Chapter 6 — 2.2 Layers of the Atmosphere

The atmosphere is not uniform from bottom to top. As one ascends, the pressure and density of the air fall continuously, but the temperature does not: it falls, then rises, then falls again and finally rises steeply. The composition also changes, from a well-mixed blend of gases near the surface to separate shells of lighter gases far above. Because of these changes scientists divide the atmosphere into layers or spheres, and this division is the subject of the present section.

There are two ways of dividing it. The first is by chemical composition, which gives two divisions, the homosphere below about 90 km, where the gases are thoroughly mixed and their proportions constant, and the heterosphere above it, where the gases separate into layers by weight. This division is described in the next topic.

The second and more important way is by the vertical change of temperature, which gives five thermal layers, one above another: the troposphere, the stratosphere, the mesosphere, the thermosphere and the exosphere. The boundary at the top of each layer, where the temperature trend reverses, is called a pause: the tropopause, the stratopause, the mesopause and the thermopause. In the troposphere temperature falls with height; in the stratosphere it is steady and then rises; in the mesosphere it falls again to the lowest value in the atmosphere; and in the thermosphere it rises to very high values. The exosphere is the outermost fringe. These five layers, their heights, their temperatures and their characteristics form the core of the section and are set out one by one in the topics that follow.

The knowledge of these layers has been built up over little more than a century. Until the 1890s scientists assumed that temperature fell steadily all the way up. Then the French meteorologist Léon Teisserenc de Bort, sending unmanned balloons carrying thermometers to great heights, discovered in 1902 that above about 11 km the temperature stopped falling; he named the lower layer the troposphere and the upper layer the stratosphere. The layers above were mapped by rockets and satellites after 1945. Today, weather balloons (radiosondes) released twice daily from stations such as Kolkata rise to about 30 km, aircraft cruise in the lower stratosphere, and satellites orbit in the thermosphere and exosphere.

The layers matter for practical reasons. All weather, and hence all agriculture and daily life, happens in the troposphere. Long-distance jets fly in the calm lower stratosphere. The ozone layer of the stratosphere protects us from ultraviolet rays. Meteors burn in the mesosphere. Radio broadcasts bounce off the ionosphere of the thermosphere, and satellites orbit through it. A clear picture of the layers is thus the key to understanding both the weather and the technologies of the modern world.

When the atmosphere is divided according to the composition of its gases, two great divisions are recognised.

The homosphere is the lower part, from the surface to a height of about 90 km. Here the gases are so thoroughly stirred by winds, convection and turbulence that they are uniformly mixed, and the proportions of nitrogen (78 per cent), oxygen (21 per cent), argon and the other permanent gases are the same at every height and every place. The name means the sphere of sameness. The homosphere contains virtually the whole mass of the atmosphere, more than 99.9 per cent, and it includes the troposphere, the stratosphere and the mesosphere of the thermal classification. Its only non-uniform constituents are water vapour, which is confined to the lowest few kilometres, ozone, which is concentrated in the stratosphere, and the dust and pollutants near the ground. The average molecular weight of the air throughout the homosphere is constant at about 29.

The heterosphere is the upper part, above about 90 km, extending to the outer limit of the atmosphere at about 10,000 km. Here the air is so thin that collisions between molecules are rare, mixing no longer takes place, and each gas settles out under gravity according to its own weight, the heavier gases lower down and the lighter gases higher up, in a series of concentric shells. The name means the sphere of difference. Four shells are recognised: a molecular nitrogen layer from about 90 to 200 km; an atomic oxygen layer from about 200 to 1,100 km, in which ultraviolet light has split oxygen molecules into single atoms; a helium layer from about 1,100 to 3,500 km; and a hydrogen layer from about 3,500 km outward, which finally merges with the hydrogen of interplanetary space. The heterosphere corresponds to the thermosphere and exosphere of the thermal classification. Its total mass is negligible, yet it is important because the ionosphere lies within it and because satellites orbit through it.

The boundary between the two, at roughly 90 km, is sometimes called the turbopause or homopause, because it is the level above which turbulent mixing stops. It is not far from the mesopause of the thermal classification and from the Kármán line at 100 km that is taken as the edge of space.

