SEBA Class 9 Social Science Chapter 7 Atmosphere: Structure, Air Pressure and Wind System

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SEBA Class 9 Social Science Chapter 7 Atmosphere: Structure, Air Pressure and Wind System

SEBA Class 9 Social Science Chapter 7 Atmosphere: Structure, Air Pressure and Wind System Also, you can read the (SCERT) in these sections Solutions by Expert Teachers as per Board of Secondary Education, Assam (SEBA) Guidelines. SEBA Class 9 Social Science Chapter 7 Atmosphere: Structure, Air Pressure and Wind System. Here we have given SEBA Textual Solutions For Class 9 Social Science English Medium.

Atmosphere: Structure, Air Pressure and Wind System

GEOGRAPHY

EXERCISE

Give Very Short Answer:

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

Ans: The atmosphere is the gaseous envelope of air that surrounds the Earth from its surface up to a certain height. It is made up of colourless gases, water vapour, and dust particles. We cannot see or feel air, but we survive by breathing it.

The atmosphere is a layer of gases that extends up to about 10,000 km from the Earth’s surface. It consists of gases, water vapour, and dust particles. Its composition and properties change with altitude, affecting air pressure, density, and temperature.

The main gases are: Nitrogen (78.08%), Oxygen (20.94%), Argon (0.93%), Carbon dioxide (0.036%). Trace gases include neon, helium, methane, krypton, hydrogen, ozone, xenon, and carbon monoxide (0.014%).

These components together form the structure and function of the atmosphere.

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

Ans: The atmosphere is a blanket of gases that surrounds the Earth and supports life by providing air, regulating temperature, and protecting from harmful solar radiation. Based on variations in temperature and altitude, the atmosphere is divided into five distinct layers, each with its own features and functions.

(i) The Troposphere (0–12 km) is the lowest layer where all weather changes occur and temperature decreases with height.

(ii) The Stratosphere (12–50 km) contains the ozone layer; temperature increases with altitude and jet planes fly here.

(iii) The Mesosphere (50–80 km) is the coldest layer; meteors burn up in this layer and temperature decreases with height.

(iv) The Thermosphere (80–500 km) has very high temperatures, contains the ionosphere, and causes auroras.

(v) The Exosphere (above 500 km) is the outermost layer with very thin air that gradually merges into outer space.

Each layer of the atmosphere plays a crucial role in maintaining life and environmental balance on Earth. From weather changes in the troposphere to communication signals in the thermosphere, the structure of the atmosphere helps us understand how our planet functions as a life-supporting system.

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

Ans: 

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Ans: Atmospheric pressure varies due to:

(i) Air Temperature: Warmer air expands and rises, lowering pressure. Cooler air contracts and sinks, increasing pressure. E.g., equatorial region has low pressure due to high temperature.

(ii) Altitude: Higher altitude means thinner air, resulting in lower pressure. E.g., pressure is higher at sea level and decreases on mountains.

(iii) Water Vapour: Moist air is lighter and has lower pressure; dry air is heavier and has higher pressure.

(iv) Seasonal Changes: Pressure changes with seasons due to heating/cooling of land and ocean surfaces.

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

Ans: There are four major pressure belts:

(i) Equatorial Low Pressure Belt: Located between 10°N to 10°S. High temperature, rising moist air, low pressure. Also known as Doldrum.

(ii) Subtropical High Pressure Belt: At 25° to 35° N/S. Descending cool air, dry conditions. Calm areas called Horse Latitudes.

(iii) Sub-Polar Low Pressure Belt: At 60° N/S. Rising air due to convergence of polar and westerlies. Causes cyclonic storms.

(iv) Polar High Pressure Belt: At 90° N/S. Cold, dense air, high pressure, dry.

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

Ans: The atmosphere is vital for maintaining life and ecological balance on Earth. 

Its importance includes:

(i) Breathable Air: Provides oxygen for humans and animals, and carbon dioxide for plants.

(ii) Climate Regulation: Distributes heat around the globe, maintaining temperature suitable for life.

(iii) Weather System: Enables weather phenomena such as rainfall, storms, and snowfall.

(iv) Protection from Radiation: The ozone layer in the stratosphere absorbs harmful UV rays.

(v) Water Cycle: Carries moisture and facilitates precipitation.

(vi) Supports Communication: The ionosphere reflects radio waves, enabling wireless communication.

Thus, the atmosphere creates a shield and an environment favourable for life to exist.

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

Ans: Wind is the horizontal movement of air from areas of high pressure to low pressure. It is a vital element of the Earth’s weather and climate system.

Factors responsible for origin of wind:

(i) Pressure Gradient Force: The difference in air pressure between regions causes air to move.

(ii) Gravitational Force: Holds the atmospheric gases and influences air movement from high to low altitude.

(iii) Coriolis Force: Caused by Earth’s rotation, it deflects wind to the right in the Northern Hemisphere and left in the Southern Hemisphere.

(iv) Frictional Force: Acts near the Earth’s surface, slowing wind and changing its direction based on terrain.

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

Ans: The most important reason for wind is the difference in air pressure, primarily caused by uneven heating of the Earth’s surface by the sun.

Velocity and direction of wind are determined by:

(i) Pressure Gradient Force: Steeper gradients produce stronger winds.

(ii) Coriolis Force: Deflects wind based on Earth’s rotation.

(iii) Frictional Force: Slows and alters direction near the surface, depending on landscape.

(iv) Gravitational Force: Contributes to vertical air movement and affects wind pressure.

These forces collectively shape the global wind system.

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

Ans: Coriolis Force is an apparent force that arises due to the rotation of the Earth. It was first mathematically described by Gaspard de Coriolis, a French scientist, in the 19th century. This force does not actually push the wind, but it causes a deflection in its path due to the Earth’s rotation.

Because the Earth rotates from west to east, any object or mass of air moving freely over the Earth’s surface appears to be deflected:

(i) To the right in the Northern Hemisphere.

(ii) To the left in the Southern Hemisphere.

This effect is zero at the equator and becomes strongest at the poles. The Coriolis force is directly proportional to the speed of the moving object and the sine of the latitude.

Contribution in Wind Direction:

Coriolis Force plays a crucial role in shaping global wind patterns. Without this force, winds would move directly from high-pressure to low-pressure areas. But due to Coriolis force:

(i) Trade Winds curve westward in both hemispheres.

(ii) Westerlies bend toward the east.

(iii) Jet Streams, which are high-speed winds in the upper atmosphere, also follow curved paths.

It also influences ocean currents and cyclonic systems, causing cyclones to rotate anticlockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.

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

Ans: Pressure Gradient Force is the force exerted due to the difference in air pressure between two places. It acts perpendicular to the isobars and from high-pressure areas to low-pressure areas.

This force initiates the movement of air, causing wind. The greater the difference in pressure, the stronger the wind produced. It is the primary force responsible for the generation of wind and helps maintain atmospheric circulation by moving air masses around the globe.

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

Ans: Earth’s gravitational force is responsible for holding the atmosphere close to its surface. It contributes to the vertical movement of air by pulling heavier, denser air downward and allowing lighter air to rise.

Gravitational force causes the air pressure to be higher at sea level and lower at higher altitudes. It also causes air to flow from elevated areas to low-lying areas, such as from mountain tops to valleys. Thus, gravity helps maintain the vertical balance of atmospheric gases and contributes to pressure differences that drive wind.

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

Ans: The naming of wind is based on the direction from which it blows. For example, a wind coming from the west and blowing towards the east is called a westerly wind. Similarly, a wind blowing from the northeast direction is called a north-easterly wind. The wind carries the physical characteristics of the region over which it travels, such as heat, moisture, and dust. For example, winds blowing over oceans contain large amounts of water vapor, whereas winds blowing over deserts are typically dry and hot. In meteorology, wind direction is crucial and can be observed using tools like the Wind Vane. The direction is expressed in degrees, with north as 0º, east as 90º, south as 180º, and west as 270º. The direction from which the wind comes is called the windward side, and the direction it moves towards is called the leeward side.

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

Ans: The velocity of wind is determined with the help of an instrument called an Anemometer. This instrument measures the speed of the wind, and the modern version of it, known as Anemograph, can automatically record both wind direction and speed. The unit of wind velocity is usually expressed in Knots. One Knot is equal to one nautical mile per hour, which is approximately 1.854 kilometers per hour or 30.9 meters per minute. For easier understanding of wind velocity and its effects, a 0–12 number Beaufort Scale was developed by Sir Francis Beaufort. According to this scale, wind with a velocity less than 1 Knot is called “Calm,” and as the wind velocity increases, it is categorized as breeze, gale, storm, etc.

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

Ans: Winds are broadly classified into three categories based on their spatial extent and characteristics: 

(1) Primary Wind Circulation.

(2) Secondary Wind Circulation. and 

(3) Local or Tertiary Wind Circulation. 

Primary winds are planetary in nature and include the Trade Winds, Westerlies, and Polar Easterlies, which blow continuously in fixed directions between permanent pressure belts. Secondary winds include cyclones, anticyclones, monsoons, and air masses that are seasonal and regional. For example, the Monsoon Winds of South Asia are secondary circulations. Tertiary or local winds are temporary and localized, influenced by topography and local conditions. Examples include land breeze, sea breeze, mountain wind, and valley wind. Each type of wind plays a critical role in determining the weather and climate of a place.

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

Ans: Primary circulation of wind refers to the large-scale, permanent movement of air that is driven by the latitudinal differences in temperature and atmospheric pressure. These circulations are also called planetary winds as they blow continuously across the earth and are responsible for the global wind system. 

There are three main types of primary winds: 

(i) Trade Winds.

(ii) Westerlies. and 

(iii) Polar Easterlies.

(i) Trade Winds: Blow from subtropical high-pressure belts (30º N/S) towards the equatorial low-pressure belt. In the northern hemisphere, they are called North-East Trade Winds, and in the southern hemisphere, South-East Trade Winds.

(ii) Westerlies: Blow from the subtropical high-pressure belt towards the sub-polar low-pressure belt. They are south-westerlies in the northern hemisphere and north-westerlies in the southern hemisphere.

(iii) Polar Easterlies: Originate from the polar high-pressure areas and blow towards the sub-polar low-pressure belt.

These wind systems form cells (Hadley Cell, Ferrel Cell, and Polar Cell) and determine the weather patterns of different zones on earth.

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

Ans: Secondary circulation of wind refers to the regional or seasonal wind patterns formed due to the differential heating of land and water, variation in topography, and shifting of pressure belts. These winds are not permanent like planetary winds but occur periodically or under specific conditions. Examples include cyclones, anticyclones, monsoons, and air masses.

Cyclones and Anticyclones influence weather conditions like rainfall, temperature, and storms.

Monsoon Winds are the best example of secondary circulation. They cause seasonal reversal of wind direction due to the differential heating of land and sea. For example, South-West Monsoon brings heavy rainfall to India during summer.

Air Masses carry specific temperature and humidity characteristics which affect the regions they pass over.

These circulations play a crucial role in altering the climatic conditions and influencing rainfall, temperature, and overall weather patterns.

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

Ans: Local wind, also known as tertiary wind, refers to wind circulation limited to a small area and caused by local geographical or thermal conditions. These winds are temporary and short-lived but significantly affect the weather of the local area. Examples include land breeze, sea breeze, mountain wind, and valley wind.

Sea Breeze occurs during daytime when land gets heated faster than the adjacent sea. Cooler air from the sea moves towards land.

Land Breeze occurs during nighttime when the land cools faster than the sea. The air from land flows towards the sea.

Mountain and Valley Winds arise due to differential heating of slopes. During day, warm air moves up the mountain (valley wind), and during night, cold air descends from mountain to valley (mountain wind).

These winds affect temperature, humidity, and often cause localized rainfall or dry weather, making them important in microclimate studies.

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

Ans: A cyclone is a large-scale atmospheric system in which winds spiral inward towards a center of low pressure, rotating counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.

There are two types of Cyclones:

(i)  Tropical Cyclone: Originates in tropical oceans during summer, such as hurricanes, typhoons, or cyclones.

(ii)  Extra-Tropical or Temperate Cyclone: Forms in mid and high latitudes between 30° to 65° in both hemispheres due to the interaction of cold and warm air masses.

Influence on Climate:

Cyclones significantly influence the climate by bringing heavy rainfall, thunderstorms, hailstorms, and sometimes destructive winds. They can cause sudden weather changes and are responsible for shifting rainfall patterns and temperature variations, especially in tropical and temperate regions.

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

Ans: An air mass is a large body of air with uniform temperature, humidity, and pressure characteristics acquired from its source region. As it moves, it influences the weather and climate of the areas it passes over.

Classification of Air Masses:

(i) Tropical Continental (CT)

(ii) Tropical Maritime (mT)

(iii) Polar Continental (cP)

(iv) Polar Maritime (mP)

Contribution in Climate:

Air masses transfer heat and moisture from one region to another, bringing changes in temperature, humidity, and rainfall. They play a vital role in balancing global temperatures and affect regional weather patterns such as droughts, snow, fog, and rainfall.

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

Ans: A Front is a boundary or zone of transition between two different air masses having contrasting temperature, humidity, and pressure characteristics.

Formation:

When two air masses meet and do not mix due to differences in density and temperature, a front is formed. The warm air rises over the cold air, leading to instability and cloud formation.

Types of Fronts:

1. Cold Front

2. Warm Front

3. Stationary Front

4. Occluded Front

Relationship with Climate:

Fronts cause sudden and significant weather changes. Cold fronts bring heavy rain and thunderstorms, warm fronts bring steady rainfall, and occluded fronts may lead to complex weather patterns. Thus, fronts play a crucial role in determining the short-term climatic conditions of a region.

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

Ans: Monsoon Wind is a seasonal wind that changes its direction with the change of seasons, blowing from sea to land in summer and land to sea in winter.

Formation:

It forms due to the differential heating of land and sea. In summer, land heats up faster creating low pressure, attracting moist oceanic winds. In winter, land cools down quickly forming high pressure, causing the dry wind to blow towards the ocean.

Regions with Distinct Impact:

The monsoon wind system has a distinct impact in South and Southeast Asia, especially in countries like India, Bangladesh, Myanmar, Thailand, Vietnam, and South China. It also affects parts of North Australia, the Gulf of Mexico, and East Africa. These regions experience heavy seasonal rainfall due to monsoons, which heavily influence agriculture and water resources.

22. Write with reasons:

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

Ans: All weather phenomena like cloud formation, rainfall, storms, and cyclones occur mainly in the troposphere because it contains about three-fourths of the total atmospheric gases, nearly all the water vapour, and dust particles. This layer is closest to the earth’s surface and directly receives the effects of solar heating, creating convection currents. The temperature in this layer decreases with height, which promotes vertical air movement necessary for cloud formation and precipitation. Moreover, the troposphere is the most dynamic layer of the atmosphere, constantly influenced by changes in temperature and pressure, thus supporting weather changes.

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SEBA Class 9 Social Science Solutions [New Editions Updated]

Sl. No.	CONTENTS
Chapter – 1	Advent of the Europeans into India
Chapter – 2	Growth of Indian Nationalism
Chapter – 3	The Moamoriya Rebellion
Chapter – 4	Burmese Invasion of Assam
Chapter – 5	Beginning of The British Administration in India
Chapter – 6	Change of the Earth’s Surface
Chapter – 7	Atmosphere: Structure, Air Pressure and Wind System
Chapter – 8	Geography of India
Chapter – 9	Geography of Assam
Chapter – 10	Political Parties in India
Chapter – 11	Types or Classification of Government
Chapter – 12	Basic Concepts of Economics
Chapter – 13	Basic Economic Problems