Chapter 5 — 2.1 Concepts of Atmosphere

The atmosphere is the envelope of gases, together with water vapour and suspended solid particles, that surrounds the earth on all sides and is held to it by the force of gravity. The word comes from the Greek atmos, vapour, and sphaira, sphere. The atmosphere is the outermost of the four great spheres of the earth, lying above the lithosphere (the rocky crust), the hydrosphere (the waters) and the biosphere (the zone of life), and it interacts continually with all three.

The atmosphere is not a separate body placed on the earth; it is part of the earth and rotates with it. It is invisible, colourless, odourless and tasteless, but it has weight and exerts pressure, it can be felt as wind, and it becomes visible in the blue of the sky, which is sunlight scattered by its molecules, and in clouds, which are its water vapour condensed. It is extremely thin compared with the size of the earth: if the earth were the size of a football, the part of the atmosphere in which all weather occurs would be thinner than a coat of paint. Yet this thin film is the difference between a living planet and a dead one; the moon, which has no atmosphere, has a surface that bakes at over 100 °C by day and freezes at below −150 °C by night, and it is pitted with craters because nothing burns up the meteors that strike it.

The atmosphere is a mixture, not a compound: its gases are mingled together but not chemically combined, and each keeps its own properties. In the lower atmosphere the gases are so thoroughly mixed by winds that their proportions are the same everywhere on earth, from Kolkata to the Antarctic. Only the variable constituents, water vapour, dust and a few trace gases, differ from place to place.

The atmosphere is in constant motion. Heated unequally by the sun, it rises here and sinks there, flows from high pressure to low pressure as wind, carries water vapour from the oceans to the continents and returns it as rain. All the phenomena that we call weather, sunshine, cloud, rain, wind, storms and fog, are events in the atmosphere, and the long-term pattern of those events at any place is its climate. The atmosphere is therefore the subject of two sciences: meteorology, the study of the atmosphere and its day-to-day changes, and climatology, the study of climate.

In this unit the atmosphere is studied in four steps: its basic concepts and composition (this section), its vertical structure in layers, its heating and temperature, and its pressure and winds.

The atmosphere that we breathe is not the one the earth was born with. It has changed greatly over the 4,600 million years of the earth's history, and scientists recognise three stages in its evolution.

The first atmosphere, when the earth formed from the cloud of gas and dust around the young sun, consisted mainly of the lightest gases, hydrogen and helium. The earth's gravity was too weak to hold these very light gases, and the intense heat of the young sun and the solar wind swept them away into space. This primordial atmosphere was thus lost almost entirely.

The second atmosphere was produced from inside the earth itself by degassing: the volcanoes that covered the young, hot earth poured out enormous quantities of water vapour, carbon dioxide, nitrogen, sulphur dioxide, methane and ammonia, just as volcanoes still do today. The earth's gravity could hold these heavier gases. As the earth cooled, the water vapour condensed and fell as rain over millions of years, filling the oceans. The carbon dioxide dissolved in the oceans and was locked up in limestone rocks. Nitrogen, being chemically inactive, stayed in the air and accumulated. This second atmosphere had almost no free oxygen; it would have been poisonous to us.

The third atmosphere, the present one, was created by life. About 3,500 million years ago, tiny blue-green algae and later green plants in the oceans began to carry out photosynthesis, taking in carbon dioxide and water and releasing oxygen. Over hundreds of millions of years this biological oxygen built up in the air to its present level of about 21 per cent. Some of the oxygen high in the atmosphere was converted by sunlight into ozone, which formed a layer that screened out the ultraviolet rays, and only then could life move from the sea on to the land. The atmosphere and life have thus shaped each other: life made the oxygen, and oxygen made life on land possible.

The atmosphere is still changing. Volcanoes add gases, the oceans exchange carbon dioxide with the air, plants and animals exchange oxygen and carbon dioxide, and lightning and bacteria convert nitrogen. Over the last two centuries human beings have become a major agent of change, burning coal, oil and gas and cutting forests, so that the carbon dioxide content has risen from about 280 parts per million before the industrial age to over 420 parts per million today, a matter taken up in a later section on global warming.

Dry, clean air near the surface is a mixture of many gases in remarkably constant proportions. By volume it is made up of:

Nitrogen and oxygen together make up about 99 per cent of the air, and argon nearly all of the rest; every other gas together accounts for a few hundredths of one per cent. The proportions of nitrogen, oxygen and argon are the same everywhere in the lower atmosphere, up to about 80 to 90 km, because the winds mix the air thoroughly; for this reason they are called the permanent or constant gases. Carbon dioxide, ozone, methane and water vapour, whose amounts change with place and time, are the variable gases, described in the next topic.

Nitrogen is the most abundant gas. It is colourless, odourless and chemically inactive, so it neither burns nor supports burning nor takes part directly in breathing. Its importance is twofold. First, it dilutes the oxygen; in pure oxygen fires would rage uncontrollably and living tissues would be damaged, and nitrogen keeps combustion and oxidation at a moderate pace. Second, it is an essential element of all proteins and therefore of all living things. Plants cannot take nitrogen from the air directly; it is fixed into the soil by lightning, by bacteria living in the root nodules of leguminous plants such as peas, beans and gram, and by industrial fertiliser manufacture, and returns to the air when organic matter decays. This circulation is the nitrogen cycle.

Oxygen is the second most abundant gas and the most important for life. It is colourless and odourless but chemically very active. All animals and most other organisms breathe it to release energy from food by respiration, and no fire can burn without it. Oxygen also weathers rocks by oxidation, rusting iron and reddening soils. It is continually removed from the air by breathing, burning and rusting, and continually replaced by the photosynthesis of green plants, so that its proportion stays constant; the forests and the microscopic plants of the sea are the lungs of the earth. Above about 100 km, ultraviolet light splits oxygen molecules into single atoms, and in the stratosphere some oxygen is converted into ozone.

Argon is an inert gas with no role in life or weather; it is used to fill electric bulbs. The other inert gases, neon, helium, krypton and xenon, are present in minute traces and are used in lighting and industry.

Several gases are present in the atmosphere in very small and changing amounts, yet they influence weather, climate and life out of all proportion to their quantity. These are the variable gases.

Carbon dioxide makes up only about 0.04 per cent of the air, or about 420 parts per million, but it is one of the most important constituents. It is the raw material of photosynthesis, from which plants build all the food on earth. It is a greenhouse gas: it is transparent to the short-wave radiation of the sun but absorbs the long-wave heat radiated back by the earth, and so it traps heat near the surface and keeps the earth warm enough for life. Without carbon dioxide and water vapour the average temperature of the earth would be about −18 °C instead of the present 15 °C. Carbon dioxide is added to the air by the breathing of animals and plants, by the decay of organic matter, by volcanoes, by forest fires and, in the last two centuries, by the burning of coal, oil and natural gas; it is removed by photosynthesis and by absorption in the oceans. Because human beings are adding it faster than plants and oceans remove it, its concentration has risen by about 50 per cent since 1800, from about 280 to over 420 parts per million, and this is the chief cause of global warming, discussed in a later section. Carbon dioxide is heavier than air and so is concentrated in the lower atmosphere.

Ozone (O₃) is a form of oxygen in which three atoms are joined instead of two. It is present in minute traces, and most of it is concentrated in the ozone layer of the stratosphere between about 15 and 35 km above the surface, where it is formed by the action of ultraviolet sunlight on ordinary oxygen. Ozone absorbs most of the harmful ultraviolet radiation of the sun and so shields living things from skin cancer, eye damage and genetic harm; life on land could not exist without it. The layer has been thinned by chlorofluorocarbons (CFCs) released from refrigerators and aerosol sprays, producing the ozone hole over Antarctica, which international agreement (the Montreal Protocol of 1987) is slowly repairing. Near the ground, ozone produced from vehicle exhausts is a pollutant that harms lungs and crops.

Methane (CH₄), from wetlands, paddy fields, cattle, landfills and gas leaks, is present at about 2 parts per million but is a very strong greenhouse gas. Nitrous oxide from fertilisers, and water vapour, the most important variable constituent of all, are also greenhouse gases; water vapour is described in the next topic.

The variable gases show that the influence of a gas does not depend on its abundance. Argon, a hundred times more plentiful than carbon dioxide, does nothing; carbon dioxide, at 0.04 per cent, feeds all plants and controls the temperature of the planet; ozone, at a few parts per million, decides whether life can survive on land.

Besides its gases, the atmosphere contains two other constituents that vary greatly from place to place and are responsible for most of what we call weather.

Water vapour is water in its invisible gaseous form. It enters the atmosphere by evaporation from the oceans, lakes, rivers and moist ground and by transpiration from plants, and it leaves it by condensation into cloud, fog, dew, rain and snow. Its amount ranges from almost nothing over the cold polar regions and the hot deserts to about 4 per cent by volume over the warm, humid tropical oceans and the monsoon lands. About 90 per cent of all the water vapour lies in the lowest 5 km of the atmosphere, and it decreases rapidly with height, because it comes from the surface and because cold air can hold very little of it. The amount of water vapour that air can hold doubles roughly for every 10 °C rise in temperature, which is why warm monsoon air over Bengal is so humid and why cold winter air is dry.

Water vapour is the most important variable constituent for three reasons. First, it is the source of all precipitation, and therefore of the fresh water on which land life depends; the whole water cycle passes through the atmosphere in this form. Second, it is the most powerful greenhouse gas, absorbing the earth's outgoing heat and keeping nights and winters from being unbearably cold; dry desert air lets the heat escape, which is why deserts are freezing at night though scorching by day, while humid Kolkata stays warm through the night. Third, it carries latent heat: when water evaporates it takes in heat, which it carries with it as vapour and releases when it condenses. In this way the atmosphere moves enormous quantities of energy from the tropical oceans towards the poles, and the energy released by condensation is what powers thunderstorms and cyclones.

Aerosols are the tiny solid and liquid particles suspended in the air, so small and light that they float for days or years. They include dust blown from deserts and dry fields, salt crystals from the spray of breaking waves, smoke and ash from forest fires and volcanoes, pollen and spores from plants, soot and sulphate particles from factories, vehicles and cooking fires, and microscopic meteoric dust from space. They are most abundant in the lower atmosphere and over land, especially over deserts and cities.

Aerosols are important in three ways. They act as condensation nuclei: water vapour cannot condense into droplets in perfectly clean air, but it condenses readily on these particles, so without them there would be no cloud, fog or rain. They scatter and absorb sunlight, giving the sky its blue colour by day, the red and orange colours of sunrise and sunset, and the haze that reduces visibility; large volcanic eruptions such as Pinatubo in 1991 threw so much dust into the stratosphere that the earth cooled by about half a degree for two years. And in cities they are the chief component of air pollution, harming the lungs; the winter smog of Delhi and Kolkata is a mixture of smoke, dust and fog held down by cold, still air.

The atmosphere has no sharp upper edge. It becomes thinner and thinner with height and fades imperceptibly into the near-vacuum of interplanetary space, so any figure for its height is a matter of definition. The following facts should be known.

About 50 per cent of the entire mass of the atmosphere lies within the lowest 5.5 km, below the height of Kanchenjunga; about 75 per cent lies below 11 km, the cruising height of a jet aircraft; about 90 per cent lies below 16 km and about 99 per cent below 32 km. Traces of gas can be detected by satellites up to about 1,000 km, and the outermost fringe, where the atmosphere merges with space, is placed at about 10,000 km. For most purposes the atmosphere is said to extend to about 1,000 km, though all weather takes place in the lowest 10 to 16 km. The Kármán line at 100 km is used internationally as the boundary of space for aviation and astronautics; a person who has crossed it is counted an astronaut.

The atmosphere is held to the earth by gravity, which pulls every molecule towards the centre of the earth. Because gases are compressible, the weight of the upper layers squeezes the lower layers, so the air is densest and its pressure greatest at sea level and both fall rapidly with height. At sea level the density of air is about 1.2 kilograms per cubic metre and the pressure is about 1,013 millibars, which is the weight of a column of air about one square centimetre in cross-section from the sea to the top of the atmosphere, roughly one kilogram, or the same as a column of mercury 76 cm high. Pressure falls by roughly half for every 5.5 km of ascent: about 500 millibars at 5.5 km, 250 at 11 km, 125 at 16 km. On the summit of Everest at 8,848 m the pressure is about a third of that at sea level, and a climber's lungs take in only a third as much oxygen with each breath, which is why most climbers carry oxygen cylinders. The total mass of the atmosphere is about 5.15 × 10¹⁸ kg, that is, about five thousand million million tonnes, but this is less than one millionth of the mass of the earth.

The atmosphere rotates with the earth, because friction and gravity carry it along; if it did not, a wind of about 1,600 km per hour would blow at the equator. The thinning of the atmosphere with height has practical consequences: aircraft fly high to reduce air resistance but must pressurise their cabins; mountain towns such as Darjeeling at 2,000 m have a pressure about a fifth lower than Kolkata, and water boils there at about 93 °C instead of 100 °C, so cooking takes longer; and the thin air of the high Himalaya offers less protection from ultraviolet rays, so sunburn is severe.

The understanding of the vertical extent of the atmosphere leads directly to the next section, which divides it into layers by the way temperature changes with height.

The atmosphere is the first condition of life on the earth. Its functions for living things can be set out under several heads.

Breathing. Every animal, including human beings, needs oxygen for respiration, the process by which food is burnt in the cells to release energy, and every green plant needs carbon dioxide for photosynthesis, the process by which food is made. The atmosphere supplies both and receives back the products of each, so that the two processes balance one another in the oxygen–carbon dioxide cycle. Nitrogen from the air, fixed in the soil, is the source of the proteins of all plants and animals. A person breathes about 11,000 litres of air a day; without it life ends within minutes.

Temperature control. The atmosphere acts like the glass of a greenhouse or the blanket on a bed. By day it absorbs and scatters part of the sun's radiation, so that the surface does not become scorching; by night its water vapour and carbon dioxide absorb the heat radiated by the earth and send part of it back, so that the surface does not freeze. Because of this the average temperature of the earth is about 15 °C and the difference between day and night is a few degrees rather than the 250 degrees of the moon. The winds and ocean currents driven by the atmosphere carry heat from the equator to the poles and make the whole planet habitable.

Protection from radiation. The ozone layer of the stratosphere absorbs almost all of the sun's harmful ultraviolet rays, which would otherwise cause skin cancer, blindness and mutation and would kill the plankton on which the sea's life depends. The upper atmosphere absorbs X-rays and gamma rays from space.

Protection from meteors. Millions of meteors enter the atmosphere every day. Friction with the air heats them white-hot and they burn up as shooting stars long before reaching the ground. Without the atmosphere the earth would be bombarded like the moon.

Water and weather. The atmosphere carries water vapour from the oceans over the land and returns it as rain and snow, feeding rivers, lakes, groundwater and all the fresh water that plants, animals and farms require. The whole hydrological cycle runs through the atmosphere, and the monsoon rains on which Indian agriculture depends are an atmospheric phenomenon.

Sound and light. Sound travels through air; in a vacuum there is no sound. The atmosphere scatters sunlight, giving the sky its blue and the sunset its red, spreading daylight into shade and lengthening it into twilight before sunrise and after sunset; without it the sky would be black even at noon and shadows would be absolutely dark.

Human use. The atmosphere is the medium of flight, from birds to aircraft; its winds turn windmills and fill sails; its nitrogen is fixed into fertiliser; its oxygen and inert gases are extracted for industry and medicine; radio waves are reflected by its ionosphere; and it is the great sink into which the smoke and gases of human activity are poured, which is its most abused function.

Besides sustaining life, the atmosphere shapes the surface of the earth itself. Almost every exogenetic process studied in the first unit of this book is driven by the atmosphere, and the landscape of every region is the product of its climate.

Weathering. The gases and moisture of the air attack rocks directly. Oxygen oxidises the iron in minerals and rots the rock; carbon dioxide dissolved in rainwater forms carbonic acid that dissolves limestone; water vapour condensing as dew and rain hydrates and hydrolyses minerals. The daily heating and cooling of the surface, which is an atmospheric process, cracks rocks by expansion and contraction, and frost, which is atmospheric water freezing, shatters them. Without the atmosphere the rocks of the earth would remain unweathered like those of the moon, and there would be no soil.

Running water. The rivers that carve valleys, build plains and deltas and carry the material of the continents to the sea are fed by rain, and rain is water vapour that the atmosphere has lifted from the oceans and carried over the land. The whole work of rivers is therefore atmospheric in origin.

Glaciers. Snow is atmospheric precipitation, and the glaciers that carved the Himalaya and the fjords of Norway exist because the atmosphere delivered snow faster than it could melt. The height of the snowline is fixed by atmospheric temperature.

Wind. Wind is the atmosphere in motion, and the mushroom rocks, yardangs, dunes and loess of the deserts are its direct work. The waves that cut cliffs and build beaches are raised by the wind.

Soil and vegetation. Soil forms from weathered rock under the action of atmospheric water, temperature and the organisms that the atmosphere sustains. The type of natural vegetation of every region, from the evergreen forests of the equator to the tundra of the Arctic, is decided by atmospheric temperature and rainfall, and vegetation in turn protects or exposes the surface to erosion.

Climate and human activity. The distribution of population, agriculture, settlement and industry follows the distribution of climate: the crowded monsoon lands, the empty deserts and the frozen wastes are all made by the atmosphere. The pattern of crops in West Bengal, rice in the wet delta and tea in the misty Darjeeling hills, is an atmospheric decision.

In short, the atmosphere is both the sustainer of life and the sculptor of the land. The two units of this book, on the exogenetic processes and on the atmosphere, are therefore two halves of one subject: the first describes what the atmosphere's agents do to the surface, and the second describes the atmosphere itself.

Two words used throughout the study of the atmosphere must be distinguished carefully, because they are often confused in everyday speech and are frequently asked in the examination.

Weather is the condition of the atmosphere at a particular place at a particular moment or over a short period, an hour, a day or a few days, described in terms of temperature, pressure, wind, humidity, cloud, precipitation and visibility. Weather changes from hour to hour and from day to day; it may be sunny in the morning and stormy in the afternoon. It refers to a small area and a short time. When we say it is raining in Kolkata today, or that yesterday was very hot, we are speaking of weather.

Climate is the average condition of the atmosphere over a large area over a long period, conventionally at least 30 years, together with the usual range of variation and the extremes. It is the sum and average of all the weather that a place experiences year after year. Climate does not change from day to day; it is the character of a region, not its mood. When we say that Kolkata has a hot and humid monsoon climate with rainy summers and dry, mild winters, we are speaking of climate. Climate is said to be what you expect and weather what you get.

The differences may be tabulated:

The elements of weather and climate are the same: temperature, the degree of heat of the air; atmospheric pressure, the weight of the air; wind, the horizontal movement of air, with its direction and speed; humidity, the amount of water vapour in the air; cloudiness; precipitation, in the form of rain, snow, hail and dew; and visibility and sunshine. Weather is the state of these elements at a moment; climate is their average and typical behaviour.

The factors that decide the climate of a place, sometimes called the controls of climate, are its latitude, its altitude, its distance from the sea, the ocean currents along its coast, the prevailing winds, the direction of mountain ranges and the nature of the surface. These factors explain, for example, why Darjeeling at 2,000 m is cool while Siliguri at its foot is hot, why Kolkata near the sea has a smaller range of temperature than Delhi, and why the Meghalaya hills facing the monsoon winds are the wettest place on earth.

The elements of weather are measured at thousands of weather stations, on ships, by balloons, aircraft, radar and satellites, and the readings are collected by national meteorological services such as the India Meteorological Department, whose regional centre for eastern India is at Alipore in Kolkata. The basic instruments should be known.

Temperature is measured with a thermometer, a glass tube containing mercury or coloured alcohol that expands with heat, graduated in degrees Celsius. The maximum and minimum thermometer, or Six's thermometer, records the highest and lowest temperature of the day; the maximum is usually reached at about 2 p.m. and the minimum just before sunrise. Thermometers are kept in a white, louvred wooden box called the Stevenson screen, about 1.25 m above grass, so that they measure the temperature of the shaded, freely moving air and not of the sun or the ground.

Pressure is measured with a barometer. In the mercury barometer, invented by Torricelli in 1643, the air pressure supports a column of mercury in a glass tube about 76 cm high at sea level, and the height of the column rises and falls with the pressure. The aneroid barometer uses a sealed metal box that flexes with pressure and moves a needle; it is portable and is used in aircraft and as an altimeter. Pressure is expressed in millibars (mb) or hectopascals (hPa), the two being equal; standard sea-level pressure is 1,013.25 mb. A barograph records pressure continuously on a chart.

Wind direction is shown by a wind vane, an arrow pivoted on a vertical rod that turns to point into the wind; a wind is always named by the direction from which it blows, so a westerly wind blows from the west. Wind speed is measured with an anemometer, usually three or four cups on horizontal arms that spin in the wind, the speed being read from a dial in kilometres per hour or knots. The Beaufort scale from 0 (calm) to 12 (hurricane) estimates wind force from its effects.

Humidity is measured with a hygrometer. The commonest is the wet-and-dry bulb thermometer or psychrometer, in which one thermometer bulb is wrapped in wet muslin; evaporation cools the wet bulb, and the difference between the two readings, looked up in tables, gives the relative humidity.

Rainfall is measured with a rain gauge, a cylinder with a funnel of known diameter that collects rain into a bottle, from which the depth of rain is read in millimetres in a measuring glass every morning at 8.30 a.m. India Standard Time. Sunshine is recorded by a Campbell-Stokes recorder, a glass sphere that focuses the sun's rays to burn a trace on a card. Cloud cover is estimated by eye in eighths of the sky, called oktas. Upper-air conditions are measured by radiosondes, instrument packages carried aloft by hydrogen balloons, and by weather satellites such as India's INSAT series, whose cloud pictures track cyclones over the Bay of Bengal. Doppler weather radar at Kolkata and other stations tracks rain and storms within 250 km.

For most of the earth's history the composition of the atmosphere changed only over millions of years. In the last two centuries human beings have begun to change it within decades, and this section of the syllabus asks the student to know the main ways in which this is happening.

Increase of carbon dioxide. The burning of coal, petroleum and natural gas in power stations, factories, vehicles and homes releases about 37,000 million tonnes of carbon dioxide a year, and the cutting and burning of forests, which removes the plants that would absorb it, adds more. As a result the carbon dioxide in the air has risen from about 280 parts per million before 1800 to over 420 parts per million, higher than at any time in at least 800,000 years. Because carbon dioxide is a greenhouse gas, this is raising the temperature of the earth; the global average has already increased by about 1.2 °C, and the consequences, melting glaciers, rising seas, stronger cyclones and shifting rains, are treated in the section on global warming.

Increase of other greenhouse gases. Methane from paddy fields, cattle, coal mines, gas pipelines and rubbish dumps has more than doubled; nitrous oxide from chemical fertilisers has increased; and entirely new gases, the chlorofluorocarbons and their relatives, have been invented and released.

Depletion of ozone. Chlorofluorocarbons (CFCs), used from the 1930s as refrigerants, aerosol propellants and foam-blowing agents, are so stable that they rise unchanged into the stratosphere, where ultraviolet light breaks them down and releases chlorine atoms; each chlorine atom destroys thousands of ozone molecules. By the 1980s an ozone hole had opened over Antarctica each spring, and the ozone layer had thinned worldwide. The Montreal Protocol of 1987 banned CFCs, and the layer is slowly recovering, with full repair expected around 2060; it is the best example of international action to protect the atmosphere.

Air pollution. Smoke, soot, sulphur dioxide, oxides of nitrogen, carbon monoxide, hydrocarbons, lead and fine particulate matter (PM_2.5 and PM₁₀) from vehicles, industries, brick kilns, thermal power stations, crop burning and cooking fires pollute the lower atmosphere of cities and even villages. Sulphur dioxide and nitrogen oxides dissolve in rain to form acid rain, which damages forests, lakes, crops and buildings, including the marble of the Taj Mahal. Particulate matter causes lung and heart disease; the winter smog of Delhi, Kolkata and the Ganga plain, when cold still air traps the pollutants near the ground, is among the worst in the world. Ground-level ozone formed from vehicle exhaust in sunlight is a further pollutant.

Deforestation and land use. Clearing forests reduces the absorption of carbon dioxide and the release of oxygen and water vapour, and bare land raises more dust. Draining wetlands and changing farming alter the exchange of gases between land and air.

The atmosphere has no boundaries, and gases released in one country spread over the whole earth within a year or two. Its protection therefore requires international agreements, such as the Montreal Protocol on ozone and the Paris Agreement of 2015 on climate, and national measures such as India's Bharat Stage emission norms, the National Clean Air Programme and the shift to solar and wind power. Every student can contribute by saving electricity and fuel, planting trees, avoiding the burning of waste and using public transport.

The concepts of the atmosphere are examined mostly through short items, but they underlie the longer questions of the following three sections, so they must be exact.

One-mark questions (multiple choice, fill in the blanks, true or false, match the columns) ask for facts and figures: the most abundant gas (nitrogen, 78%), the percentage of oxygen (21%), the gas that absorbs ultraviolet rays (ozone), the height of the ozone layer (15–35 km), the gas used by plants for photosynthesis (carbon dioxide, 0.04%), the inert gas third in abundance (argon), the constituent that varies from 0 to 4 per cent (water vapour), the particles on which water vapour condenses (aerosols or condensation nuclei), the instrument for pressure (barometer), for humidity (hygrometer), the standard sea-level pressure (1,013 mb), the height below which half the atmosphere lies (5.5 km), the force that holds the atmosphere (gravity), the gases responsible for ozone depletion (CFCs), and the average temperature of the earth (about 15 °C). A short list of these numbers should be memorised.

Two-mark questions ask for definitions and reasons: What is the atmosphere? What is meant by weather? What is climate? Why is water vapour called a variable constituent? Why is the atmosphere thicker near the surface? What is the importance of ozone? Why is nitrogen important though inactive? Name two greenhouse gases. Each answer should be two or three precise sentences.

Three-mark questions ask for explanations and distinctions: Distinguish between weather and climate in three points. Explain the importance of carbon dioxide. Describe the role of aerosols. Explain why the atmosphere is called a mixture. Describe how the present atmosphere evolved.

Five-mark questions ask for a fuller treatment: Describe the composition of the atmosphere with a pie diagram. Explain the importance of the atmosphere for life and for the earth's surface. Describe the human activities that are changing the composition of the atmosphere and their effects. Discuss the roles of the variable gases.

Guidance for answering. State the exact percentages of nitrogen, oxygen, argon and carbon dioxide whenever composition is asked, and draw the pie chart; it is worth a mark. Group the constituents as permanent gases, variable gases, water vapour and aerosols, and give one role for each. For weather and climate, always mention time, area and variability, and give a West Bengal example of each. For importance, list functions under sub-headings (breathing, temperature, protection, water, sound and light) rather than as a paragraph. For human impact, name the gas, its source and its effect together in one line, for example: carbon dioxide, from burning fossil fuels, causes global warming; CFCs, from refrigerants, deplete ozone. Avoid the common errors of writing that oxygen is the most abundant gas, that ozone is in the troposphere, that carbon dioxide is a large fraction of the air, or that weather and climate mean the same thing.