The compositional division is less used than the thermal division and is examined chiefly as a one-mark or two-mark item: the student should know the names of the two spheres, their heights, the reason for the difference between them and the order of the four layers of the heterosphere from bottom to top, nitrogen, oxygen, helium, hydrogen, which can be remembered by their decreasing molecular weight.

The troposphere is the lowest layer of the atmosphere, in contact with the surface of the earth, and the one in which all living things and almost all weather are found. The name was given by Teisserenc de Bort from the Greek tropos, meaning turning or mixing, because the air in this layer is constantly turning over by convection.

Height. The troposphere extends from the surface to an average height of about 12 km, but its thickness varies with latitude and season. It is about 16 to 18 km thick over the equator and only about 8 km over the poles, and it is a little higher in summer than in winter. The reason is that the equatorial air is strongly heated and expands upward in vigorous convection, while the cold polar air is dense and shallow.

Temperature. Temperature decreases with height throughout the troposphere, because the air is heated from below by the earth's surface, not directly by the sun, and because the air becomes thinner and holds less water vapour and dust to absorb heat. The average rate of decrease is 6.5 °C per kilometre (about 1 °C for every 165 m), and this is called the normal lapse rate. At the top of the troposphere the temperature reaches about −60 °C over the poles and about −80 °C over the equator, where the layer is thickest.

Composition. The troposphere contains about 75 per cent of the total mass of the atmosphere and almost all of its water vapour, cloud and dust. The air is dense and turbulent, with strong vertical currents that carry heat, moisture and dust upward and mix the layer thoroughly.

Weather. Because it holds the water vapour and the dust, and because its temperature falls with height so that rising air cools and its vapour condenses, the troposphere is the layer of all weather phenomena: cloud, rain, snow, hail, fog, dew, thunderstorms, cyclones and winds. Meteorologists therefore call it the weather sphere. Life is confined to it, and the whole of the hydrological cycle takes place within it.

The tropopause. The upper boundary of the troposphere, where the fall of temperature stops, is the tropopause, a transition zone about 1 to 2 km thick. The temperature at the tropopause is the lowest in the lower atmosphere, so cold that almost no water vapour passes through it, which is why clouds do not rise above it; the flat anvil tops of thunderclouds are spread out along the tropopause. Its height, like that of the troposphere, is about 17 km at the equator and 8 km at the poles, and it is not continuous but broken in the middle latitudes, where the fastest jet streams blow. Its name means the place where the turning stops.

For the examination the troposphere should be described under the heads of height, temperature and lapse rate, composition, weather and the tropopause, with the figures given above.

The most important property of the troposphere is that its temperature decreases with height, and this deserves a fuller explanation because it underlies the study of weather.

The rate at which the temperature of the free air decreases with height is called the lapse rate. The average value throughout the troposphere, over the whole earth and the whole year, is 6.5 °C per 1,000 m, or 1 °C for about every 165 m of ascent, and this average is called the normal lapse rate or the environmental lapse rate. The actual rate at a given place and time may be more or less, depending on the season, the time of day and the weather, but the average is remarkably constant.

There are three reasons why temperature falls with height. First, the atmosphere is heated mainly from below: sunlight passes through the clear air with little heating and is absorbed by the ground, which then warms the air in contact with it by conduction, radiation and convection, so the air nearest the ground is the warmest. Second, water vapour and dust, which absorb the earth's heat radiation, are concentrated in the lowest few kilometres and thin out rapidly above, so the upper air has less to absorb heat with. Third, rising air expands as the pressure around it falls, and expanding air cools because it does work in pushing the surrounding air aside; this cooling is the adiabatic cooling that produces clouds.

The lapse rate explains many familiar facts. Hill stations are cool: Darjeeling at about 2,000 m is roughly 12 °C cooler than the plains of Siliguri at its foot. Mountains carry snow: above the snowline, about 4,500 m in the eastern Himalaya, the temperature never rises enough to melt it. The vegetation changes with altitude from tropical forest at the foot of the Himalaya through temperate forest to alpine meadows and bare rock, a sequence that mirrors the change from the equator to the poles. And clouds form when rising air cools below its dew point.

Sometimes the normal order is reversed and the temperature increases with height for a short distance; this is a temperature inversion. It happens on clear, calm winter nights when the ground loses heat rapidly by radiation and cools the air in contact with it, so that a layer of cold air lies beneath warmer air aloft. In mountain valleys, cold dense air from the slopes drains down at night and collects on the valley floor, producing a valley inversion; frost then damages crops on the valley floor while the slopes above remain frost-free, which is why the tea gardens and orange orchards of the Darjeeling hills are planted on the slopes rather than in the valley bottoms. An inversion is very stable, because the cold air below cannot rise through the warm air above, and it traps smoke, dust and fog near the ground; the winter smog of Delhi and Kolkata and the fog that closes airports and delays trains in the Ganga plain in December and January are held down by inversions. Inversions are treated again in the section on heat and temperature.

Above the tropopause lies the stratosphere, the second layer of the atmosphere, extending from the tropopause at about 12 km (8 to 18 km) to a height of about 50 km. The name comes from the Latin stratum, a layer, because the air in it lies in horizontal layers without the vertical turning of the troposphere.

Temperature. In the lower stratosphere the temperature stays constant with height, at about −55 to −60 °C, up to about 20 km, and this steady zone is called the isothermal layer. Above 20 km the temperature rises with height, slowly at first and then rapidly, reaching about 0 °C at the top of the layer. This increase of temperature with height, the opposite of the troposphere, is caused by the ozone layer: the ozone concentrated between 15 and 35 km absorbs the sun's ultraviolet radiation and is heated by it, and the heating is greatest near the top where the radiation is strongest. The stratosphere is thus heated from above, while the troposphere is heated from below.

Composition. The stratosphere contains about 19 per cent of the mass of the atmosphere, so that the troposphere and stratosphere together hold about 99 per cent. It is almost entirely free of water vapour, cloud and dust, apart from the very thin and rare nacreous or mother-of-pearl clouds of ice crystals at 20 to 30 km over the polar regions, and the sulphate dust thrown up by great volcanic eruptions, which may stay in the stratosphere for two or three years and cool the earth. Its most important constituent is ozone, described in the next topic.

Calmness. Because the temperature does not fall with height, there is no convection; the air is stable and stratified, winds are horizontal and steady, and there are no storms, clouds or turbulence. This is why long-distance jet aircraft cruise in the lower stratosphere at 10 to 13 km: the flight is smooth, the air resistance is low, and the aircraft is above the weather. Weather balloons rise to 30 km, and the highest manned balloon flights and some military aircraft have reached the middle stratosphere. The stratosphere is the transition between the weather zone below and the near-space above.

The stratopause. The upper boundary of the stratosphere, at about 50 km, where the rise of temperature stops, is the stratopause. Its temperature is about 0 °C, the warmest level between the surface and the thermosphere, and it marks the top of the ozone heating.

For the examination the stratosphere should be described under height (12 to 50 km), temperature (constant then rising to 0 °C because of ozone), composition (dry, dust-free, ozone layer), calm and stratified air suitable for jet flight, and the stratopause.

Within the stratosphere lies the most important shield of life on earth, the ozone layer or ozonosphere. Ozone (O₃) is a form of oxygen in which three atoms are bonded together instead of two. It is a pale blue gas with a sharp smell, and near the ground it is a pollutant, but in the stratosphere it is a protector.

Formation. When the sun's short-wave ultraviolet radiation strikes ordinary oxygen molecules (O₂) high in the stratosphere, it splits some of them into single oxygen atoms; each free atom then joins another oxygen molecule to form ozone (O + O₂ → O₃). Ozone is in turn broken down by ultraviolet light back into O₂ and O, and the balance between formation and destruction keeps a steady concentration. The layer lies between about 15 and 35 km, with its greatest density at about 20 to 25 km. Even there ozone is very rare, a few molecules in every million; if all the ozone of the atmosphere were brought down to sea level pressure it would form a layer only about 3 mm thick.

Importance. This thin layer absorbs about 97 to 99 per cent of the sun's biologically harmful ultraviolet radiation, especially the UV-B band. If this radiation reached the surface unchecked it would cause skin cancer, cataracts and blindness, suppress the immune system, damage the genetic material of plants and animals, reduce crop yields and kill the plankton that is the foundation of life in the sea. Life could not leave the oceans until the ozone layer had formed some 600 million years ago. The absorption of ultraviolet radiation by ozone is also the reason why the temperature of the stratosphere rises with height.

Depletion. From the 1970s scientists found that the ozone layer was thinning, and in 1985 British scientists reported a large seasonal ozone hole over Antarctica, where up to two-thirds of the ozone disappears each spring (September to November). The cause is a group of man-made chemicals, chiefly chlorofluorocarbons (CFCs), used since the 1930s as refrigerants in fridges and air conditioners, as propellants in aerosol sprays, as solvents and in foam plastics, together with halons from fire extinguishers and some other chlorine and bromine compounds. These gases are so stable that they drift unchanged up into the stratosphere, where ultraviolet light breaks them apart and releases chlorine atoms. A single chlorine atom acts as a catalyst and can destroy up to 100,000 molecules of ozone before it is removed. The destruction is worst over Antarctica because the very cold polar stratospheric clouds of the southern winter provide surfaces on which the chlorine reactions are speeded up, and the spring sunlight then sets them going. Supersonic aircraft, nuclear explosions and nitrogen fertilisers contribute smaller amounts of ozone-destroying oxides of nitrogen.

Effects and remedies. Ozone depletion has increased ultraviolet exposure in the southern hemisphere, raising the incidence of skin cancer in Australia, Chile and New Zealand, damaging Antarctic plankton, and affecting crops. In 1987 the nations of the world signed the Montreal Protocol, which phased out the production of CFCs and halons; India signed in 1992. Because CFCs last for decades, recovery is slow, but the ozone hole has stopped growing and is expected to close by about 2060. The Montreal Protocol is regarded as the most successful international environmental agreement, and 16 September is observed as World Ozone Day.

Above the stratopause lies the third thermal layer, the mesosphere, extending from about 50 km to about 80 km. The name comes from the Greek mesos, middle, because it lies in the middle of the five layers.

Temperature. In the mesosphere the temperature falls with height once again, from about 0 °C at the stratopause to about −90 °C or lower at the top. The reason is that the mesosphere has no ozone to absorb ultraviolet radiation and be heated by it, and the air is far too thin to absorb the heat radiated from below; it loses heat by radiating it to space and cools. The top of the mesosphere, the mesopause at about 80 km, is the coldest level of the entire atmosphere, with temperatures of −90 to −100 °C, colder than anything on the surface of the earth, even in the Antarctic winter. The temperature curve of the atmosphere thus has a peak at the stratopause and a trough at the mesopause.

Composition and density. The air of the mesosphere is extremely thin; the pressure at 80 km is about a hundred-thousandth of that at sea level, and the layer contains only about 0.1 per cent of the mass of the atmosphere. It is still part of the homosphere, so the gases are mixed in the usual proportions. Strong winds and the largest atmospheric tides occur here.

Meteors. The mesosphere is the layer in which most meteors burn up. Meteoroids, the fragments of rock and metal that the earth sweeps up in its orbit, enter the atmosphere at 10 to 70 km per second. In the exosphere and thermosphere the air is too thin to affect them, but at 80 to 100 km it becomes dense enough for friction and compression to heat them white-hot, and they vaporise as the streaks of light we call shooting stars, typically between 75 and 100 km high. The mesosphere thus protects the surface from the bombardment that pits the moon. The dust left by burnt meteors settles slowly and provides some of the nuclei for the rare noctilucent clouds, the highest clouds on earth, which form of ice crystals at about 80 to 85 km near the mesopause and shine silvery blue after sunset in high-latitude summers.

Exploration. The mesosphere is the least explored layer. It is too high for aircraft and balloons, which cannot fly above about 40 km, and too low for satellites, which would be dragged down by the air below about 160 km; only sounding rockets pass through it briefly. It is sometimes called the ignorosphere for this reason.

The mesopause, the upper boundary at about 80 km, also marks approximately the top of the homosphere; above it the gases begin to separate by weight and the temperature begins to rise steeply into the thermosphere.

Above the mesopause lies the thermosphere, the fourth layer, extending from about 80 km to about 500 to 700 km (some texts take it to about 480 km). The name comes from the Greek thermos, heat, because its temperature rises to very high values.

Temperature. In the thermosphere the temperature rises rapidly with height, from about −90 °C at the mesopause to about 1,000 °C at 200 km and to 1,500 °C or more in the upper part, the exact value depending on the activity of the sun; during solar storms it may exceed 2,000 °C. The cause is the absorption by the thin gas of the sun's most energetic radiation, the extreme ultraviolet and X-rays, which strip electrons from the atoms. It is important to understand, however, that this temperature would not feel hot: the air is so extremely thin, a million-millionth of sea-level density, that the few molecules present, though moving very fast, carry almost no heat to a body placed among them. An astronaut or a satellite in the thermosphere is not roasted; it is warmed by direct sunshine and cooled in shadow.

The ionosphere. The most important feature of the thermosphere is that the ultraviolet and X-radiation of the sun knocks electrons out of the gas atoms and molecules, leaving them as electrically charged ions. The region of charged particles, extending from about 80 km up to about 400 km and overlapping the upper mesosphere and the thermosphere, is called the ionosphere. It is arranged in several layers named D (about 60 to 90 km), E or the Kennelly–Heaviside layer (about 90 to 150 km) and F or the Appleton layer (about 150 to 400 km), which vary between day and night. These electrified layers reflect radio waves back to the earth. Medium- and short-wave radio signals that would otherwise travel straight out into space are bounced back by the ionosphere and can be received thousands of kilometres away, on the other side of the earth; this is the basis of long-distance radio broadcasting and of the radio communication of ships and aircraft. The D layer, which absorbs radio waves, disappears at night, which is why distant medium-wave stations come in clearly after dark. Solar flares disturb the ionosphere and cause radio blackouts.

The aurora. Near the magnetic poles, charged particles streaming from the sun are guided down by the earth's magnetic field into the thermosphere, where they strike the oxygen and nitrogen atoms at 100 to 300 km and make them glow green, red and violet. These are the aurora borealis (northern lights) of the Arctic and the aurora australis (southern lights) of the Antarctic, seen as shimmering curtains in the sky of Norway, Alaska, Canada and Antarctica.

Satellites. The thermosphere is the region of low earth orbit. The International Space Station orbits at about 400 km and most earth-observation and weather satellites between 300 and 800 km, so they fly within the thermosphere, where the faint air drag slowly lowers their orbits and requires periodic boosting. The thermopause, the upper boundary at 500 to 700 km, marks the level above which temperature no longer rises and the exosphere begins.

The fifth and outermost layer is the exosphere, beginning at the thermopause, about 500 to 700 km up, and extending outward to roughly 10,000 km, where it merges imperceptibly with interplanetary space. The name comes from the Greek exo, outside, since it is the exit of the atmosphere.

Composition and density. The exosphere is made of the lightest gases, chiefly hydrogen and helium, with some atomic oxygen in its lower part; it corresponds to the upper shells of the heterosphere. The gas is unimaginably thin, thinner than the best vacuum that can be made in a laboratory; individual atoms are kilometres apart and almost never collide. Instead of behaving like a gas, they move in long ballistic curves under gravity, like tiny projectiles, and the fastest of them, especially the light hydrogen atoms, reach escape velocity and leave the earth for ever. The exosphere is thus the layer from which the atmosphere slowly leaks into space; the earth loses about three kilograms of hydrogen every second, a loss made good by the breakdown of water vapour below.

Temperature. The temperature in the exosphere is very high, about 1,000 to 1,500 °C or more, in the sense that the atoms move at great speed, but as in the thermosphere the gas is too thin for this to have any meaning as heat; the temperature no longer rises with height, and the concept of temperature itself gradually loses its meaning. The exosphere is the region of the outer satellites: navigation satellites at about 20,000 km and geostationary communication and weather satellites at 36,000 km lie beyond even its conventional limit, in space.

The magnetosphere. Beyond and enclosing the exosphere lies the magnetosphere, which is not a layer of gas but the region in which the earth's magnetic field controls the movement of charged particles. It extends about 60,000 km towards the sun and stretches out into a long tail on the night side. It is not part of the atmosphere, but it is often mentioned with it because it performs a protective function: it deflects the solar wind, the stream of charged particles blowing from the sun, and traps some of them in two doughnut-shaped zones around the earth called the Van Allen radiation belts, discovered by the first American satellites in 1958. Without the magnetosphere the solar wind would strip away the upper atmosphere, as it has done on Mars, and would bathe the surface in harmful radiation. The particles that leak from the magnetosphere down the magnetic field lines into the polar thermosphere cause the aurora.

With the exosphere the survey of the layers is complete, and it may be summed up in the order from below: troposphere (weather), stratosphere (ozone, jets), mesosphere (coldest, meteors), thermosphere (ionosphere, aurora, satellites), exosphere (hydrogen and helium, escape to space), with the magnetosphere as the outer shield.

The essential facts of the five thermal layers should be committed to memory in tabular form, because the examination asks for them both in objective items and in the labelled diagram.

Several patterns in the table help in remembering it. The temperature curve zig-zags: down in the troposphere, up in the stratosphere, down in the mesosphere, up in the thermosphere. The reasons alternate too: the troposphere is heated from below by the ground; the stratosphere from above by ozone absorbing ultraviolet; the mesosphere has no heat source; the thermosphere is heated by extreme ultraviolet and X-rays. The two warm levels are the surface and the stratopause (0 °C); the two cold levels are the tropopause (−60 to −80 °C) and the mesopause (−90 to −100 °C), the latter being the coldest point in the whole atmosphere.

The mass is concentrated at the bottom: the troposphere holds 75 per cent, the stratosphere 19 per cent, and the three upper layers together only about 1 per cent. Everything that concerns weather and life is in the lowest 12 km; everything that concerns ultraviolet protection is in the next 40 km; and everything that concerns radio, aurora and satellites is above 80 km.

The pauses can be remembered as the four boundaries, each named after the layer beneath it: tropopause (about 12 km), stratopause (about 50 km), mesopause (about 80 km), thermopause (about 500–700 km). A common examination trick is to ask which layer lies between two named pauses, or which pause is the coldest (the mesopause) or the warmest (the stratopause, among those below the thermosphere).

Finally, the compositional division overlaps the thermal one: the homosphere (0–90 km) covers the troposphere, stratosphere and mesosphere; the heterosphere (above 90 km) covers the thermosphere and exosphere; and the ionosphere (80–400 km) straddles the upper mesosphere and the thermosphere.

Two questions of understanding are often asked about the structure of the atmosphere, and they deserve separate treatment.

Why is the troposphere thicker at the equator than at the poles? The troposphere is the layer in which air is turned over by convection, and its thickness depends on the vigour of that convection. At the equator the sun is overhead throughout the year, the surface is strongly heated, and the warm air rises in powerful currents to great heights, carrying the top of the mixed layer up to 16 to 18 km. At the poles the sun is low, the surface is cold, the dense air hardly rises at all, and the mixed layer is shallow, only about 8 km. In the same way, the troposphere is thicker in summer than in winter at any given place, and the tropopause dips downward from the equator to the poles in steps, with breaks in the middle latitudes where the jet streams blow. A further effect is that the tropopause is coldest where it is highest: the air rising over the equator cools by the lapse rate through 17 km to about −80 °C, while over the poles it cools through only 8 km to about −60 °C, so the equatorial tropopause, paradoxically, is colder than the polar one although the equatorial surface is far warmer.

Why does the study of the layers matter? Each layer has a direct bearing on human life. The troposphere gives us the air we breathe, the rain that grows our food and every kind of weather, good and bad; the whole science of weather forecasting is the study of this one layer, and all pollution we emit accumulates in it. The stratosphere holds the ozone shield without which we could not walk in the sun, and its calm air is the highway of world aviation; the volcanic dust it retains can change the climate for years. The mesosphere destroys the meteors that would otherwise strike the earth. The thermosphere, through its ionosphere, makes long-distance radio possible and is the orbit of the satellites that give us weather pictures, television, telephone links and navigation; its aurora is the visible sign of the sun's influence on the earth. The exosphere and the magnetosphere beyond it are the outer defences against the solar wind and cosmic radiation.

The layers are also the framework for the rest of this unit. The section on heat and temperature explains why the troposphere is heated from below and what happens to the sun's energy in it; the section on pressure and winds describes the circulation of the troposphere and the jet streams at the tropopause; and the discussion of global warming and ozone depletion concerns the troposphere and stratosphere respectively. A student who can draw the layers with their heights and temperature trends from memory has the map on which all these later topics are placed.

In West Bengal the practical face of the layers is seen every year: the nor'wester thunderclouds of April and May climbing to the tropopause at 16 km, the winter fog and smog trapped by inversions in the lowest few hundred metres of the troposphere over Kolkata, the satellite pictures of Bay of Bengal cyclones taken from the thermosphere and relayed through it, and the short-wave radio signals bouncing off the ionosphere.

The layers of the atmosphere are among the most predictable topics in the Madhyamik geography paper, and preparation should be exact.

One-mark questions ask for facts: the layer nearest the earth (troposphere); the layer of all weather (troposphere); the layer containing ozone (stratosphere); the height of the ozone layer (15–35 km); the coldest layer or pause (mesosphere, mesopause); the layer where meteors burn (mesosphere); the layer that reflects radio waves (ionosphere, in the thermosphere); the layer of the aurora (thermosphere); the outermost layer (exosphere); the gases of the exosphere (hydrogen and helium); the boundary between troposphere and stratosphere (tropopause); the normal lapse rate (6.5 °C per km); the height of the troposphere at the equator (16–18 km) and the poles (8 km); the upper limit of the homosphere (90 km); the layer suitable for jet flight (stratosphere); the chemicals that deplete ozone (CFCs); the year of the Montreal Protocol (1987). True-or-false items test statements such as 'temperature rises with height in the stratosphere' (true) and 'the tropopause is the coldest level of the atmosphere' (false; the mesopause is).

Two-mark questions ask for definitions and short reasons: What is the tropopause? What is the normal lapse rate? What is the ionosphere? Why is the stratosphere suitable for aircraft? Why is the troposphere called the weather sphere? What is the ozone hole? Why is the mesopause the coldest level?

Three-mark questions ask for explanations: Why does temperature rise with height in the stratosphere? Why is the troposphere thicker over the equator? Explain the importance of the ozone layer. Explain the importance of the ionosphere. Distinguish between homosphere and heterosphere. Describe the causes of ozone depletion.

Five-mark questions ask for the full structure: Describe with a labelled diagram the different layers of the atmosphere. Describe the characteristics of the troposphere and the stratosphere. Explain the vertical variation of temperature in the atmosphere with a diagram. The answer should treat each layer in order under a sub-heading with its height, temperature trend and two or three characteristics, and the diagram must show the five layers, the four pauses, the heights and the temperature curve.

Guidance for the diagram. Draw a vertical column and mark 0, 12, 50, 80 and 500 km with the layer names between them and the pause names at the lines; shade the ozone layer at 15–35 km; draw the temperature curve beside the column and write the temperatures at the surface (15 °C), tropopause (−60 °C), stratopause (0 °C), mesopause (−90 °C) and thermosphere (over 1,000 °C); add small symbols, a cloud in the troposphere, an aircraft in the stratosphere, a meteor in the mesosphere, a radio wave and an aurora in the thermosphere, a satellite in the exosphere. Label neatly; the diagram carries as many marks as the text. Avoid the common mistakes of placing ozone in the troposphere, writing that temperature falls throughout, calling the tropopause the coldest level, confusing the ionosphere (a region of charged particles) with a separate thermal layer, and giving the height of the troposphere as a single fixed figure without mentioning its variation from equator to poles.