Atmosphere | ISC Class 11 Geography Notes

This note covers the composition and structure of the atmosphere, atmospheric temperature and the heat budget, the factors controlling temperature and isotherms, atmospheric pressure and winds, atmospheric moisture and rainfall types, and climate change with its natural and human causes and India's adaptation measures, with tables, diagrams, worked examples and exam-style answers.

What is the atmosphere and what is it made of?

The atmosphere is a mixture of different gases that envelopes the earth all round. It contains life-giving gases like oxygen for humans and animals and carbon dioxide for plants.

Air is essential to the survival of all organisms: humans may survive for some time without food and water but cannot survive even a few minutes without breathing air.

The air is an integral part of the earth's mass, and 99 per cent of the total mass of the atmosphere is confined to the height of 32 km from the earth's surface.

Air is colourless and odourless and can be felt only when it blows as wind.

The atmosphere is composed of gases, water vapour and dust particles. The proportion of gases changes in the higher layers: oxygen is almost negligible at the height of 120 km, and carbon dioxide and water vapour are found only up to 90 km from the surface.

Carbon dioxide and ozone

Carbon dioxide is meteorologically a very important gas. It is transparent to the incoming solar radiation but opaque to the outgoing terrestrial radiation.

It absorbs a part of terrestrial radiation and reflects back some part of it towards the earth's surface.

The volume of other gases is constant, but the volume of carbon dioxide has been rising in the past few decades mainly because of the burning of fossil fuels, and this has also increased the temperature of the air.

Ozone is another important component, found between 10 and 50 km above the earth's surface. It acts as a filter and absorbs the ultra-violet rays radiating from the sun and prevents them from reaching the surface of the earth.

Water vapour and dust particles

Water vapour is a variable gas which decreases with altitude. In the warm and wet tropics it may account for four per cent of the air by volume, while in the dry and cold areas of desert and polar regions it may be less than one per cent.

It also decreases from the equator towards the poles. It absorbs parts of the insolation from the sun and preserves the earth's radiated heat, acting like a blanket that allows the earth neither to become too cold nor too hot.

Water vapour also contributes to the stability and instability in the air.

Dust particles originate from different sources and include sea salts, fine soil, smoke-soot, ash, pollen, dust and disintegrated particles of meteors. They are generally concentrated in the lower layers, yet convectional air currents may transport them to great heights.

Their concentration is higher in subtropical and temperate regions due to dry winds, in comparison to equatorial and polar regions. Dust and salt particles act as hygroscopic nuclei around which water vapour condenses to produce clouds.

Definition: The atmosphere is a mixture of gases, water vapour and dust particles that envelopes the earth all round, with 99 per cent of its total mass confined to 32 km above the surface.

What are the layers of the atmosphere and their special characteristics?

The atmosphere consists of different layers with varying density and temperature. Density is highest near the surface of the earth and decreases with increasing altitude.

The column of atmosphere is divided into five layers depending upon the temperature condition: troposphere, stratosphere, mesosphere, thermosphere and exosphere. The ozonosphere is the ozone-bearing part of the stratosphere, between 10 and 50 km.

Character of each layer

The troposphere is the lowermost layer. Its thickness is greatest at the equator because heat is transported to great heights by strong convectional currents. This is the most important layer for all biological activity.

The stratosphere extends up to 50 km and holds the ozone layer. In the mesosphere temperature starts decreasing with the increase in altitude.

The ionosphere contains ions, and radio waves transmitted from the earth are reflected back to the earth by this layer. The exosphere is the highest layer, and whatever contents are there are extremely rarefied in this layer.

Draw a line graph with altitude in km on the vertical axis (mark 0, 10, 50, 80 and 400) and temperature in °C on the horizontal axis (mark -100, -80, -60, -40, -20, 0 and 20).

Plot the zig-zag temperature profile and label, from the bottom, Troposphere, Tropopause, Stratosphere, Stratopause, Mesosphere, Mesopause, Thermosphere and Exosphere.

What is ozone depletion and why does it matter?

