UPSC Prelims 2023 Q62: Insolation and Infrared Radiation — Explained
With reference to the Earth's atmosphere, which one of the following statements is correct?
- The total amount of insolation received at the equator is roughly about 10 times of that received at the poles.
- Infrared rays constitute roughly two-thirds of insolation.
- Infrared waves are largely absorbed by water vapour that is concentrated in the lower atmosphere.
- Infrared waves are a part of visible spectrum of electromagnetic waves of solar radiation.
Answer: (c) Infrared waves are largely absorbed by water vapour that is concentrated in the lower atmosphere.
Statement-wise explanation
- a: incorrect — This statement is wrong. The NCERT Class XI chapter 'Solar Radiation, Heat Balance and Temperature' says insolation at the surface ranges from about 320 W/m² in the tropics to about 70 W/m² at the poles. That is roughly 4–5 times, not 10 times. NASA's Earth Observatory gives an even smaller gap: the Arctic and Antarctic Circles get about 40% of the energy the equator gets.
- b: incorrect — This statement is wrong. Roughly half of solar radiation is infrared (about 49%, at wavelengths of 700 nm to 1 mm). About 43% is visible light and about 7% is ultraviolet. 'Two-thirds' overstates the infrared share.
- c: correct — This is the correct statement. Water vapour is the strongest gaseous absorber of infrared radiation. