Air

This is Chapter 4 of Our Environment (Class 7 Geography): 10 pages (20–29), of which about six are text. It covers the composition of air (a pie chart, Fig. 4.1), the roles of nitrogen, oxygen and carbon dioxide, the greenhouse effect and global warming, the five layers of the atmosphere (Fig. 4.2), weather and climate, temperature and insolation, air pressure, winds and the pressure belts (Fig. 4.4), a two-page box on the 1999 Odisha super cyclone, and moisture, clouds and rainfall (Fig. 4.5). It has seven ‘Do you know?’ boxes, a newspaper collage with a ‘Read and Ponder’, a picture of four weather instruments (Fig. 4.3), a ‘Let's do’, and exercises ending in a crossword and a weather calendar. This page has 9 concept sections.

• Before this: gess201 (Environment) named the atmosphere as one of the four domains; gess202 (Inside Our Earth) looked down — this chapter looks up. Next: gess205 (Water) — the water cycle, oceans, waves, tides and currents.
• Goes further in Class 9 and 11: iest103 (Atmosphere and Climate, Class 9); kegy207 (Composition and Structure of Atmosphere), kegy208 (Solar Radiation, Heat Balance and Temperature — insolation, heat budget, inversion), kegy209 (Atmospheric Circulation and Weather Systems — pressure belts, Coriolis force, cyclones), kegy210 (Water in the Atmosphere — humidity, condensation, rainfall types) and kegy211 (World Climate and Climate Change). India's monsoon and the loo are in kegy104 (Climate).
• UPSC, honestly: GS1 physical geography and GS3 environment. Prelims asked these basics directly — the order of the layers (1984, 1998), the ionosphere and radio waves (1996), the make-up of dry air (1988), insolation, pressure and the barometer (1987, 1989, 2001), humidity (1981), the planetary winds, doldrums and horse latitudes (1980, 1991, 2007), the ozone layer (1986), the greenhouse effect (1989, 2025) — and Mains asked about the troposphere (2022) and urban heat islands (2013). We link 19 (17 Prelims, 2 Mains).

• Air (Fig. 4.1): nitrogen 78%, oxygen 21%, argon 0.93%, CO₂ 0.03% (about 0.042% now †), others 0.04%; plus dust.
• CO₂ traps the earth's heat = greenhouse effect; more CO₂ from fuels = global warming → melting snow, rising seas, coastal floods.
• Layers: troposphere (avg 13 km, all weather) → stratosphere (to 50 km, ozone, aeroplanes) → mesosphere (to 80 km, meteorites burn) → thermosphere (ionosphere 80–400 km, radio) → exosphere.
• Weather = hour-to-hour, day-to-day; climate = long-term average. Insolation falls from equator to poles.
• Pressure is highest at sea level; warm → rising air → low (cloudy, wet); cold → sinking air → high (clear). Wind blows high → low.
• Winds: permanent (trades, westerlies, easterlies), seasonal (monsoon), local (land and sea breeze, loo). Rainfall: convectional, orographic, cyclonic.

The air we breathe is a mixture of many gases. Nitrogen and oxygen make up the bulk of the atmosphere; carbon dioxide, helium, ozone, argon and hydrogen are found in smaller quantities, and tiny dust particles are also present. The pie chart (Fig. 4.1) gives the shares: nitrogen 78%, oxygen 21%, argon 0.93%, carbon dioxide 0.03% and all others 0.04%. Nitrogen is the most plentiful gas; oxygen is the second.

Nitrogen: we inhale some and exhale it again. Plants need nitrogen to survive but cannot take it directly from the air. Bacteria living in the soil and in the roots of some plants take nitrogen from the air and change its form so that plants can use it. Oxygen: humans and animals take it in as they breathe; green plants produce it during photosynthesis; in this way the oxygen content of the air stays constant — until we cut trees and the balance is disturbed. Carbon dioxide: green plants use it to make their food and release oxygen; humans and animals release it. The amounts seem to balance, but burning fuels such as coal and oil adds billions of tons of carbon dioxide to the atmosphere every year, and the increased volume is affecting the earth's weather and climate.

A ‘Do you know?’ box explains: carbon dioxide in the atmosphere creates a greenhouse effect by trapping the heat radiated from the earth. It is therefore called a greenhouse gas, and without it the earth would have been too cold to live in. But when its level rises because of factory smoke or car fumes, more heat is retained and the earth's temperature rises — global warming. The rise melts snow in the coldest parts of the world; the sea level rises, causing floods in coastal areas; the climate of a place may change drastically, and in the long run some plants and animals may become extinct.

Starting from the earth's surface, the atmosphere has five layers: troposphere, stratosphere, mesosphere, thermosphere and exosphere (Fig. 4.2). The troposphere is the most important layer: its average height is 13 km; the air we breathe is here, and almost all weather — rainfall, fog, hailstorms — happens in it. The stratosphere above extends to 50 km; it is almost free from clouds and weather, which makes it ideal for flying aeroplanes, and it contains the ozone layer that protects us from the sun's harmful rays. The mesosphere extends to 80 km; meteorites burn up in it on entering from space. In the thermosphere the temperature rises very rapidly with height; the ionosphere, part of it, lies between 80 and 400 km and helps radio transmission — radio waves sent from the earth are reflected back by it. The exosphere is the uppermost layer: very thin air, from which light gases like helium and hydrogen float into space.

Weather is the hour-to-hour, day-to-day condition of the atmosphere. The book compares our body to a radio and the mind to its speaker: weather fiddles with the control knobs — hot, humid weather makes us irritable; pleasant, breezy weather makes us cheerful. Weather can change dramatically from day to day, which is why we have daily weather forecasts. The average weather of a place over a longer period is its climate. Temperature is the degree of hotness or coldness of the air; it changes between day and night and from season to season. An important factor in its distribution is insolation — the incoming solar energy intercepted by the earth — which decreases from the equator towards the poles, so temperature does too; that is why the poles are covered with snow. If the earth's temperature rose too high, some crops could not grow. Cities are much warmer than villages: concrete, metals and the asphalt of roads heat up by day and release the heat at night, and crowded high-rise buildings trap the warm air.

The air above us presses on our bodies with great force, but we do not feel it, because it presses from all directions and our body exerts a counter pressure. Air pressure is the pressure exerted by the weight of air on the earth's surface. It is highest at sea level and falls rapidly as we go up. Horizontally, pressure depends on temperature: where it is hot, the air is heated and rises, creating a low-pressure area, associated with cloudy skies and wet weather; where it is cold, the air is heavy and sinks, creating a high-pressure area, associated with clear and sunny skies. Air always moves from high pressure to low pressure.

Wind is the movement of air from a high-pressure area to a low-pressure area. You see it blowing dry leaves, carrying away smoke and fine dust, or uprooting trees in a storm; it can make walking or holding an umbrella hard. A wind is named after the direction from which it blows — a wind from the west is a westerly. Winds are of three types: permanent winds — the trade winds, westerlies and easterlies — blow constantly throughout the year in a particular direction; seasonal winds change direction with the seasons, like the monsoons in India; and local winds blow only at a particular time of day or year in a small area, like land and sea breezes and the hot, dry loo of the northern plains of India. Fig. 4.4 shows the major pressure belts and wind system.

A box tells the story of Odisha, on India's east coast, which is prone to cyclones from the Bay of Bengal. On 17–18 October 1999 a cyclone hit five districts; then a super cyclone struck on 29 October 1999. The damage came from three factors: wind velocity, rain and tidal surge. Winds of up to 260 km per hour lasted over 36 hours, uprooting trees, damaging kutcha houses, blowing away roofs and snapping power and telecom lines. Heavy rain for three days flooded the major rivers. The cyclonic winds pushed sea water 20 km inland; a 7 to 10 m high wave ruined standing paddy. The storm began as a ‘depression’ on 25 October 1999 and moved north-westward, intensifying into a super cyclone that hit between Erasama and Balikuda on 29 October at 10.30 a.m. It swept the whole coast including Bhubaneswar, Cuttack and 28 coastal towns; about 13 million people were affected; livestock were killed and crops destroyed; salt water made farmland infertile; sal, teak and bamboo plantations disappeared; the mangroves between Paradeep and Konark vanished.

Water evaporating from land and water bodies becomes water vapour. The moisture in the air at any time is humidity; when the air is full of water vapour we call it a humid day. The book says that as air gets warmer its capacity to hold water vapour increases, so it becomes more and more humid. On a humid day clothes take longer to dry and sweat does not evaporate easily, so we feel uncomfortable. When water vapour rises it cools and condenses into droplets; clouds are masses of such droplets; when they become too heavy to float, they come down as precipitation. Jet planes leave white trails because the moisture from their engines condenses. Precipitation in liquid form is rain; other forms are snow, sleet and hail. Most groundwater comes from rain; plants help preserve water, and when hillside trees are cut, rainwater rushes down and can flood low-lying areas. By mechanism there are three types of rainfall: convectional, orographic and cyclonic (Fig. 4.5). Too little rain brings water scarcity and drought; too much brings floods.

The ‘Read and Ponder’, the questions asked inside the text, and the ‘Let's do’.

Six short answers, five ticks, a match, two reasons and two ‘For fun’ tasks. Try first, then open.

Numbers

One-liners

• The atmosphere gives us air, shields us from harmful rays and keeps the earth's temperature liveable.
• Nitrogen is the most plentiful gas in the air; oxygen is the second.
• Bacteria in soil and roots make the air's nitrogen usable by plants.
• Carbon dioxide traps the earth's heat; more of it causes global warming.
• Almost all weather occurs in the troposphere; the ozone layer is in the stratosphere.
• The ionosphere, part of the thermosphere, reflects radio waves back to earth.
• Weather is day-to-day; climate is the long-term average.
• Insolation decreases from the equator to the poles.
• Air pressure is highest at sea level and falls with height.
• Wind blows from high pressure to low pressure and is named after where it comes from.
• Warm air can hold more water vapour; rising vapour cools, condenses and forms clouds.

Traps to avoid

• Argon, not carbon dioxide, is the third most plentiful gas in dry air.
• The ionosphere lies above the mesosphere, in the thermosphere.
• The ozone layer is in the stratosphere; it does not reflect radio waves.
• Horse latitudes are high-pressure belts at about 30°–35°, not lows.
• The monsoon is a seasonal wind, not a planetary (permanent) wind.
• A westerly blows from the west; the north-east trades blow towards the south-west.
• A low-pressure area means cloudy, wet weather; a high means clear skies.
• Warming air without adding water lowers its relative humidity.
• The greenhouse effect traps heat radiated by the earth; without it the earth would be too cold.
• The 1999 'tidal wave' in Odisha was a storm surge, not a tide or a tsunami.
• Fig. 4.2's dotted lines are not at the text's heights — use 13, 50 and 80 km.

Prelims — what UPSC asks

• From this chapter: the make-up of dry air (1988); the order of layers (1984, 1998) and the ionosphere and radio waves (1996); jets in the lower stratosphere (2011); the ozone layer's protection (1986); the greenhouse effect (1989) and the atmosphere's warming role (2025); insolation (1989); pressure from gravity (1989) and the falling barometer (1987, 2001); humidity and temperature (1981); the monsoon as a non-planetary wind (1991); doldrums (1980) and horse latitudes (2007); meteors (1995).
• Elsewhere: the ozone layer and UV (1982, 1987) and water vapour (2024) on kegy207 and iest103; troposphere thickness (2024) on kegy207; moisture and latitude (2003) on kegy207, kegy210 and iest103; the Coriolis force, local winds and cyclones on kegy209; convectional rain at the equator (1996, 2003) on kegy210; climate treaties on kegy211 and leps106.
• Method: these are order-and-definition questions. Learn the layer order with one feature each, the gas order (N > O > Ar > CO₂), the belt sequence from equator to pole (low–high–low–high) with latitudes, and the three-way sort of winds and rainfall.

Mains — where it appears and how to answer

• GS1 2022: ‘Troposphere is a very significant atmosphere layer that determines weather processes. How?’ — this chapter gives the base (the air we breathe, almost all weather, average 13 km); add the lapse rate, convection and water vapour from kegy207. GS1 2013: causes of urban heat islands — the book's concrete, asphalt and high-rise buildings, plus loss of greenery and waste heat †.
• Use the 1999 Odisha super cyclone as a ready case study for GS1 (cyclones) and GS3 (disaster management): wind, rain and surge; salinised farmland and lost mangroves; and the later shift to evacuation and shelters (Phailin 2013 †). A labelled sketch of the layers or of the three rainfall types earns marks.

Tap an option. NCERT's own exercise questions come first; the rest test the figures and common confusions.

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