The ozone layer lies in the stratosphere between 10 and 50 km and acts as a filter that absorbs the ultra-violet rays radiating from the sun, preventing them from reaching the earth's surface.

Without ozone, this intense and harmful form of energy would reach the surface and damage life. The layer is therefore described as a shield for life on the earth.

Case study: the Antarctic ozone hole

Ozone depletion means the thinning of this ozone shield, so that more ultra-violet radiation reaches the surface.

The main cause is the release of chlorofluorocarbons (CFCs) and similar halogen gases used in refrigerants, air conditioners, aerosol sprays and some industrial foams.

These gases rise slowly into the stratosphere, where ultra-violet light breaks them down and releases chlorine atoms; each chlorine atom can then destroy many ozone molecules in a chain reaction.

The thinning is greatest over the polar regions, especially Antarctica, where the winter polar vortex and polar stratospheric clouds make the chlorine chemistry far more effective, producing the seasonal "ozone hole". Scientists of the British Antarctic Survey reported this hole in 1985.

The consequences include more ultra-violet radiation at the surface, which raises the risk of skin cancer and eye damage in humans, harms phytoplankton and crops, and damages materials.

The world response was the Montreal Protocol of 1987, an international agreement to phase out ozone-depleting substances; it is regarded as one of the most successful environmental treaties, and the ozone layer is slowly recovering.

India is a party to the Protocol and has phased out CFC production in stages under its provisions.

Note: Ozone in the stratosphere shields life, but ozone near the ground is a pollutant. The two effects must not be confused.

How is the atmosphere heated and cooled?

The earth receives almost all of its energy from the sun and in turn radiates back to space the energy received. As a result the earth neither warms up nor cools down over a period of time.

The energy received by the earth is known as incoming solar radiation, or insolation. On an average the earth receives 1.94 calories per sq. cm per minute at the top of its atmosphere.

The earth is farthest from the sun (152 million km) on 4th July, a position called aphelion, and nearest (147 million km) on 3rd January, called perihelion; the annual insolation on 3rd January is therefore slightly more, but the effect is masked by the distribution of land and sea and atmospheric circulation.

Radiation, conduction, convection and advection

There are four ways in which the atmosphere is heated and cooled:

1. Radiation: the earth receives short wave solar radiation and, after being heated, radiates energy to the atmosphere in long wave form as terrestrial radiation.
2. Conduction: the air in contact with the land gets heated slowly, and the upper layers in contact with the lower layers also get heated; heat flows from the warmer to the cooler body until both reach the same temperature or the contact is broken. Conduction is important in heating the lower layers of the atmosphere.
3. Convection: the air in contact with the earth rises vertically on heating in the form of currents and further transmits the heat of the atmosphere; this vertical heating is confined only to the troposphere.
4. Advection: the transfer of heat through horizontal movement of air, which is relatively more important than vertical movement; in middle latitudes most of the diurnal variation in daily weather is caused by advection alone, and the local wind called loo in northern India in summer is the outcome of advection.

The long wave radiation from the earth is absorbed by atmospheric gases, particularly carbon dioxide and other greenhouse gases, so the atmosphere is indirectly heated by the earth's radiation.

The atmosphere in turn radiates and transmits heat to space, and finally the amount of heat received from the sun is returned to space, maintaining constant temperature at the earth's surface and in the atmosphere.

Definition: Insolation is the incoming solar radiation received by the earth, most of it in short wavelengths.

What factors influence insolation and how does the heat budget work?

The amount and intensity of insolation vary during a day, in a season and in a year. The factors are: (i) the rotation of the earth on its axis; (ii) the angle of inclination of the sun's rays;

(iii) the length of the day; (iv) the transparency of the atmosphere; (v) the configuration of land in terms of its aspect. The last two have less influence.

Angle of the sun's rays and length of day

The earth's axis makes an angle of 66½° with the plane of its orbit round the sun, and this has a greater influence on the amount of insolation received at different latitudes.

The angle of the sun's rays depends on the latitude of a place. The higher the latitude, the less is the angle the rays make with the surface, giving slant rays.

