Structure and Composition of the Atmosphere

The atmosphere is the layer of gases surrounding the Earth, held in place by gravity, that protects life and regulates climate and weather. It is divided into five layers.

Composition of the Atmosphere

• Nitrogen (N₂) – 78%: most abundant gas; dilutes oxygen, prevents rapid combustion
• Oxygen (O₂) – 21%: essential for respiration and combustion
• Argon – 0.93%: an inert gas present in small quantity
• Carbon Dioxide (CO₂) – 0.04%: traps heat (greenhouse effect); used by plants in photosynthesis
• Water vapour: variable; responsible for humidity, clouds, precipitation
• Dust particles: help condensation of water vapour to form clouds

Elements of Weather and Climate

The main elements are Temperature, Atmospheric Pressure, Wind, Humidity, and Precipitation, measured using these instruments:

• Temperature — measured by a Thermometer
• Atmospheric Pressure — measured by a Barometer
• Wind direction — measured by a Wind vane; Wind speed — measured by an Anemometer
• Humidity — measured by a Hygrometer
• Precipitation — measured by a Rain gauge

Local Winds and their Impacts

Local winds blow over a small area due to differences in temperature and air pressure caused by local features such as mountains, deserts, and water bodies. They are usually seasonal or occur at specific times of day.

• The Loo: a hot, dry summer wind over northern India and Pakistan, raising temperatures and sometimes causing heatstroke and crop damage
• The Chinook wind: a warm, dry wind down the eastern Rocky Mountains in North America that rapidly melts snow, benefiting agriculture but sometimes causing discomfort like headaches
• The Foehn wind: produces similar warming effects in the Alps in Europe
• Sea breeze and land breeze: coastal areas experience sea breezes during the day and land breezes at night, moderating temperatures and sometimes bringing rainfall

Local winds can have positive impacts (improving farming conditions, regulating temperature) and negative impacts (health problems, drought, crop damage).

The Loo — a hot, dry summer wind over northern India

Seasons of India

India experiences four main seasons: Hot Weather Season, Cold Weather Season, Advancing Monsoon (Southwest Monsoon), and Retreating Monsoon.

1. Hot Weather Season (March to May)

• Characterised by rising temperature and falling air pressure
• The Loo, strong hot, dusty, and dry winds, blow during the day over northern and north-western India
• Mango showers occur towards the end of this season in the south, helping ripen mangoes

2. Cold Weather Season (mid-November to February)

• Sets in north India in mid-November and prevails till February
• North-east trade winds prevail over the country, blowing from land to sea, causing most parts of India to experience a dry season

The Indian Monsoon

The southwest monsoon enters India in two branches: the Arabian Sea Branch and the Bay of Bengal Branch.

Characteristics of the Bay of Bengal Branch

• This branch is naturally directed towards parts of southeast Bangladesh and coastal Myanmar, to be deflected later
• This deflection occurs due to the Arakan Hills along Myanmar’s coast; a major part enters West Bengal and Bangladesh from the south and southeast
• Low temperature in northwest India causes the branch to split into two — one moves westward, the other moves to the north and northeast India
• The westward branch reaches the Punjab Plains along the Ganga Plains, while the other branch moves up the Brahmaputra valley causing heavy and widespread rains in northeastern India

The relief of the country determines rainfall distribution: the Western Ghats receive over 250 cm, while the leeward side gets less than 50 cm. Northern Plains experience decreased rainfall moving westward — Kolkata: 120 cm, Patna: 102 cm, Prayagraj: 91 cm, Delhi: 56 cm.

The monsoon has ‘breaks’ in rainfall causing wet and dry spells, caused by frequent tropical depressions forming at the head of the Bay of Bengal that follow the monsoon trough’s axis. When the axis is over the plains, the region gets ample rainfall; when it shifts closer to the Himalayas, dry spells increase and can cause floods in mountainous catchment areas.

Tropical depressions and cyclones forming over the Bay of Bengal influence monsoon rainfall

Retreating Monsoon (Transition Season)

The monsoon trough of low pressure grows weaker by October–November and is slowly replaced by high pressure. The monsoon’s outreach becomes unsustainable and gradual withdrawal begins — this is the ‘retreat of monsoon’. By the start of October, monsoons withdraw from the Northern Plains.

• Period of transition occurs in October–November
• Clear skies and rising temperature are prominent features
• During the day temperature remains high, but nights are pleasant and cool; the land remains moist
• High temperature and humidity make daytime weather oppressive — known as October Heat, replaced in late October by low temperature, especially in Northern India

The northwestern India’s low pressure conditions move to the Bay of Bengal area by early November, caused by cyclonic depressions originating over the Andaman Sea. Depressions crossing the eastern coast cause heavy rains; destructive tropical cyclones strike the densely populated Godavari, Krishna and Kaveri deltas, also reaching Odisha, West Bengal, and Bangladesh coasts. Most rainfall of the Coromandel Coast is a consequence of these depressions and cyclones.

Distribution of Precipitation

• India receives over 400 cm annual rainfall on parts of the western coast and northeastern India
• Western Rajasthan, Punjab, Haryana and Gujarat record only 60 cm
• Low rainfall also recorded in the interiors of the Deccan Plateau east of the Sahyadris, and around Leh in Jammu and Kashmir
• The rest of India gets moderate rainfall; snowfall is restricted to the Himalayan region
• Regions of low rainfall (Rajasthan, Gujarat, leeward Western Ghats) have high rainfall variability — prone to droughts, while high-rainfall areas are prone to floods

Monsoons — A Unifying Bond

The Himalayas act as protective barriers stopping cold winds from Central Asia, allowing northern India to experience uniform high temperature. The sea’s influence from three sides moderates temperature on the peninsular plateau. The monsoon is a unifying bond — it influences agriculture (the occupation of most Indians), festivals, lifestyle, dances, music, paintings, and literature across the country.

Climate Change

Climate change is one of the most important environmental issues facing the world today — the long-term change in Earth’s average temperature and weather patterns, mainly caused by human activities such as burning fossil fuels and deforestation, which increase greenhouse gases in the atmosphere.

Causes of Climate Change

• Burning fossil fuels: coal, oil, and natural gas burned for electricity, transport, and industry release greenhouse gases like CO₂, which trap heat
• Deforestation: trees absorb CO₂; cutting/burning forests releases stored carbon and removes future absorption capacity
• Industrial and agricultural activities: factories emit greenhouse gases; farming produces methane (from livestock) and nitrous oxide (from fertilisers), both powerful heat-trapping gases
• Waste and overconsumption: landfills emit methane; producing goods at large scale increases energy use and emissions

Melting of ice due to high temperature

Effects of Climate Change

1. Rising global temperatures — leading to hotter days and more frequent heatwaves
2. Melting ice and rising sea levels — glaciers and polar ice caps are melting, threatening coastal cities and islands
3. More extreme weather — increased intensity and frequency of storms, floods, droughts, and wildfires
4. Harm to ecosystems and wildlife — many species struggle to adapt quickly, leading to habitat loss and extinction
5. Impacts on humans — food and water shortages, health problems (heat stress, spread of diseases), economic losses, and climate-driven migration

Droughts, wildfires, storms, and floods are consequences of climate change

Wildfires increase in frequency and intensity due to climate change

The good news is that reducing emissions, protecting forests, and switching to renewable energy can slow its impacts.

Floods

Floods are inundations of land with water caused by rains, rivers overflowing their banks, increase in reservoirs of water, winds, cyclones, tsunami, melting of glaciers, and sea tides. A violent splash of water from a breach or burst is called a flash flood.

Key Terms

• Drainage basin: the area drained by the waters of a river
• Flood plain: the flattish area alongside river channels covered by water during floods
• Catchment area: the area in the upper course of the river that catches rainfall and feeds the river
• Tributary: a stream or smaller river joining the main river
• Origin: the point from where the river begins its course
• Mouth: the point or place where the river joins the sea
• Water divide: the high relief feature that separates two drainage basins
• Run off: the surface flow of water

Onset Type

Floods are regarded as ‘slow movements’ in terms of their origin but not in terms of their happening. Most floods are preceded by heavy rainfall and melting of snow over several days — this period is known as the ‘probability’ period. Globally, statistics reveal floods have a probability of occurring once during a ten-year period, with milder floods possibly every two years in between. Even floods from cyclones or tsunamis, producing storm surges, have a certain probability period.

Warning

The Central Water Commission (CWC) has about 132 forecasting stations covering almost all interstate flood-prone rivers, issuing over 6000 forecasts annually. Hydrographs (charts showing lag-time between heavy rainfall and peak discharge) help forecast floods and issue warnings via TV and computer networks. The interval between heavy rainfall and flooding is called lag-time.

