MPSC 2022 Prelims Paper-I · Question 52 of 100

Correct — option (4), the troposphere. The atmosphere is divided into layers according to the way temperature changes with height, and the troposphere is the lowest of them, extending from the surface of the earth to a ceiling called the tropopause which lies at roughly eighteen kilometres above the equator and about eight kilometres above the poles, the difference arising because rising air at the equator pushes the boundary further out. The reason this thin lowest layer holds the great bulk of the atmosphere's air, the figure the stem gives being about seventy per cent, is that air is compressible and is pulled down by gravity, so it packs most densely at the bottom. Pressure and density therefore fall very rapidly with height rather than evenly: the atmosphere has no sharp outer edge, but it thins so quickly that most of its mass is crowded into the first ten or fifteen kilometres, which is a small fraction of its total vertical extent. The troposphere is also where almost everything that matters to human life takes place. It contains nearly all the water vapour and nearly all the dust and other suspended particles, and because water vapour is present here and almost nowhere else, all clouds, rain, snow, fog, thunderstorms and cyclones occur within this layer. Temperature in the troposphere falls with height at an average rate of about six and a half degrees Celsius per kilometre, the normal lapse rate, which is why mountain tops are cold and why aircraft cruising above the weather report very low outside temperatures. The layer is also turbulent, being heated from below by the earth's surface rather than from above, so warm air rises and cold air sinks through it constantly — the very convection that produces weather. Its name reflects this: tropos means turning or mixing. Above the tropopause the pattern reverses, temperature beginning to rise again with height through the stratosphere because ozone there absorbs ultraviolet radiation, and that inversion acts as a lid which prevents the turbulent tropospheric air from mixing upwards, so the water vapour, the weather and the great majority of the air are all held below it. Option (4) is the answer.

• (1)Stratosphere — The stratosphere is the layer immediately above the troposphere, extending from the tropopause to about fifty kilometres, and although it is much thicker than the troposphere it contains only a small fraction of the atmosphere's air, because the air within it is already very thin. Its importance lies elsewhere. It contains the ozone layer, concentrated roughly between fifteen and thirty-five kilometres, which absorbs most of the sun's ultraviolet radiation and shields life at the surface from it; that absorption is why temperature rises with height in the stratosphere instead of falling. Because the temperature profile is inverted there is almost no vertical mixing, so the stratosphere is free of cloud and of turbulence, which is why long-distance jet aircraft fly in its lower part. A candidate who chooses this option has probably confused the layer that matters most for protection from radiation with the layer that holds most of the air.
• (2)Mesosphere — The mesosphere lies above the stratosphere, from about fifty kilometres to about eighty kilometres, and the air in it is exceedingly thin — a minute fraction of the atmosphere's total mass, nothing remotely approaching seventy per cent. It is distinguished by two things worth remembering for their own sake. Temperature falls with height throughout it and reaches the lowest values found anywhere in the atmosphere at its upper boundary, the mesopause. And it is the layer in which meteors entering the atmosphere are heated by friction until they burn up, producing the streaks of light called shooting stars, so the mesosphere is in effect the earth's shield against small incoming debris. Neither fact has anything to do with the quantity of air the layer holds, and choosing it here means recalling a layer by its most memorable phenomenon rather than by its share of the atmosphere's mass.
• (3)Thermosphere — The thermosphere lies above the mesopause and extends outward for hundreds of kilometres, so it is by far the thickest of the layers named in this question, and that thickness is exactly why it can mislead. It contains almost no air at all. The molecules within it are so widely separated that the layer is nearly a vacuum by ordinary standards, and although temperatures in it are recorded as extremely high, the figure refers to the kinetic energy of individual particles and not to any quantity of heat that could be felt, because there are too few particles to carry it. The thermosphere's significance is that it contains the ionised layers, collectively called the ionosphere, which reflect radio waves back to the earth and make long-distance radio communication possible, and that the aurora occurs within it. A layer that is thick but almost empty cannot hold the bulk of the atmosphere's mass, which is concentrated where the air is densest, at the bottom.

The atmosphere is layered according to how temperature behaves with increasing height, and the five layers in ascending order are the troposphere, the stratosphere, the mesosphere, the thermosphere and the exosphere, separated by boundaries named the tropopause, the stratopause and the mesopause. In the troposphere temperature falls with height at an average of about six and a half degrees per kilometre; the layer reaches roughly eighteen kilometres over the equator and eight over the poles, holds nearly all the water vapour and dust, and is the seat of all weather. In the stratosphere temperature rises with height because ozone absorbs ultraviolet radiation, which makes the layer stable and cloud-free and suitable for jet flight, and the ozone layer within it is the shield that made life on land possible. In the mesosphere temperature falls again to the coldest values in the whole atmosphere at the mesopause, and meteors burn up there. In the thermosphere temperature rises steeply with height while the air becomes vanishingly thin; the ionised layers within it reflect radio waves, and the aurora is seen there. The exosphere is the outermost region, merging gradually into space. Composition is a separate question from layering: dry air is about seventy-eight per cent nitrogen and about twenty-one per cent oxygen by volume, with argon and carbon dioxide making up most of the remainder, and this composition is roughly uniform through the lower layers while water vapour, which is highly variable, is confined almost entirely to the troposphere.