The area covered by vertical rays is always less than that covered by slant rays; if more area is covered, the energy gets distributed and the net energy received per unit area decreases.

Slant rays must also pass through greater depth of the atmosphere, resulting in more absorption, scattering and diffusion.

The duration of the day decides how long a place is heated, and the transparency of the atmosphere decides how much is absorbed, scattered or lost before reaching the ground.

Insolation at the surface varies from about 320 Watt/m² in the tropics to about 70 Watt/m² at the poles. Maximum insolation is received over the subtropical deserts, where cloudiness is least.

The equator receives comparatively less insolation than the tropics, and at the same latitude insolation is more over the continent than over the oceans.

The heat budget

The heat budget is the balance between insolation and terrestrial radiation. Take the insolation at the top of the atmosphere as 100 per cent.

Roughly 35 units are reflected back to space even before reaching the surface: 27 units from the top of the clouds and 2 units from snow and ice-covered areas, while 6 units are scattered to space.

The reflected amount is called the albedo of the earth. The remaining 65 units are absorbed, 14 within the atmosphere and 51 by the earth's surface.

The earth radiates back 51 units as terrestrial radiation: 17 units to space directly and 34 units absorbed by the atmosphere (6 directly, 9 through convection and turbulence, and 19 through latent heat of condensation).

The 48 units absorbed by the atmosphere (14 from insolation plus 34 from terrestrial radiation) are also radiated back into space. The total returning, 17 + 48 = 65 units, balances the 65 units received. This is the heat budget or heat balance of the earth.

Draw two panels. The left panel shows short wave solar radiation, total 100 per cent, with 6 scattered to space, 27 reflected by clouds and 2 by the earth, 14 absorbed by the atmosphere, and 17 and 34 absorbed by the earth.

The right panel shows long wave earth radiation: 17 radiated to space from the earth, 48 radiated to space by the atmosphere, and the atmosphere's gains of 6 from the earth, 9 from turbulence and convection and 19 from condensation.

Latitudinal heat balance

Because the earth as a whole does not accumulate or lose heat, it maintains its temperature.

But the amount of radiation received varies from place to place: there is a surplus of net radiation balance between 40 degrees north and south, and the regions near the poles have a deficit.

The surplus heat energy from the tropics is redistributed polewards, so the tropics do not get progressively heated up and the high latitudes do not get permanently frozen.

Plot energy in watts/m² on the vertical axis (50 to 350) and latitude on the horizontal axis (70 N to 70 S).

Draw a solid curve for solar radiation and a dashed curve for terrestrial radiation, shade the middle as Surplus and both ends near the poles as Deficit.

What factors control the distribution of temperature?

The interaction of insolation with the atmosphere and the earth's surface creates heat, which is measured in terms of temperature.

Heat represents the molecular movement of particles comprising a substance, while temperature is the measurement in degrees of how hot or cold a thing or place is.

The temperature of air at any place is influenced by (i) the latitude of the place; (ii) the altitude of the place; (iii) distance from the sea; (iv) the air-mass circulation; (v) the presence of warm and cold ocean currents; (vi) local aspects.

• Latitude: temperature depends on the insolation received, which varies with latitude, so temperature varies accordingly.
• Altitude: the atmosphere is indirectly heated by terrestrial radiation from below, so places near sea level record higher temperature than places at higher elevations. The rate of decrease of temperature with height is the normal lapse rate, 6.5°C per 1,000 m.
• Distance from the sea: compared to land, the sea gets heated slowly and loses heat slowly, while land heats up and cools down quickly. The variation in temperature over the sea is less than over land, and coastal places come under the moderating influence of sea and land breezes.
• Air masses and ocean currents: places under warm air masses experience higher temperature and places under cold air masses experience low temperature; coasts washed by warm currents record higher temperature than coasts washed by cold currents.

Vertical distribution and inversion of temperature

Temperature also varies vertically. In the troposphere it falls at 1°C for every 165 m, while in the stratosphere it rises with height because of ozone absorption.

