The ________ part of the Earth's atmosphere contains about 70% of the total air in the atmosphere.

Question

UnlockIAS · Accepted Answer

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.

Why the other options are wrong — (a) 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.  (b) 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.  (c) 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.

Concept: 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.

State PCS 2022 Q52 — Geography PYQ with Answer

State PCS 2022 Q52 — Geography PYQ with Answer