Samacheer Kalvi 9th Social Science Atmosphere Exercise Solutions | KalviGavani

Atmosphere — Exercise Solutions Guide

Solve every exercise on your own first, then cross-check your answers here. Understanding why an answer is correct matters more than memorising it.

I. Multiple Choice Questions (MCQs)

**Q1. The layer of the atmosphere where weather phenomena occur is the

(a) Stratosphere (b) Troposphere (c) Mesosphere (d) Thermosphere

**Q2. The ozone layer is found in the

(a) Troposphere (b) Stratosphere (c) Mesosphere (d) Exosphere

**Q3. Which gas makes up the largest percentage of the Earth's atmosphere?

(a) Oxygen (b) Carbon dioxide (c) Nitrogen (d) Argon

**Q4. The instrument used to measure atmospheric pressure is the

(a) Barometer (b) Thermometer (c) Hygrometer (d) Anemometer

**Q5. Winds that blow from the subtropical high-pressure belts towards the equator are called

(a) Westerlies (b) Trade winds (c) Polar winds (d) Monsoons

**Q6. Which type of rainfall is caused when moist air is forced to rise over a mountain?

(a) Convectional rainfall (b) Cyclonic rainfall (c) Orographic rainfall (d) Acid rain

**Q7. The boundary between the troposphere and the stratosphere is called the

(a) Stratopause (b) Tropopause (c) Mesopause (d) Thermopause

**Q8. Clouds that are feathery and high-altitude are called

(a) Cumulus (b) Stratus (c) Cirrus (d) Nimbus

**Q9. The standard atmospheric pressure at sea level is approximately

(a) 760 mb (b) 1013 mb (c) 500 mb (d) 1100 mb

**Q10. Relative humidity is expressed as a

(a) Fraction (b) Decimal (c) Percentage (d) Ratio

**Q11. The hot local wind that blows in the northern plains of India during summer is called

(a) Mango shower (b) Loo (c) Chinook (d) Harmattan

**Q12. The weight of the vertical column of air above a point on the Earth's surface is called

(a) Wind (b) Atmospheric pressure (c) Humidity (d) Temperature

II. True or False

1. The troposphere is the thickest layer of the atmosphere.

2. Temperature decreases with altitude in the troposphere.

3. The stratosphere is the coldest layer of the atmosphere.

4. Trade winds blow from east to west.

5. Humidity refers to the temperature of the air.

6. Cyclonic rainfall is caused by the meeting of warm and cold air masses.

7. The exosphere gradually merges with outer space.

8. Low atmospheric pressure is associated with clear skies and dry weather.

9. Cumulonimbus clouds are associated with fair weather.

10. The ionosphere is part of the thermosphere.

III. Fill in the Blanks

1. The most abundant gas in the atmosphere is ___.

2. The layer of the atmosphere that contains the ozone layer is the ___.

3. The line joining places of equal atmospheric pressure on a map is called an ___.

4. The hot, dry wind blowing in the northern plains of India during summer is called the ___.

5. The type of rainfall caused by heated air rising from the ground is called ___ rainfall.

6. The instrument used to measure humidity is called a ___.

7. The boundary between the troposphere and stratosphere is called the ___.

8. Winds that blow from the subtropical high-pressure belts towards the poles are called ___.

9. The percentage of oxygen in the atmosphere is approximately ___.

10. The layer of the atmosphere where meteors burn up is the ___.

IV. Match the Following

1 → b — The troposphere (0-12 km) is where virtually all weather phenomena occur. It contains most of the atmosphere's water vapour and dust, and temperature decreases with altitude.

2 → a — The stratosphere (12-50 km) contains the ozone layer (15-35 km), which absorbs harmful ultraviolet radiation from the sun, protecting life on Earth. Temperature increases with altitude here.

3 → d — The mesosphere (50-80 km) is the coldest layer of the atmosphere. Meteors burn up here due to friction with air molecules, creating shooting stars.

4 → c — The thermosphere (80-700 km) has very high temperatures (up to 1,500 degrees Celsius) due to absorption of solar radiation. The International Space Station and many satellites orbit in this layer.