Sometimes the normal lapse rate is reversed, and temperature increases with height; this is the inversion of temperature.

A long winter night with clear skies and still air is the ideal situation, because the heat of the day is radiated off during the night and by early morning the earth is cooler than the air above.

Over polar areas inversion is normal throughout the year. Surface inversion promotes stability in the lower layers, and smoke and dust collect beneath the inversion layer and spread horizontally, so dense morning fogs are common in winter.

In hills and mountains, cold air produced during the night flows down the slope under gravity and piles up in valley bottoms with warm air above; this is air drainage, and it protects plants from frost damage.

What are isotherms and what do they show?

Isotherms are lines joining places having equal temperature. The global distribution of temperature is generally shown on maps with their help. In general the effect of latitude is well pronounced, as the isotherms are generally parallel to the latitude.

The deviation from this trend is more pronounced in January than in July, especially in the northern hemisphere, because the northern hemisphere has much more land surface than the southern hemisphere, so the effects of land mass and ocean currents are well pronounced.

January and July isotherms

In January the isotherms deviate to the north over the ocean and to the south over the continent.

Over the North Atlantic Ocean the warm currents, the Gulf Stream and North Atlantic Drift, make the ocean warmer and the isotherms bend towards the north; over the land the temperature decreases sharply and the isotherms bend towards the south in Europe, most pronounced in the Siberian plain.

The mean January temperature along 60° E longitude is minus 20°C both at 80° N and 50° N latitudes.

The mean monthly temperature for January is over 27°C in equatorial oceans, over 24°C in the tropics, 2°C to 0°C in the middle latitudes and -18°C to -48°C in the Eurasian continental interior.

In the southern hemisphere the isotherms are more or less parallel to the latitudes and the variation is more gradual; the isotherms of 20°C, 10°C and 0°C run parallel to 35° S, 45° S and 60° S respectively.

In July the isotherms generally run parallel to the latitude. The equatorial oceans record more than 27°C, and over the land more than 30°C is noticed in the subtropical continental region of Asia along 30° N.

Along 40° N runs the isotherm of 10°C, and along 40° S the temperature is 10°C.

The range of temperature between January and July is highest, more than 60°C, over the north-eastern part of the Eurasian continent, due to continentality, and least, 3°C, between 20° S and 15° N.

Draw a world outline with the Equator and a dashed thermal equator. Mark isotherms at -25°C, -12.5°C, 0°C, 10°C, 20°C and 30°C, bending north over the North Atlantic and south over Europe and the Siberian plain.

Draw a world outline with the Equator and a dashed thermal equator. Mark isotherms at 0°C, 10°C, 20°C and 30°C, running generally parallel to the latitude, with the 10°C isotherm along 40° N and 40° S.

Temperature anomalies

Temperature anomalies are departures of the actual temperature of a place from the average for that latitude.

They are positive where the place is warmer than the latitude average, as on a coast washed by a warm current or under a warm air mass, and negative where it is colder, as in the Eurasian continental interior in January.

Their nature is decided by the same factors that control temperature: latitude sets the average, and altitude, distance from the sea, air masses, ocean currents and aspect create the departure.

How is temperature measured and graphed?

Temperature is measured with a thermometer kept in a Stevenson screen, a louvred white box that admits air but shields the instrument from direct sunshine and rain, placed about 1.2 m above the ground over grass.

A maximum thermometer records the highest temperature of the day and a minimum thermometer the lowest; their readings give the daily range.

The daily mean is the average of the maximum and minimum, and the monthly mean is the average of the daily means of the month.

Worked example 1. At a station in north India the mean daily maximum in January, its coldest month, is 21.1°C and the mean daily minimum is 7.3°C; in May, its warmest month, they are 39.6°C and 25.9°C. Find the mean monthly temperature for each month and the annual range of temperature.

Answer: January mean = (21.1 + 7.3) ÷ 2 = 14.2°C. May mean = (39.6 + 25.9) ÷ 2 = 32.75°C. Annual range = 32.75 - 14.2 = 18.55°C.

To show the variation of temperature of one or more Indian cities, plot a line graph with months on the horizontal axis and temperature in °C on the vertical axis, and draw three lines for the mean maximum, the mean minimum and the mean temperature.

Use the same scale for all cities so that the curves can be compared. A bar graph may be used for the annual range of each city.

The highest daily range is usually found in the dry pre-monsoon months and the lowest in the humid monsoon months.

Plot months January to December on the horizontal axis and temperature in °C on the vertical axis. Draw three lines for mean maximum, mean minimum and mean temperature, and label the axes and the city.

How is atmospheric pressure distributed and what are pressure belts?

Atmospheric pressure is the weight of the column of air above a unit area of the earth's surface.

It is highest at sea level and decreases with altitude, because the column of air above becomes shorter and thinner; this is its vertical distribution.

Its horizontal distribution depends mainly on temperature, because warm air expands and becomes lighter while cold air contracts and becomes heavier.

The factors affecting the distribution are temperature, altitude, the rotation of the earth, water vapour in the air and the distribution of land and sea.

Lines joining places of equal pressure are called isobars. Their characteristics are: they never cross one another; closely spaced isobars show a steep pressure gradient and strong winds, while widely spaced isobars show a gentle gradient and light winds; and they bend where land and sea or warm and cold surfaces meet.

Pressure belts and planetary winds

The general pattern of pressure belts is:

1. The equatorial low pressure belt, where intense heating makes air rise, giving the doldrums.
2. The subtropical high pressure belts near 30° N and 30° S, where descending air gives the horse latitudes.
3. The subpolar low pressure belts near 60° N and 60° S, where warm and cold air meet.
4. The polar high pressure areas at the poles, where cold dense air sinks.

These belts shift north and south with the apparent movement of the sun. The winds that blow between them over large parts of the globe all year round are called planetary winds: the trade winds blow from the subtropical highs towards the equatorial low, the westerlies from the subtropical highs towards the subpolar lows, and the polar easterlies from the polar highs towards the subpolar lows.

Seasonal winds such as the monsoon, and local winds such as land and sea breezes, mountain and valley breezes and the loo, complete the picture.

Air masses, fronts and disturbances

An air mass is a large body of air with fairly uniform temperature and humidity, formed over a source region such as a warm ocean or a cold continent.

Where two air masses of different properties meet, the boundary is a front. Atmospheric disturbances are organised low pressure systems: temperate cyclones form along fronts in middle and high latitudes and bring mild, cloudy, rainy weather, while tropical cyclones form over warm tropical oceans and bring violent winds and heavy rain.

Anticyclones are high pressure systems with descending air, giving clear, dry and calm weather.

Draw a globe with the Equator, 30° N and S, 60° N and S and the poles.

Shade the equatorial low, the two subtropical highs, the two subpolar lows and the polar highs, and draw arrows for the trade winds, westerlies and polar easterlies.

How do evaporation, condensation and precipitation work?

Evaporation is the change of water into water vapour from the surface of water bodies, soil and plants. It depends on temperature, the area of the water surface, the dryness of the air and wind speed.

Absolute humidity is the actual amount of water vapour present in a given volume of air, while relative humidity is the ratio of the actual amount of water vapour in the air to the maximum it can hold at that temperature, expressed as a percentage.

Warm air can hold more vapour than cold air, so relative humidity rises as air cools.

Forms of condensation

Condensation is the change of water vapour into liquid water or ice. It needs the air to be cooled to its dew point and needs hygroscopic nuclei such as dust and salt particles around which the vapour can condense. The forms of condensation are:

• Dew: moisture condensed on cool surfaces such as grass and leaves on a clear, calm night.
• Frost: ice crystals formed when the dew point is below freezing.
• Fog: a cloud at ground level, formed when air near the surface cools below its dew point.
• Cloud: visible mass of tiny water droplets or ice crystals formed around nuclei at height.

