UPSC Prelims 2024 Q1: Heating of the Atmosphere — Explained
Consider the following statements:
Statement-I:
The atmosphere is heated more by incoming solar radiation than by terrestrial radiation.
Statement-II:
Carbon dioxide and other greenhouse gases in the atmosphere are good absorbers of long wave radiation.
Which one of the following is correct in respect of the above statements?
- Both Statement-I and Statement-II are correct and Statement-II explains Statement-I
- Both Statement-I and Statement-II are correct, but Statement-II does not explain Statement-I
- Statement-I is correct, but Statement-II is incorrect
- Statement-I is incorrect, but Statement-II is correct
Answer: (d) Statement-I is incorrect, but Statement-II is correct
Statement-wise explanation
- Statement-I: incorrect — The atmosphere is mostly transparent to incoming short-wave solar radiation, so sunlight passes through it and heats the Earth's surface first. The warmed surface then gives off long-wave (terrestrial) radiation, and this heats the air from below. The NCERT chapter 'Solar Radiation, Heat Balance and Temperature' says the atmosphere is 'indirectly heated by the earth's radiation'. In NCERT's heat budget, the atmosphere takes in only 14 of every 100 units of insolation directly, but 34 units from the Earth.
- Statement-II: correct — Carbon dioxide, water vapour, methane and other greenhouse gases let incoming short-wave sunlight through but absorb the long-wave infrared radiation given off by the Earth's surface. NCERT describes carbon dioxide as transparent to incoming solar radiation but opaque to outgoing terrestrial radiation. Statement-I is false, so Statement-II cannot explain it. In fact, Statement-II is the reason the opposite of Statement-I is true: the air is heated mainly by terrestrial radiation.
How to eliminate
Statement-II is the textbook definition of the greenhouse effect. Most candidates can mark it correct, which leaves three choices: both statements correct with II explaining I, both correct without II explaining I, or I incorrect and II correct. Next, test Statement-I against the normal lapse rate. In the troposphere, temperature falls by about 6.5°C for every 1,000 m of height. If sunlight heated the air directly, the upper air, which meets the sunlight first, would be warmer. It is colder because the heat comes from the ground. A second clue lies inside the question. If greenhouse gases are good absorbers of long-wave radiation, and the long-wave radiation in the atmosphere comes from the Earth, then Statement-II points against Statement-I, not toward it. Both statements together suggest the answer. The fact to remember is that the atmosphere is heated from below, mainly by terrestrial (long-wave) radiation, not directly by insolation.
Know more
How the atmosphere gets its heat
The Sun sends out energy mostly as short waves. The atmosphere absorbs only a small part of these. Most of the energy passes through and warms the land and oceans. The warm surface then gives off energy as long waves (infrared), and greenhouse gases absorb these well. So the air is heated from below. This is why temperature in the troposphere normally falls with height, at about 6.5°C per 1,000 m (the normal lapse rate).
The surface passes heat to the air in four ways:
- Terrestrial radiation: long-wave radiation absorbed by CO₂, water vapour and other greenhouse gases
- Conduction: the layer of air touching the warm ground is heated directly
- Convection: heated air rises and carries heat upward (vertical transfer)
- Latent heat: heat used to evaporate water is released when the vapour condenses
(Advection is the horizontal movement of heat by wind. It shifts heat from place to place but does not heat the atmosphere as a whole.)
NCERT heat budget (100 units of insolation)
| Flow | Units |
|---|---|
| Reflected to space (albedo) | 35 |
| Absorbed directly by the atmosphere | 14 |
| Absorbed by the Earth's surface | 51 |
| Surface energy sent straight to space | 17 |
| Surface energy absorbed by the atmosphere | 34 (6 radiation, 9 convection and turbulence, 19 latent heat) |
| Radiated to space by the atmosphere | 48 (14 + 34) |
The atmosphere gets 34 units from the Earth and only 14 directly from the Sun. NASA's energy budget shows the same pattern: the atmosphere absorbs about 23% of incoming sunlight directly, while the surface absorbs about 48%. NASA sums it up this way: 'Most solar heating happens at the surface, while most radiative cooling happens in the atmosphere.'
Why short waves pass and long waves are trapped: gases such as CO₂ and water vapour absorb strongly at infrared wavelengths and weakly at visible wavelengths. Adding more of them traps more outgoing heat. That is the basis of the enhanced greenhouse effect.
NCERT Concepts to Revise
- Terrestrial radiation: Statement-I sets this long-wave Earth radiation against incoming sunlight as the source of the atmosphere's heat.
- Greenhouse effect: Statement-II describes how greenhouse gases treat long-wave radiation, which is the core of the greenhouse effect.
- Insolation: Statement-I is about incoming solar radiation, so it helps to know what insolation is and how it reaches the ground.
- Heat budget of the Earth: The heat budget gives the share of energy the atmosphere gets directly from the Sun and from the Earth's surface.