Vulnerability

• Settlements in low-lying flood plain areas (known as peneplain) at the mouth of a river, or on steep slopes, are at risk
• Houses built on stilts near the mouth of a river
• Industries and multistoreyed housing in flood plains are vulnerable and are also a cause of floods, since surfaced roads, drains, and parking lots prevent water from seeping underground

Typical Effects

• Panic: weak physique, old people, children, and pregnant mothers die more due to panic
• Disease: decomposition of organic matter in floodwater leads to epidemics, viral infections, malaria, and diarrhoea
• Physical damage: sediments transported by floods damage standing crops; houses, dams, and animal shelters are at risk
• Soil erosion: severe floods cause erosion even in areas where rivers normally deposit sediment, especially due to non-degradable materials washed out

Main Mitigation Strategies

Non-Structural Mitigation

• Mapping: most major rivers, flood plains, basins, and water divides are fully mapped using computer databases for flood control
• Land-Use Control: reduces danger to life and property; no major development should be permitted in flood-prone areas; hospitals should be in safer, elevated areas
• Flood Forecasting and Warning: better facilities for forecasting reduce the impact of floods

Structural Mitigation

• Straightening of channels: artificial straightening helps control floods, since meanders are more open to flooding
• Dikes and levees: artificial embankments to divert flood water in a particular direction
• Stream behaviour: engineering structures divert water courses; zoning and flood-tracts are earmarked as a non-structural strategy too
• Flood proofing: artificial reservoirs with sluice gates, and sand bags to block water flow
• Constructing platforms and elevations: during the Indus Valley Civilisation, houses were built on elevated platforms as a safety measure; many Nordic countries below sea level still use raised platforms today

Dikes — artificial embankments used for flood mitigation

Levees diverting river floodwater in a particular direction

A dam creates a reservoir to contain floodwater and can also generate hydroelectric power

Carbon Footprint

Human activities have greatly increased the amount of greenhouse gases in the atmosphere. One important way to understand this impact is through the concept of carbon footprint.

Meaning of Carbon Footprint

A carbon footprint is the total amount of greenhouse gases, mainly carbon dioxide (CO₂), released into the atmosphere due to human activities. It is usually measured in tonnes of carbon dioxide equivalent (CO₂e). In simple words, it shows how much we contribute to air pollution and global warming through our daily actions.

Pollution from vehicle exhaust contributes to carbon footprint

Sources of Carbon Footprint

• Burning of fossil fuels (coal, petroleum, natural gas)
• Transportation (cars, buses, airplanes)
• Electricity generation in thermal power plants
• Industries and factories
• Deforestation
• Use of household appliances like air conditioners and refrigerators
• Agriculture and livestock farming

Factory smoke polluting the air

Types of Carbon Footprint

Direct Carbon Footprint — emissions from activities we directly control, such as driving vehicles, burning fuels at home, using LPG or petrol.

Indirect Carbon Footprint — emissions produced during manufacturing and transportation of goods/services we use, such as clothes, electronic items, and food products.

Effects of High Carbon Footprint

• Global warming
• Climate change
• Melting of glaciers and polar ice
• Rise in sea level
• Increase in extreme weather events
• Loss of biodiversity
• Health problems due to air pollution

Ways to Reduce Carbon Footprint

Reducing carbon footprint is a shared responsibility. Steps individuals can take:

• Use public transport, cycling, or walking
• Save electricity and use energy-efficient appliances
• Switch to renewable energy sources
• Reduce, reuse, and recycle materials
• Avoid unnecessary use of plastic
• Plant more trees
• Consume local and seasonal products

Role of Students in Reducing Carbon Footprint

• Spreading awareness
• Practicing energy conservation at home
• Participating in tree plantation drives
• Supporting eco-friendly habits at home and school

Role of Students in Reducing Carbon Footprint

• Spreading awareness
• Practicing energy conservation at home
• Participating in tree plantation drives
• Supporting eco-friendly habits at home and school

Glossary

Exercises

A. Multiple Choice Questions

B. Fill in the Blanks

C. Very Short Answer Type Questions

D. Short Answer Type Questions

E. Long Answer Type Questions

Take the Quiz

Test what you’ve learned with this 25-question quiz covering the entire chapter: