Atmosphere and ClimateClass 9 Social Science NCERT Solutions
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Q1Questions and activities
What is atmosphere? Explain its composition with the help of a pie diagram.
Solution
The atmosphere is a blanket of air that surrounds the Earth, held in place by the planet's gravity. It is essential for life as it provides the air we breathe, protects us from harmful solar radiation, and regulates the Earth's temperature.
The Earth's atmosphere is a mixture of various gases, water vapour, and dust particles. Its composition is as follows:
- Nitrogen (N₂): This is the most abundant gas, making up approximately 78% of the atmosphere. It is crucial for plant life.
- Oxygen (O₂): The second most abundant gas, constituting about 21% of the atmosphere. It is vital for the respiration of most living beings.
- Other Gases (1%): The remaining 1% consists of several other gases in smaller quantities. These include:
- Argon (Ar): The most significant part of the remaining 1%.
- Carbon Dioxide (CO₂): A greenhouse gas essential for photosynthesis in plants.
- Neon (Ne), Helium (He), Krypton (Kr), Xenon (Xe), Hydrogen (H₂), and Ozone (O₃): These are present in trace amounts.
Besides these gases, the atmosphere also contains variable amounts of water vapour (ranging from 0.1% to 0.4%), which is crucial for cloud formation and precipitation, and tiny dust particles.
Q2Questions and activities
Draw a labelled diagram of the structure of atmosphere.
Solution
Since a diagram cannot be drawn here, the structure of the atmosphere is described below with its layers in order from the Earth's surface upwards.
Structure of the Atmosphere
The atmosphere is divided into five major layers based on temperature changes with increasing altitude.
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Troposphere:
- Altitude: Extends from the Earth's surface up to an average height of 12 km.
- Characteristics: This is the densest layer. Temperature decreases with increasing altitude. It contains the air we breathe, most of the water vapour, and clouds. Almost all weather phenomena, such as rain, fog, and hail, occur in this layer.
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Stratosphere:
- Altitude: Extends from about 12 km to 50 km.
- Characteristics: Temperature increases with altitude in this layer. It contains the ozone layer, which absorbs the Sun's harmful ultraviolet (UV) radiation. This layer is free from clouds and weather disturbances, making it ideal for flying aeroplanes.
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Mesosphere:
- Altitude: Extends from about 50 km to 80 km.
- Characteristics: Temperature decreases with increasing altitude. Most meteorites that enter the Earth's atmosphere burn up in this layer due to friction with air particles.
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Thermosphere:
- Altitude: Extends from about 80 km to 700 km.
- Characteristics: Temperature rises very rapidly with increasing altitude as gas molecules absorb X-rays and short-wave UV radiation. This layer helps in radio transmission by reflecting radio waves back to Earth. The ionosphere is a part of this layer, and auroras (Northern and Southern Lights) occur here.
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Exosphere:
- Altitude: The uppermost layer, extending from about 700 km outwards into space.
- Characteristics: The air in this layer is extremely thin. Light gases like helium and hydrogen float into space from here due to the weak gravitational pull.
Q3Questions and activities
Which are the four main seasons of India?
Solution
The Indian Meteorological Department (IMD) has recognised four distinct seasons in India, based on the tropical monsoon climate. The four main seasons are:
- Winter: Lasts from December to early April.
- Summer or pre-monsoon: Spans from April to June (or July in north-western India).
- Monsoon or rainy (Advancing monsoon): Generally spans from June to September.
- Post-monsoon (Retreating monsoon): Lasts from October to December.
Q4Questions and activities
Why do you not feel the pressure of the atmosphere?
Solution
We do not feel the significant pressure of the atmosphere on our bodies for two main reasons. Firstly, the air pressure is exerted on us from all directions, not just from above. Secondly, and more importantly, our bodies exert a counter-pressure from within. The pressure inside our bodies is equal to the atmospheric pressure outside, which cancels out the external force. This balance of pressures ensures that we are not crushed by the weight of the air above us.
Q5Questions and activities
In which layer of the atmosphere do aeroplanes fly and why?
Solution
Aeroplanes typically fly in the stratosphere. The primary reason for this is that the stratosphere is above the troposphere, where most weather phenomena like clouds, rain, and storms occur. Flying in the stratosphere provides a smoother ride as it is free of clouds and other weather disturbances. This makes air travel safer, more efficient, and more comfortable for passengers.
Q6Questions and activities
Distinguish between the following: a. The troposphere and stratosphere b. The south-west monsoon and north-east monsoon
Solution
a. The Troposphere and Stratosphere
| Feature | Troposphere | Stratosphere |
|---|---|---|
| Altitude | From the Earth's surface up to about 12 km. | Lies above the troposphere, extending up to 50 km. |
| Temperature | Temperature decreases with increasing altitude. | Temperature generally increases with increasing altitude. |
| Weather | Contains almost all weather phenomena like clouds, rain, and storms. | It is free of clouds and other weather disturbances. |
| Key Feature | It is the layer where we live and breathe. | It contains the ozone layer, which protects us from UV radiation. |
| Suitability for Flying | Not ideal for long-distance flights due to weather turbulence. | Ideal for flying aeroplanes due to stable conditions. |
b. The South-West Monsoon and North-East Monsoon
| Feature | South-West Monsoon | North-East Monsoon |
|---|---|---|
| Season | Occurs during the summer, from June to September. | Occurs during the winter, from October to February. |
| Wind Direction | Winds blow from sea to land (from the south-west). | Winds blow from land to sea (from the north-east). |
| Pressure System | Caused by a low-pressure area over the Indian landmass and high pressure over the Indian Ocean. | Caused by a high-pressure area over the landmass and low pressure over the surrounding seas. |
| Moisture | Winds are moisture-laden as they blow over the Arabian Sea and Bay of Bengal. | Winds are generally cold and dry as they originate over land. |
| Rainfall | Brings widespread and heavy rainfall to most parts of India. | Brings rainfall mainly to the south-eastern coast of India, particularly Tamil Nadu and Andhra Pradesh, after picking up moisture from the Bay of Bengal. |
Q7Questions and activities
Do it yourself: Table 3.3 shows the average monthly temperatures and rainfall amounts for 10 representative stations. Study these figures on your own and convert them into 'temperature and rainfall' graphs. The visual representations will help you grasp their similarities and differences at a glance. One such graph (Fig. 3.14) is already prepared for you. See if you can arrive at some broad generalisations about our diverse climatic conditions. 7.1. Now look at Table 3.3 again. Re-arrange the 10 stations according to their distance from the equator. 7.2. Find out: a. Two stations with the most extreme climate. b. Two stations influenced by retreating monsoons. c. The two hottest stations in the months of (i) February (ii) June 7.3. Now find out: a. Why does Shillong experience more rainfall than Kolkata? b. Why does Delhi receive more rainfall than Jodhpur? 7.4. Now think why a. Thiruvananthapuram has an equable climate? b. Chennai has more rainfall only after the fury of the monsoon is over in most parts of the country? c. Leh has moderate precipitation almost throughout the year? 7.5. Despite these differences across regions, can you observe any substantial evidence to conclude that the monsoons provide a very strong framework, lending overall climatic unity to the whole country?
Solution
7.1. Stations arranged by distance from the equator (south to north based on latitude):
- Thiruvananthapuram (8°29'N)
- Bengaluru (12° 58' N)
- Chennai (13° 4' N)
- Mumbai (19°N)
- Nagpur (21° 9' N)
- Kolkata (22°34'N)
- Shillong (24° 34' N)
- Jodhpur (26° 18' N)
- Delhi (29°N)
- Leh (Latitude not given, but it is the northernmost station in India)
7.2. Findings from the table:
- a. Two stations with the most extreme climate: An extreme climate has a large difference between the highest and lowest temperatures.
- Leh: Has a temperature range from -8.5°C in January to 17.2°C in July.
- Delhi: Has a temperature range from 14.4°C in January to 33.3°C in May and June.
- b. Two stations influenced by retreating monsoons: These stations receive significant rainfall in October and November.
- Chennai: Receives 30.6 cm in October and 35.0 cm in November.
- Thiruvananthapuram: Receives 27.3 cm in October and 20.6 cm in November.
- c. The two hottest stations:
- (i) February: Thiruvananthapuram (27.3°C) and Chennai (25.7°C).
- (ii) June: Jodhpur (33.9°C) and Delhi (33.3°C).
7.3. Reasons for rainfall differences:
- a. Why does Shillong experience more rainfall than Kolkata? Shillong is located at a high altitude (1461 metres) in a hilly region. The mountains act as a barrier to the moisture-laden south-west monsoon winds, forcing them to rise. As the air rises, it cools, condenses, and results in very heavy rainfall (orographic rainfall). Kolkata is located on a plain at a low altitude (6 metres) and does not benefit from this orographic effect, thus receiving less rainfall.