Precipitation and types of rainfall

Precipitation is the fall of water from clouds to the ground. Its forms are rain (liquid drops), snow (ice crystals) and hail (lumps of ice formed by strong updraughts in cumulonimbus clouds). The types of rainfall are:

1. Orographic rainfall: moist air is forced to rise over a mountain barrier, cools and rains on the windward slope, leaving a rain shadow on the leeward side.
2. Cyclonic rainfall: air rises along the fronts of a cyclone, giving widespread and often prolonged rain.
3. Convectional rainfall: heated air rises in strong convection currents, cools and gives heavy showers, often with thunder, typical of equatorial and summer afternoons.

Definition: Relative humidity is the ratio of the actual amount of water vapour in the air to the maximum it can hold at that temperature, expressed as a percentage.

How is rainfall measured and graphed?

Rainfall is measured with a rain gauge, an open cylindrical vessel with a funnel that collects rain, read against a graduated measuring jar in millimetres. The gauge is placed in an open space away from trees and buildings.

The daily rainfall is the total collected in 24 hours, the monthly total is the sum of the daily amounts, and the annual total is the sum of the monthly totals.

Worked example 2. A station records monthly rainfall of 20 mm, 15 mm, 10 mm, 5 mm, 30 mm, 200 mm, 320 mm, 280 mm, 150 mm, 40 mm, 10 mm and 5 mm.

Find the annual total and the percentage of the annual total that falls in June to September.

Answer: Annual total = 20 + 15 + 10 + 5 + 30 + 200 + 320 + 280 + 150 + 40 + 10 + 5 = 1,085 mm.

June to September = 200 + 320 + 280 + 150 = 950 mm. Percentage = (950 ÷ 1,085) × 100 = 87.6 per cent.

To show the variation of rainfall in one or more Indian cities, draw a bar graph with months on the horizontal axis and rainfall in mm on the vertical axis, one bar for each month, and use the same scale for all cities so that the bars can be compared.

The bars show the concentration of rain in the monsoon months and the dry winter and pre-monsoon months.

Plot months January to December on the horizontal axis and rainfall in mm on the vertical axis. Draw one bar for each month and label the axes, the scale and the city.

What causes climate change and how is India adapting?

Climate change means a long-term shift in the average state of the climate. Its causes are both natural and man-made.

The natural factors are changes in solar output, changes in the earth's orbit and tilt, volcanic eruptions that throw dust and ash into the atmosphere, and natural changes in ocean circulation.

The man-made factors are the burning of fossil fuels, which raises carbon dioxide; deforestation, which removes the trees that absorb carbon dioxide; agriculture and livestock, which release methane; and industry, which releases other greenhouse gases.

These gases are transparent to incoming solar radiation but opaque to outgoing terrestrial radiation, so they trap heat and raise the temperature of the air, the greenhouse effect.

Climate change in India

In India the signs of change in the recent past include rising average temperatures, more frequent and longer heat waves, heavier short-duration rainfall with more floods, more frequent droughts in some regions, a rising sea level along the coast, and the retreat of Himalayan glaciers.

India's large population, long coastline and dependence on monsoon rain make it one of the most vulnerable countries.

Adapting in urban and rural India

Measures taken to adapt in urban India include heat action plans in cities, cool roofs and reflective paints, more urban trees and parks, rainwater harvesting and recharge of groundwater, flood-proof drainage, and the Nagar Van Yojana for urban forests.

Case study: Ahmedabad's Heat Action Plan

Ahmedabad launched India's first city Heat Action Plan in 2013, after a severe heat wave in May 2010 caused a sharp rise in deaths in the city. It was also the first such plan in South Asia.

The plan issues early warnings when very high temperatures are forecast, alerts hospitals and trains health workers to treat heat illness, opens drinking water points and cool shelters, and promotes cool roofs for poor households. Many Indian cities and states have since prepared similar plans.

Measures in rural India include drought-resistant and flood-tolerant crop varieties, changing sowing dates, drip and sprinkler irrigation, watershed development, crop insurance, and the use of solar pumps and biogas.

India has also set up a National Action Plan on Climate Change with missions on solar power, water, agriculture and the Himalayan ecosystem.

Note: Adaptation means adjusting to the changes that are already happening; mitigation means reducing the greenhouse gases that cause them. Both are needed.