Layers of the atmosphere are among the most frequently examined topics in the geography portion of MPSC papers, and the Commission asks them from several directions: the order of the layers, the height at which one ends and the next begins, the temperature behaviour within each, the phenomenon associated with each, and questions of the kind asked here about where the mass or the water vapour is concentrated. All of these come from a single diagram, and the most efficient preparation is to draw that diagram — five layers, four boundaries, the approximate heights, the direction in which temperature moves in each and one distinguishing feature apiece — until it can be reproduced from memory. The specific idea this question tests is that the atmosphere is not evenly distributed through its vertical extent: the layers above the troposphere are far thicker but almost empty, because gravity compresses the air towards the surface and pressure falls away rapidly with height. Once that is understood, the question answers itself without any recall of percentages, which is useful because different sources quote somewhat different figures for the share of the atmosphere's mass held in the troposphere.

• The troposphere is the lowest layer of the atmosphere, extending to about eighteen kilometres over the equator and about eight kilometres over the poles, and it holds the great bulk of the atmosphere's air because air is compressed towards the surface by gravity.
• Nearly all the water vapour and dust of the atmosphere is confined to the troposphere, which is why all clouds, rainfall, storms and other weather phenomena occur within this layer.
• Temperature in the troposphere falls with height at an average rate of about six and a half degrees Celsius per kilometre, known as the normal lapse rate, and the layer's upper boundary is the tropopause.
• In the stratosphere, which extends to about fifty kilometres, temperature rises with height because ozone absorbs ultraviolet radiation; the layer is stable and cloud-free, which is why jet aircraft fly in its lower part.
• The mesosphere contains the coldest temperatures in the atmosphere at its upper boundary and is where meteors burn up, while the thermosphere contains the ionised layers that reflect radio waves and is where the aurora occurs.

• Assuming that a thicker layer must hold more air, when the upper layers are vastly thicker than the troposphere and yet nearly empty
• Confusing the layer that shields the earth from ultraviolet radiation, the stratosphere, with the layer that holds the atmosphere's air and its weather
• Treating the very high temperatures recorded in the thermosphere as a measure of heat, when they describe the energy of individual particles in an almost airless region
• Learning the layers by name and order alone, without the temperature behaviour of each, which is what most questions on the topic actually turn on

The atmosphere is a standing item in MPSC geography, asked as the sequence and heights of the layers, as which layer contains the ozone or the ionosphere, as where meteors burn up or where aircraft fly, and as the value of the normal lapse rate. The Commission also connects the topic to environment questions on ozone depletion and to current affairs on climate, so the layers should be learnt together with what happens in each rather than as a list of names. A related family of questions concerns the composition of air by volume and the special position of water vapour, carbon dioxide and ozone as variable constituents, and it is worth being able to state the nitrogen and oxygen proportions exactly. Since the whole topic is carried by one labelled diagram, drawing that diagram is the single most productive thing a candidate can do with it.

No directly related past PYQ was found.

• practice — not a real PYQ

  In which layer of the atmosphere does the temperature increase with increasing height because of the absorption of ultraviolet radiation ?

  • (a)Troposphere
  • (b)Stratosphere
  • (c)Mesosphere
  • (d)Exosphere

  Answer(b) Stratosphere — the ozone concentrated in this layer absorbs most of the sun's ultraviolet radiation, and the energy absorbed warms the surrounding air, so temperature rises rather than falls as one goes higher. That inverted temperature profile makes the layer stable and free of vertical mixing, which is why it has no clouds and no weather and why jet aircraft cruise in its lower part. In the troposphere and the mesosphere temperature falls with height instead.

• practice — not a real PYQ

  All weather phenomena such as clouds, rainfall and thunderstorms are confined to the troposphere chiefly because

  • (a)it is the layer closest to the sun
  • (b)almost all the water vapour of the atmosphere is present in it
  • (c)it contains the ozone layer
  • (d)the air within it is electrically charged

  Answer(b) Almost all the water vapour of the atmosphere is present in it — weather is essentially the behaviour of water in the air, so the layer holding the water vapour is the layer in which clouds form and precipitation occurs. The troposphere is also heated from below by the earth's surface, which drives the convection currents that lift moist air and produce cloud, and it is this combination of available moisture and vertical mixing that confines weather to the lowest layer. The ozone layer belongs to the stratosphere, and the electrically charged layers to the thermosphere.