5 → e — The exosphere (above 700 km) is the outermost layer where gas density is so low that molecules rarely collide. It gradually merges with the vacuum of outer space.

V. Short Answer Questions (2-3 marks)

Q1. What is the atmosphere? Name its composition.

The atmosphere is a thin envelope of gases, water vapour, and dust particles surrounding the Earth, held in place by gravity. Its composition by volume is: nitrogen (78.08%), oxygen (20.95%), argon (0.93%), carbon dioxide (0.04%), and trace gases (neon, helium, methane, ozone, etc.). It also contains variable amounts of water vapour (0-4%) and dust particles. The atmosphere protects life by absorbing harmful solar radiation, regulating temperature, and burning up meteors.

Q2. Distinguish between weather and climate.

Weather refers to the short-term state of the atmosphere at a specific place and time, including conditions like temperature, humidity, rainfall, wind, and cloud cover. It changes frequently — from day to day or even hour to hour. Climate, on the other hand, is the average weather conditions of a place measured over a long period, typically 30 years or more. For example, "today is rainy" describes weather, while "Chennai has a tropical wet-and-dry climate" describes climate.

Q3. What are the five layers of the atmosphere? State one feature of each.

The five layers from bottom to top are: (1) Troposphere (0-12 km) — the lowest layer where all weather occurs; temperature decreases with altitude. (2) Stratosphere (12-50 km) — contains the ozone layer that absorbs UV radiation; temperature increases with altitude. (3) Mesosphere (50-80 km) — the coldest layer where meteors burn up; temperature decreases with altitude. (4) Thermosphere (80-700 km) — contains the ionosphere important for radio communication; temperature rises sharply. (5) Exosphere (above 700 km) — the outermost layer that gradually merges with outer space; extremely low gas density.

Q4. What are trade winds? Why are they called so?

Trade winds are steady winds that blow from the subtropical high-pressure belts (around 30 degrees latitude) towards the equatorial low-pressure belt. In the Northern Hemisphere, they blow from the northeast (NE trade winds), and in the Southern Hemisphere, from the southeast (SE trade winds), due to the Coriolis effect. They are called "trade winds" because they were historically used by sailing ships on trade routes across the Atlantic and Pacific oceans, providing reliable wind for merchant vessels.

Q5. What is relative humidity? How is it different from absolute humidity?

Absolute humidity is the actual amount of water vapour present in a given volume of air, expressed in grams per cubic metre (g/m3). Relative humidity is the ratio of the actual amount of water vapour in the air to the maximum amount the air could hold at that temperature, expressed as a percentage. Relative humidity changes with temperature — warmer air can hold more moisture, so relative humidity decreases if temperature increases without adding more moisture.

Q6. Name and briefly describe three types of clouds.

Three main cloud types are: (1) Cirrus clouds — thin, feathery, wispy clouds found at high altitudes (above 6,000 m), composed of ice crystals. They usually indicate fair weather but can signal an approaching change. (2) Cumulus clouds — fluffy, cotton-like clouds with flat bases and rounded tops, found at low to middle altitudes. They are associated with fair weather, but when they grow tall they can become cumulonimbus clouds that bring thunderstorms. (3) Stratus clouds — flat, grey, uniform layers of cloud that cover much of the sky, resembling fog above the ground. They often produce light drizzle or overcast conditions.

Q7. What are isobars? Why are they important?

Isobars are imaginary lines on a weather map that connect places of equal atmospheric pressure, usually measured in millibars (mb). They are important because: (1) They show the distribution of atmospheric pressure across a region. (2) The spacing of isobars indicates wind speed — closely spaced isobars mean strong winds, while widely spaced isobars mean calm conditions. (3) Isobars help identify high-pressure and low-pressure areas, which determine weather conditions. (4) Wind flows from high-pressure to low-pressure areas along isobars, influenced by the Coriolis effect.

Q8. What are the main types of winds? Give one example of each.

