NDA (II) 2018 General Ability Test · Question 76 of 146

Correct — D, temperature falls with height in the troposphere. The atmosphere is warmed from below, not from above: sunlight passes largely unabsorbed through the air and heats the ground, and the warmed ground then radiates long-wave infrared that the air absorbs. So the air closest to the surface is the warmest, and every kilometre you climb inside the troposphere takes you further from the heat source. The rate of that fall is the normal or environmental lapse rate, and textbooks put it at roughly 6·5 °C per kilometre — the ICAO standard atmosphere fixes it at exactly 6·50 °C/km from sea level to 11 km, and NCERT states the same thing as about 1 °C for every 165 m. The booklet prints 6·4 °C, a shade under the usual figure, but this is still the only option that gets the right direction in the right layer, and that is what the item turns on.

• (a)Temperature decreases with height in the stratosphere. — The stratosphere does the opposite — it warms with height, from about −51 °C near the tropopause to about −15 °C at its top. The reason is the ozone layer, which absorbs the Sun's ultraviolet radiation directly and so heats the stratosphere from above rather than from below.
• (b)Temperature is constant at different heights in the stratosphere. — True only of the lowest slice of it. In the standard atmosphere the temperature holds steady at about −56·5 °C from 11 km to 20 km, which is the isothermal layer at the base of the stratosphere — but above 20 km ozone heating takes over and the temperature climbs steadily, so the claim fails for the layer as a whole.
• (c)Temperature increases with height in the troposphere at an average rate of 6·5 °C per kilometre. — This is the trap option. It carries the textbook number, 6·5 °C per kilometre, but attaches it to the wrong direction. In the troposphere the temperature decreases at that rate; if it increased, the coldest air would sit at the surface and there would be no convection, no clouds and no weather.

The atmosphere is layered by how its temperature behaves with height, and the sign of that change flips at every boundary. In the troposphere, the bottom 8–18 km that holds almost all the water vapour and all the weather, temperature falls with height because the layer is heated from below by long-wave radiation from the ground. Above the tropopause the stratosphere reverses this: ozone there absorbs incoming ultraviolet directly, so the higher you go the warmer it gets, which is why the stratosphere is stable and free of convective cloud. The mesosphere above it cools again, and the thermosphere warms again.

Only two facts are needed and both are about direction, not about decimals. Troposphere down, stratosphere up. That single pair kills options (a) and (c) outright and leaves (b), which sounds plausible because the lowest part of the stratosphere really is isothermal — but the question asks about the stratosphere, not about its base. One honest wrinkle worth naming: the surviving option prints the lapse rate as 6·4 °C per kilometre, while the standard value quoted almost everywhere, including the ICAO standard atmosphere, is 6·5 °C per kilometre. The booklet's figure is as printed and the key stands, because no other option is even directionally right; treat the number here as approximate and remember 6·5 for your own use.

• The ICAO International Standard Atmosphere sets the tropospheric lapse rate at 6·50 °C per kilometre from sea level to 11 km.
• The atmosphere absorbs only about a quarter of incoming shortwave sunlight but roughly ninety per cent of the long-wave infrared radiated by the ground — which is why the troposphere is heated from below.
• In the standard atmosphere the temperature stays at about −56·5 °C between 11 km and 20 km, the isothermal base of the stratosphere, and rises above that.
• Stratospheric warming with height is caused by ozone absorbing solar ultraviolet radiation, the same absorption that shields the surface.

• Assuming that being nearer the Sun means being warmer — the troposphere is heated by the ground beneath it, not by the Sun above it.
• Reading the isothermal 11–20 km slice as though it described the whole stratosphere.
• Chasing the decimal. A question that offers 6·4 and 6·5 in options pointing opposite ways is testing direction, not arithmetic.

NDA asks either for the layer in which a stated temperature behaviour occurs, or for the value of the normal lapse rate, or for the correct bottom-to-top order of the layers.

• practice — not a real PYQ

  In which layer of the atmosphere does temperature rise with increasing altitude because of the absorption of ultraviolet radiation?

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

  Answer(b) Stratosphere — its ozone absorbs solar ultraviolet directly, so the layer is heated from above and warms with height.

• practice — not a real PYQ

  If the surface temperature at a station is 30 °C, the approximate air temperature 4 km above it, using the normal lapse rate, would be about

  • (a)30 °C
  • (b)16 °C
  • (c)4 °C
  • (d)−10 °C

  Answer(c) 4 °C — a fall of about 6·5 °C per kilometre over 4 km is roughly 26 °C, leaving about 4 °C.