Atmosphere Exercise Solutions — Class 11 Geography (TN State Board)

Complete solutions for the Samacheer Kalvi 11th Geography chapter on Atmosphere, covering layers, pressure belts, winds, humidity, and precipitation — with MCQs, short answers, and detailed explanations aligned to the TN State Board syllabus.

How to Use This Page

• MCQs — Read the question, try your own answer, then reveal the correct choice and reasoning.
• Fill in the Blanks / True or False — Test your recall before checking the answers.
• Brief Answers (2 marks) — Write your own points first, then compare.
• Detailed Answers (5 marks) — Use the structured sub-headings to organise your own answer before reading the model.
• Common Mistakes and Exam Checklist — Review these before your exam to avoid losing marks.

Key Concepts (Quick Recap)

Atmosphere — Definition and Composition

The atmosphere is a thin envelope of gases surrounding the Earth, held in place by gravity. It extends roughly 10,000 km from the surface but about 99 % of its mass lies within 32 km.

Water vapour and dust particles are variable components — crucial for weather phenomena.

Layers of the Atmosphere

Temperature Inversion and Lapse Rate

• Normal Lapse Rate: Temperature drops ~6.5 °C per km rise in the troposphere.
• Temperature Inversion: Occasionally, temperature increases with altitude instead of decreasing. This traps pollutants near the surface, causing smog and fog. Common on calm, clear winter nights in valleys.

Atmospheric Pressure and Its Variation

• Atmospheric pressure is the force exerted by the weight of air on a unit area of the Earth's surface.
• At sea level: 1013.25 mb (1 atmosphere).
• Pressure decreases with altitude — roughly 1 mb per 10 m in the lower troposphere.
• Measured with a barometer; expressed in millibars (mb) or hectopascals (hPa).

Pressure Belts of the Earth

These belts shift northward in July and southward in January following the apparent movement of the Sun.

Winds

Wind = horizontal movement of air from high pressure to low pressure.

Planetary winds are also called primary winds. Seasonal winds like monsoons are secondary winds.

Humidity

Humidity = amount of water vapour in the air.

Precipitation Types

Weather vs Climate

Ozone Layer and Its Significance

• Found in the stratosphere (20 – 35 km).
• O₃ absorbs harmful ultraviolet (UV-B and UV-C) radiation from the Sun.
• Protects life from skin cancer, cataracts, and genetic mutations.
• Ozone depletion caused by CFCs, halons, and other ozone-depleting substances — addressed by the Montreal Protocol (1987).
• Ozone hole most prominent over Antarctica during spring (September–October).

I. Choose the Best Answer (MCQs)

1. The percentage of nitrogen in the atmosphere is approximately

(a) 21 % (b) 78 % (c) 1 % (d) 0.04 %

Nitrogen makes up about 78.08 % of the atmosphere by volume. It is the most abundant gas and plays a key role in the nitrogen cycle essential for life.

2. Which layer of the atmosphere contains the ozone layer?

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

The ozone layer is found between 20 km and 35 km within the stratosphere. It absorbs harmful ultraviolet radiation and protects life on Earth.

3. The normal lapse rate in the troposphere is approximately

(a) 3.5 °C per km (b) 6.5 °C per km (c) 10 °C per km (d) 15 °C per km

Temperature decreases with altitude in the troposphere at an average rate of about 6.5 °C per kilometre. This is called the normal lapse rate.

4. The pressure belt at 30° North and South latitudes is known as

(a) Equatorial Low (b) Subtropical High (c) Subpolar Low (d) Polar High

The subtropical high pressure belt lies at approximately 30° N and 30° S. Air descending from the Hadley cell creates this high-pressure zone, also called the Horse Latitudes.

5. Westerlies blow from

(a) 30° towards the equator (b) 30° towards 60° latitude (c) 60° towards the poles (d) Poles towards 60° latitude

Westerlies are winds that blow from the subtropical high pressure belt (30°) towards the subpolar low pressure belt (60°). They are called westerlies because they blow from the west in both hemispheres.

6. Which of the following is a local wind?

(a) Trade wind (b) Westerly (c) Chinook (d) Polar easterly

The Chinook is a warm, dry local wind that descends the eastern slopes of the Rocky Mountains. Trade winds, westerlies, and polar easterlies are planetary winds.

7. Relative humidity is expressed as

(a) g/m³ (b) g/kg (c) Percentage (%) (d) Millibar

Relative humidity = (Actual water vapour / Maximum possible water vapour at that temperature) × 100. It is expressed as a percentage. Absolute humidity uses g/m³ and specific humidity uses g/kg.

II. Fill in the Blanks

1. The most abundant gas in the atmosphere is nitrogen (78 %).

2. The layer of the atmosphere closest to the Earth's surface is the troposphere.

3. The ozone layer is found in the stratosphere.

4. Temperature inversion occurs when temperature increases with altitude instead of decreasing.

5. The unit of atmospheric pressure is millibar (mb) or hectopascal (hPa).

6. The pressure belt at the equator is called the equatorial low pressure belt (Doldrums).

7. Trade winds are also known as tropical easterlies.

8. The hot and dry local wind blowing over the northern plains of India is called Loo.

9. Absolute humidity is measured in grams per cubic metre (g/m³).

10. Orographic rainfall occurs on the windward side of mountains.

11. The average weather conditions over a long period (30+ years) describe the climate of a region.

12. The Montreal Protocol (1987) aims to protect the ozone layer.

III. True or False

1. Oxygen is the most abundant gas in the atmosphere.

False. Nitrogen (78 %) is the most abundant gas. Oxygen constitutes about 21 %.

2. Temperature always decreases with altitude in the atmosphere.

False. In the stratosphere and thermosphere, temperature increases with altitude. Temperature inversion can also cause a temporary increase in the troposphere.

3. The Subtropical High Pressure Belt is also called the Horse Latitudes.

True. The calm conditions and high pressure at 30° N/S earned the name "Horse Latitudes" — historically, sailors allegedly threw horses overboard when stuck in these windless zones.

4. Westerlies blow from east to west.

False. Westerlies blow from west to east. They originate in the subtropical high and move towards the subpolar low, coming from the west in both hemispheres.

5. Climate refers to the day-to-day condition of the atmosphere.

False. Climate refers to the long-term average weather conditions over 30+ years. Day-to-day conditions describe weather.

IV. Match the Following

Key: 1-c, 2-e, 3-a, 4-d, 5-f, 6-b

V. Answer Briefly (2 Marks Each)

1. Define atmosphere.

The atmosphere is the thin layer of gases surrounding the Earth, held by gravitational force. It consists mainly of nitrogen (78 %) and oxygen (21 %) along with traces of argon, carbon dioxide, water vapour, and dust particles. It extends to about 10,000 km but 99 % of its mass is concentrated within 32 km of the surface. The atmosphere protects life by absorbing harmful solar radiation, regulating temperature, and enabling weather processes.

2. What is temperature inversion? State its effects.

Temperature inversion is a condition where temperature increases with altitude instead of the normal decrease, within the troposphere. It occurs on calm, clear nights when the ground cools rapidly by radiation.

Effects:

• Traps pollutants, smoke, and dust near the surface → causes smog and fog.
• Disrupts normal convection, reducing vertical mixing of air.
• Can cause frost in valleys while higher slopes remain warmer.
• Affects aviation — creates turbulent flying conditions at the inversion boundary.

3. Name the pressure belts of the Earth.

The four major pressure belts are:

These belts shift slightly north and south with the apparent movement of the Sun across seasons.

4. Differentiate absolute humidity and relative humidity.

VI. Answer in Detail (5 Marks Each)

1. Describe the layers of the atmosphere with their characteristics.

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

(a) Troposphere

• Extends from the Earth's surface up to 12 km on average (8 km at poles, 16 km at the equator).
• Temperature decreases with altitude at the normal lapse rate of ~6.5 °C/km.
• Contains about 75 % of atmospheric mass and nearly all water vapour and dust particles.
• All weather phenomena — clouds, rain, thunderstorms, cyclones — occur here.
• The boundary with the stratosphere is called the tropopause, where temperature stops decreasing.

(b) Stratosphere

• Extends from 12 km to 50 km.
• Temperature increases with altitude — from about −55 °C at the tropopause to about 0 °C at the stratopause.
• The ozone layer (20–35 km) absorbs UV radiation, warming this layer.
• Very stable layer with almost no vertical mixing — ideal for commercial aviation.
• The top boundary is the stratopause.

(c) Mesosphere

• Extends from 50 km to 80 km.
• Temperature decreases again, reaching about −90 °C at the mesopause (the coldest point in the atmosphere).
• Meteors entering the Earth's atmosphere burn up in this layer due to friction with air molecules.
• Noctilucent clouds (rare ice-crystal clouds) may form near the mesopause.

(d) Thermosphere

• Extends from 80 km to about 700 km.
• Temperature rises sharply due to absorption of high-energy solar radiation by sparse gas molecules — can exceed 1,500 °C.
• Contains ionised gases (ions and free electrons) that reflect radio waves — vital for communication.
• Auroras (Aurora Borealis and Aurora Australis) occur here near the poles.

(e) Exosphere

• Extends from 700 km and gradually merges with outer space.
• Gases are extremely thin — molecules move so fast that some escape Earth's gravity.
• Artificial satellites orbit within this layer.
• No distinct upper boundary — it transitions into interplanetary space.

2. Explain the pressure belts and wind systems of the world.

Pressure Belts

Atmospheric pressure is not uniform across the Earth. It varies with latitude, creating alternating belts of high and low pressure:

1. Equatorial Low Pressure Belt (0° – 5° N/S)

• Also called the Doldrums.
• Intense solar heating causes air to rise (convection), creating a low-pressure zone.
• Calm conditions with little surface wind — sailing ships historically got stuck here.

2. Subtropical High Pressure Belt (30° N/S)

• Also called the Horse Latitudes.
• Air rising from the equator cools at height and descends at 30° (Hadley cell), creating high pressure.
• Generally calm, clear skies, and dry conditions — major deserts lie along this belt (Sahara, Thar, Atacama).

3. Subpolar Low Pressure Belt (60° N/S)

• Warm air from the subtropics meets cold polar air here, creating a zone of low pressure.
• Associated with cyclonic activity and stormy weather in mid-latitudes.

4. Polar High Pressure Belt (90° N/S)

• Extreme cold makes air dense and heavy → it sinks, creating high pressure.
• Very dry, cold conditions with minimal precipitation.

Wind Systems

Winds are caused by pressure differences. The Coriolis effect (due to Earth's rotation) deflects winds — to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

Planetary (Primary) Winds:

Secondary Winds:

• Monsoons — seasonal reversal of wind direction (summer: ocean → land; winter: land → ocean). Dominate the Indian subcontinent.

Local Winds:

• Land and Sea Breezes — diurnal reversal along coasts.
• Mountain and Valley Breezes — diurnal reversal in hilly terrain.
• Chinook / Foehn — warm, dry winds descending mountain slopes; raise temperature rapidly ("snow eater").
• Harmattan — hot, dry wind from the Sahara blowing over West Africa.
• Loo — hot, dry wind over the northern Indian plains in summer.

3. Differentiate between weather and climate; explain types of precipitation.

Difference between Weather and Climate

In short: Climate is what you expect; weather is what you get.

Types of Precipitation

Precipitation occurs when moist air rises, cools, and condenses. The three main types differ by the lifting mechanism:

Common Mistakes to Avoid

1. Confusing weather and climate — Weather is short-term; climate is long-term average. Do not use them interchangeably in exam answers.

2. Wrong lapse rate value — The normal lapse rate is 6.5 °C/km, not 10 °C/km. Always specify "approximately 6.5 °C per kilometre."

3. Misidentifying ozone layer location — The ozone layer is in the stratosphere (20–35 km), not the troposphere or mesosphere.

4. Confusing absolute and relative humidity — Absolute humidity is g/m³ (changes with volume); relative humidity is a percentage (changes with temperature). Relative humidity is what weather reports use.

5. Reversing wind direction — Trade winds blow from subtropical high towards the equator (not the other way). Westerlies blow from subtropical high towards subpolar low (from the west).

6. Ignoring Coriolis effect — Wind direction is always modified by the Coriolis force. Trade winds are deflected to the east (hence called easterlies), westerlies to the west.

7. Rain shadow on wrong side — In orographic rainfall, the windward side gets rain and the leeward side is the rain shadow. Do not reverse this.

8. Attributing convectional rain to mountains — Convectional rain is caused by surface heating, not by mountains. Orographic rain is the type associated with mountain barriers.

Exam Checklist

Before your exam, make sure you can:

1. [ ] List the composition of the atmosphere (N₂ 78 %, O₂ 21 %, others 1 %). 2. [ ] Name all five layers of the atmosphere with altitude ranges and temperature trends. 3. [ ] Define lapse rate (6.5 °C/km) and temperature inversion with its effects. 4. [ ] Identify the four pressure belts with their latitudes and types (high/low). 5. [ ] Explain trade winds, westerlies, and polar easterlies — origin, direction, and Coriolis deflection. 6. [ ] Give at least three examples of local winds (Chinook, Loo, Harmattan) with brief descriptions. 7. [ ] Distinguish absolute, relative, and specific humidity with units. 8. [ ] Describe the three types of precipitation — convectional, orographic, cyclonic — with mechanism and region examples. 9. [ ] Clearly differentiate weather vs climate with at least three points of comparison. 10. [ ] Explain the ozone layer — location, function, depletion causes, and the Montreal Protocol.