The main types of winds are: (1) Planetary (global) winds — large-scale winds that blow consistently across the planet, such as trade winds, westerlies, and polar winds. (2) Seasonal winds (monsoons) — winds that reverse direction seasonally, such as the Southwest and Northeast monsoons in India. (3) Local winds — winds that blow over a small area due to local temperature and pressure differences, such as the Loo (hot wind in northern India), Chinook (warm, dry wind on the leeward side of the Rocky Mountains), and Mistral (cold wind in southern France).

VI. Detailed Answer Questions (5 marks)

Q1. Explain the five layers of the atmosphere with their characteristics.

The atmosphere is divided into five layers based on temperature variation with altitude:

Troposphere (0-12 km): The lowest and most important layer for life. It contains about 75% of the atmosphere's mass and nearly all its water vapour and dust particles. Virtually all weather phenomena — clouds, rain, storms, and wind — occur here. Temperature decreases with altitude at a rate of about 6.5 degrees Celsius per kilometre (lapse rate). The boundary at the top, where temperature stops decreasing, is the tropopause.

Stratosphere (12-50 km): Above the troposphere, temperature increases with altitude due to the absorption of ultraviolet radiation by the ozone layer (located between 15-35 km). This layer is very stable with almost no vertical air movement, making it ideal for commercial flights. The ozone layer protects life on Earth by blocking harmful UV rays. The boundary at the top is the stratopause.

Mesosphere (50-80 km): Temperature decreases again with altitude, making this the coldest layer (temperatures can drop to about -90 degrees Celsius at the mesopause). Meteors entering the atmosphere burn up here due to friction with air molecules, producing shooting stars. The upper boundary is the mesopause.

Thermosphere (80-700 km): Temperature rises sharply in this layer (up to 1,500 degrees Celsius) due to the absorption of high-energy solar radiation by sparse gas molecules. The ionosphere, a region within the thermosphere, contains electrically charged particles (ions) that reflect radio waves back to Earth, enabling long-distance radio communication. The International Space Station orbits in the lower thermosphere.

Exosphere (above 700 km): The outermost layer where gas molecules are extremely sparse and can escape into outer space. There is no clear boundary between the exosphere and space. Light gases like hydrogen and helium are found here. Artificial satellites and space debris orbit in the exosphere.

Q2. Explain the three types of rainfall with diagrams (descriptions) and examples.

Rainfall occurs when moist air rises, cools, and condenses to form water droplets that fall as precipitation. The three types differ in the force that causes the air to rise:

Convectional Rainfall: Caused by intense solar heating of the ground surface. The ground heats the air above it, which expands, becomes lighter, and rises (convection currents). As the air rises, it cools, and moisture condenses to form cumulonimbus clouds, leading to heavy, short-duration rainfall, often accompanied by thunder and lightning. This type of rainfall is common in tropical regions during summer afternoons. Example: afternoon thunderstorms in Tamil Nadu and equatorial regions.

Orographic (Relief) Rainfall: Caused when moist air is forced to rise over a mountain barrier. As the wind approaches a mountain range, the air rises along the windward slope, cools, and condenses to produce heavy rainfall on the windward side. After crossing the mountain summit, the air descends on the leeward side, warms up, and becomes dry, creating a rain shadow area with very little rainfall. Example: the Western Ghats cause heavy rainfall on the western coast (windward), while the Deccan Plateau (leeward) lies in a rain shadow.

Cyclonic (Frontal) Rainfall: Caused when a warm, moist air mass meets a cold, dense air mass along a front. The lighter warm air is forced to rise above the heavier cold air, cooling and condensing to produce rainfall along the frontal zone. There are two types: warm fronts (where warm air gently rises over cold air, producing light but widespread rain) and cold fronts (where cold air forcefully pushes under warm air, producing heavy rain and thunderstorms). This type of rainfall is common in temperate regions. Example: rainfall along the polar front in mid-latitude regions and winter rainfall in north India caused by Western Disturbances.

Q3. Explain the factors affecting atmospheric pressure and the distribution of global pressure belts.

Factors Affecting Atmospheric Pressure: 1. Altitude: Atmospheric pressure decreases with increasing altitude because there is less air above to exert weight. At sea level, pressure is about 1013 mb; at the top of Mount Everest (8,848 m), it is only about 330 mb. 2. Temperature: Warm air is less dense than cold air, so atmospheric pressure decreases when temperature increases. Heated air expands and rises, creating low-pressure areas. Cold air is denser and sinks, creating high-pressure areas. 3. Moisture content: Moist air is lighter than dry air because water vapour molecules are lighter than nitrogen and oxygen molecules. Therefore, humid conditions are associated with lower pressure, and dry conditions with higher pressure. 4. Earth's rotation: The Coriolis effect and centrifugal force slightly reduce the effective weight of air at the equator, contributing to the equatorial low-pressure belt.

Global Pressure Belts: 1. Equatorial Low-Pressure Belt (0-5 degrees): Also called the Doldrums, this belt receives intense solar heating, causing air to rise (convection), creating low pressure and calm or light, variable winds. 2. Subtropical High-Pressure Belts (25-35 degrees N and S): Also called Horse Latitudes, these are zones of descending air from the Hadley cell, creating high pressure, calm winds, and dry conditions (responsible for major deserts). 3. Subpolar Low-Pressure Belts (60-65 degrees N and S): Where warm air from the subtropics meets cold polar air, creating low pressure, stormy weather, and strong westerly winds. 4. Polar High-Pressure Belts (at the poles): Extremely cold air sinks at the poles, creating permanent high-pressure zones with cold, dry conditions. Polar easterly winds blow from these high-pressure areas towards the subpolar lows.

Q4. Describe the different types of winds — planetary, seasonal, and local winds.

Planetary (Global) Winds: Large-scale winds that blow consistently across the planet due to global pressure belts and the Coriolis effect. They include: 1. Trade Winds — blow from the subtropical high-pressure belts (30 degrees) towards the equatorial low-pressure belt. NE trades in the Northern Hemisphere, SE trades in the Southern Hemisphere. 2. Westerlies — blow from the subtropical high-pressure belts towards the subpolar low-pressure belts (60 degrees). They blow from the southwest in the Northern Hemisphere and from the northwest in the Southern Hemisphere (also called Roaring Forties, Furious Fifties, and Shrieking Sixties in the Southern Ocean). 3. Polar Winds — blow from the polar high-pressure belts towards the subpolar low-pressure belts. They are cold, dry easterly winds.

Seasonal Winds (Monsoons): Winds that reverse direction with the seasons, caused by differential heating of land and sea. In summer, land heats faster than sea, creating a low-pressure area over land and drawing moist air from the sea (Southwest Monsoon in India, bringing heavy rainfall). In winter, land cools faster, creating a high-pressure area over land, and dry air blows from land to sea (Northeast Monsoon in India, bringing dry weather to most of the country except the southeast coast).

Local Winds: Small-scale winds caused by local geographical and temperature differences. Examples: 1. Loo — hot, dry wind blowing across northern India in summer. 2. Chinook — warm, dry wind on the leeward side of the Rocky Mountains that melts snow. 3. Foehn — similar to Chinook, found in the Alps. 4. Mistral — cold, strong wind blowing from the Alps down the Rhone Valley in France. 5. Harmattan — hot, dry, dusty wind blowing from the Sahara across West Africa. 6. Mango showers — pre-monsoon showers in Kerala and Karnataka that help mango ripening.

Q5. Explain humidity, clouds, and the process of precipitation.

Humidity: Humidity refers to the amount of water vapour present in the air. It is measured in two ways: (1) Absolute humidity — the total mass of water vapour in a given volume of air (g/m3). (2) Relative humidity — the ratio of the actual water vapour content to the maximum the air can hold at that temperature, expressed as a percentage. Warm air can hold more moisture than cold air, so relative humidity decreases if temperature rises without adding moisture, and increases if temperature drops. A hygrometer measures humidity.