Glossary

• Atmosphere: the envelope of gases, water vapour and dust particles around the earth, 99 per cent of whose mass lies within 32 km of the surface.
• Insolation: the incoming solar radiation received by the earth, mostly in short wavelengths.
• Albedo: the percentage of radiation reflected back to space by clouds, snow, ice and the earth's surface.
• Heat budget: the balance between the insolation received by the earth and the terrestrial radiation returned to space.
• Normal lapse rate: the rate of decrease of temperature with height, 6.5°C per 1,000 m.
• Inversion of temperature: a reversal of the normal lapse rate, so that temperature increases with height.
• Isotherm: a line on a map joining places having equal temperature.
• Isobar: a line on a map joining places having equal atmospheric pressure.
• Air mass: a large body of air with fairly uniform temperature and humidity, formed over a source region.
• Hygroscopic nucleus: a tiny dust or salt particle around which water vapour condenses to form cloud droplets.
• Relative humidity: the ratio of the actual water vapour in the air to the maximum it can hold at that temperature, as a percentage.
• Advection: the transfer of heat through horizontal movement of air.
• Air drainage: the flow of cold dense air down a slope at night to pile up in valley bottoms.

Common errors and misconceptions

• Misconception: The atmosphere is heated directly by the sun's rays. Correct: It is heated indirectly, mainly by long wave terrestrial radiation from the earth, with conduction, convection and advection redistributing the heat.
• Misconception: Temperature falls with height through all layers. Correct: It falls in the troposphere and mesosphere but rises in the stratosphere and ionosphere.
• Misconception: The equator receives the maximum insolation. Correct: Maximum insolation is received over the subtropical deserts, where cloudiness is least.
• Misconception: The earth is nearest the sun in July. Correct: It is farthest on 4th July (aphelion) and nearest on 3rd January (perihelion).
• Misconception: Isotherms run straight east to west everywhere. Correct: They bend north over warm ocean currents and south over cold continents, especially in January.
• Misconception: All rainfall is caused by rising air over mountains. Correct: Rainfall may be orographic, cyclonic or convectional.
• Misconception: Ozone is harmful everywhere. Correct: Stratospheric ozone shields life from ultra-violet rays; ozone near the ground is a pollutant.

Exam-style questions with model answers

Q1. State the average height of the troposphere and its rate of fall of temperature. [2 marks]

1. The average height is 13 km, about 8 km near the poles and 18 km at the equator.
2. Temperature falls at 1°C for every 165 m of height.

Q2. Distinguish between conduction and convection as methods of heating the atmosphere. [3 marks]

1. Conduction is the flow of heat between two bodies of unequal temperature in contact, from the warmer to the cooler body, until both reach the same temperature or the contact is broken.
2. Convection is the vertical heating of the atmosphere, in which air in contact with the heated earth rises in currents and carries heat upward.
3. Conduction heats the lower layers in contact with the ground, while convection is confined to the troposphere.

Q3. Explain why the annual range of temperature is high in the Siberian plains. [3 marks]

1. The Siberian plain lies deep inside the Eurasian continent, far from the moderating influence of the sea.
2. Land heats up and cools down quickly, so summers are very hot and winters very cold.
3. The January isotherms bend far south over the plain, and the range between January and July exceeds 60°C over the north-eastern part of Eurasia.

Q4. Describe the composition of the atmosphere. [5 marks]

1. The atmosphere is composed of gases, water vapour and dust particles.
2. Nitrogen forms the major portion; oxygen is almost negligible at 120 km, and carbon dioxide and water vapour are found only up to 90 km.
3. Carbon dioxide is transparent to incoming solar radiation but opaque to outgoing terrestrial radiation, and is largely responsible for the greenhouse effect.
4. Ozone, between 10 and 50 km, absorbs ultra-violet rays and shields life.
5. Water vapour varies from four per cent by volume in the warm wet tropics to less than one per cent in deserts and polar regions, and dust and salt particles act as hygroscopic nuclei for condensation.