- b. Why does Delhi receive more rainfall than Jodhpur? Delhi receives rainfall from the Bay of Bengal branch of the monsoon. As these monsoon winds travel westwards across northern India, they gradually lose moisture. By the time they reach Jodhpur, which is further west and in a more arid region, the winds are much drier. Therefore, Jodhpur receives significantly less rainfall than Delhi.
7.4. Explanations for climatic conditions:
- a. Why does Thiruvananthapuram have an equable climate? Thiruvananthapuram has an equable (or maritime) climate with very little temperature variation throughout the year because of two main factors: its proximity to the sea, which has a moderating influence, and its location near the equator (8°29'N), where solar insolation is high and consistent all year round.
- b. Why does Chennai have more rainfall only after the fury of the monsoon is over in most parts of the country? Chennai receives most of its rainfall from the north-east monsoon, also known as the retreating monsoon, during October and November. During this time, dry winds from the land blow towards the sea. However, as these winds pass over the Bay of Bengal, they pick up moisture and bring rain to the south-eastern coast, where Chennai is located. This occurs after the south-west monsoon has already withdrawn from most of India.
- c. Why does Leh have moderate precipitation almost throughout the year? The data shows that Leh has very low, not moderate, precipitation (annual total of 8.5 cm), making it a cold desert. The precipitation it receives is low but spread out over the year rather than being concentrated in a single season. This is because Leh is in the rain-shadow of the Himalayas, which blocks the monsoon winds. The little precipitation it gets is mainly from western disturbances and local phenomena, which can occur throughout the year.
7.5. Monsoon as a unifying framework:
Yes, despite the diverse climatic conditions, the monsoon provides a strong framework that unifies the entire country.
- Seasonal Rhythm: For most of India, the climate is defined by the seasonal reversal of winds and the associated wet (June-September) and dry seasons. This rhythm dictates the agricultural cycles, with farmers across the country depending on the arrival of the south-west monsoon for sowing crops.
- Economic and Cultural Unity: The monsoon's performance affects the entire country's economy, water supply in rivers and reservoirs, and even cultural festivals that are celebrated to mark the arrival of rains.
- Shared Experience: The data table shows that for a majority of stations (like Mumbai, Kolkata, Delhi, Nagpur, Bengaluru), the rainfall peaks significantly between June and September, demonstrating a shared climatic pattern. This common reliance on a single weather phenomenon creates a climatic unity across diverse geographical regions.
Q8Questions and activities
Collect pictures of houses and clothing of people from different regions of India. Examine whether they reflect any relationship with the climatic conditions or the relief of those regions.
Solution
This is an activity-based question. The relationship between climate, relief, and human lifestyle (houses and clothing) can be explained through the following examples:
1. Region: Rajasthan (Hot and Dry Climate)
- Houses: Traditional houses have thick mud walls and small windows to keep the interiors cool during the scorching heat of the day. Flat roofs are common, sometimes used for sleeping at night when it is cooler and for collecting the scarce rainwater.
- Clothing: People wear light-coloured, loose-fitting cotton clothes. The light colours reflect sunlight, and the loose fit allows air to circulate, keeping the body cool. Men often wear turbans (pagri) to protect their heads from the intense sun, while women wear long skirts (ghagra) and cover their heads with a veil (odhni).
2. Region: Kerala (Hot and Humid Climate, Heavy Rainfall)
- Houses: Houses traditionally have sloping roofs made of tiles or thatch to allow heavy monsoon rainwater to run off easily. They often feature large windows, open verandas, and internal courtyards to allow for cross-ventilation, which helps in the humid conditions.
- Clothing: People wear light and airy cotton clothes that absorb sweat and dry quickly. The traditional attire is the mundu for men and saree or mundum neriyathum for women, which are typically made of white or off-white cotton.
3. Region: Ladakh (Cold Desert Climate, High Altitude)
- Houses: Houses are built with thick stone or mud-brick walls and flat roofs to provide insulation against the extreme cold. The flat roofs are used for drying grains and other agricultural produce in the sun during the short summer. The design aims to maximize heat from the sun.
- Clothing: People wear multiple layers of heavy woollen clothing to protect themselves from the severe cold. The traditional dress, the goncha, is a thick woollen robe worn by both men and women. They also wear woollen caps, socks, and boots.
Conclusion: These examples clearly show a strong relationship between the climatic conditions and relief of a region and the housing and clothing styles adopted by its people. These adaptations are essential for survival and comfort in diverse environments.