Clouds: Clouds are visible masses of tiny water droplets or ice crystals suspended in the atmosphere, formed when moist air rises, cools, and condenses around dust or salt particles (condensation nuclei). The main cloud types are: (1) Cirrus — high, thin, feathery clouds made of ice crystals. (2) Cumulus — fluffy, cotton-like clouds with flat bases and rounded tops. (3) Stratus — flat, grey, uniform cloud layers covering the sky. (4) Nimbus — dark, thick rain-bearing clouds. Combinations produce cirrocumulus, cirrostratus, cumulonimbus (thunderstorm clouds), etc. Clouds are classified by altitude: high (above 6,000 m), middle (2,000-6,000 m), and low (below 2,000 m).

Precipitation: When water vapour in the atmosphere condenses and falls to the ground, it is called precipitation. Forms include rain (liquid water droplets), snow (ice crystals), sleet (frozen raindrops), and hail (balls of ice). The process: moist air rises (by convection, orographic lift, or frontal lifting), cools adiabatically, reaches the dew point, and water vapour condenses around nuclei to form cloud droplets. When the droplets grow too heavy to stay aloft, they fall as rain. The type of precipitation depends on temperature — rain in warm conditions, snow in cold conditions, hail in violent thunderstorms with strong updrafts.

Common Mistakes

1. Confusing weather and climate: Weather is a short-term condition (today's temperature), while climate is the long-term average (30+ years). Saying "Chennai has a rainy climate today" is wrong — that describes weather. Climate would be "Chennai has a tropical wet-and-dry climate."

2. Saying temperature increases with altitude in the troposphere: Temperature decreases with altitude in the troposphere (about 6.5 degrees Celsius per km). It increases in the stratosphere due to ozone absorption of UV radiation. This is a very common exam mistake.

3. Confusing trade winds and westerlies: Trade winds blow from the subtropical highs towards the equator (east to west). Westerlies blow from the subtropical highs towards the poles (west to east). Remember: trade winds = tropical, westerlies = mid-latitudes.

4. Mixing up orographic and convectional rainfall: Convectional rainfall is caused by solar heating of the ground (air rises by convection, common in tropics). Orographic rainfall is caused by mountains forcing air upward (windward = wet, leeward = dry). The key difference is the force causing the air to rise.

5. Saying the mesosphere is the hottest layer: The mesosphere is the coldest layer (temperatures down to -90 degrees Celsius). The thermosphere is the hottest in terms of kinetic temperature (over 1,000 degrees Celsius), but it would feel cold because the gas density is extremely low.

6. Confusing absolute and relative humidity: Absolute humidity is the actual amount of water vapour in the air (g/m3). Relative humidity is the percentage of moisture relative to the air's maximum capacity at that temperature. A change in temperature changes relative humidity even if the actual moisture content stays the same.

7. Thinking low pressure means dry weather: Low-pressure areas are associated with cloudy, wet weather because air rises, cools, and condenses. High-pressure areas bring clear, dry weather because descending air suppresses cloud formation.

Exam Checklist

• [ ] Can name and describe all five layers of the atmosphere with altitude ranges and key features
• [ ] Can state the composition of the atmosphere (nitrogen 78%, oxygen 21%, etc.)
• [ ] Can distinguish between weather and climate with examples
• [ ] Can explain temperature variation with altitude in each atmospheric layer
• [ ] Can describe the ozone layer's location and function
• [ ] Can define atmospheric pressure and state the factors affecting it
• [ ] Can identify and explain global pressure belts (equatorial low, subtropical high, subpolar low, polar high)
• [ ] Can name planetary winds (trade winds, westerlies, polar winds) and their directions
• [ ] Can explain monsoons as seasonal winds with examples from India
• [ ] Can give examples of local winds (Loo, Chinook, Mistral, Harmattan)
• [ ] Can explain the three types of rainfall (convectional, orographic, cyclonic) with examples
• [ ] Can describe cloud types (cirrus, cumulus, stratus, nimbus) and humidity concepts

Internal Links

• [Class 9 Social Science — Atmosphere Guide](/school/tamil-nadu-state-board/class/9/social/atmosphere)
• [All Class 9 Social Solutions](/school/tamil-nadu-state-board/class/9/social/solutions)
• [Class 9 Social Science Subject Guide](/school/tamil-nadu-state-board/class/9/social)