Q5. Explain the heat budget of the earth with its figures. [5 marks]

1. Of 100 units of insolation at the top of the atmosphere, 35 are reflected or scattered back to space, of which 27 are reflected from cloud tops and 2 from snow and ice.
2. The remaining 65 units are absorbed, 14 in the atmosphere and 51 by the earth's surface.
3. The earth radiates back 51 units, of which 17 go directly to space and 34 are absorbed by the atmosphere.
4. The atmosphere absorbs 48 units in all and radiates them back to space.
5. The total returning, 17 + 48 = 65 units, balances the 65 units received, so the earth neither warms up nor cools down.

Q6. Why is the troposphere the most important layer of the atmosphere? [4 marks]

1. It contains dust particles and water vapour, which are needed for clouds and rainfall.
2. All changes in climate and weather take place in this layer.
3. It is the layer in which convection and conduction heat the air and in which we live.
4. It is the most important layer for all biological activity.

Q7. Explain the factors that influence the amount of insolation received at a place. [5 marks]

1. The rotation of the earth on its axis changes the angle and duration of sunlight through the day.
2. The angle of inclination of the sun's rays depends on latitude; slant rays spread over a larger area and pass through a greater depth of atmosphere, so less energy reaches the ground.
3. The length of the day decides how long a place is heated.
4. The transparency of the atmosphere decides how much is absorbed, scattered or diffused before reaching the surface.
5. The configuration of land in terms of aspect decides how directly the rays strike a slope; the last two factors have less influence.

Q8. State the characteristics of isotherms and explain why they bend in January. [4 marks]

1. Isotherms are lines joining places of equal temperature and generally run parallel to the latitude.
2. In January they bend north over the ocean and south over the continent.
3. Over the North Atlantic the warm Gulf Stream and North Atlantic Drift make the ocean warmer, so the isotherms bend north.
4. Over Europe and the Siberian plain the land cools sharply, so the isotherms bend south.

Key takeaways

• The atmosphere is a mixture of gases, water vapour and dust, with 99 per cent of its mass within 32 km of the surface.
• The five layers are the troposphere, stratosphere, mesosphere, thermosphere and exosphere, each with its own temperature behaviour.
• Ozone between 10 and 50 km absorbs ultra-violet rays, and its depletion by CFCs lets harmful radiation reach the surface.
• The atmosphere is heated indirectly by terrestrial radiation, through conduction, convection and advection.
• The heat budget balances 65 units received with 65 units returned, so the earth's temperature stays constant.
• Latitude, altitude, distance from the sea, air masses, ocean currents and aspect control temperature.
• Isotherms run roughly parallel to latitude but bend north over warm oceans and south over cold continents in January.
• Pressure belts and planetary winds, air masses, cyclones and anticyclones make up the pressure and wind system.
• Condensation forms dew, frost, fog and cloud, and rainfall may be orographic, cyclonic or convectional.
• Climate change has natural and man-made causes, and India is adapting through urban and rural measures.

Test yourself

At what height is oxygen almost negligible in the atmosphere?

At the height of 120 km, while carbon dioxide and water vapour are found only up to 90 km from the surface.

What is the normal lapse rate?

The rate of decrease of temperature with height, which is 6.5°C per 1,000 m.

What is albedo?

The reflected amount of radiation, about 35 units of every 100 units of insolation, of which 27 are reflected from cloud tops and 2 from snow and ice.

Between which latitudes is there a surplus of net radiation balance?

Between 40 degrees north and south, while the regions near the poles have a deficit.

What is an air mass?

A large body of air with fairly uniform temperature and humidity, formed over a source region such as a warm ocean or a cold continent.

Name the three types of rainfall.

Orographic, cyclonic and convectional rainfall.

What is the greenhouse effect?

The trapping of outgoing terrestrial radiation by carbon dioxide and other greenhouse gases, which are transparent to incoming solar radiation but opaque to outgoing radiation.

What is air drainage?

The flow of cold dense air down a slope at night, piling up in valley bottoms with warm air above, which protects plants from frost damage.