) Typhoon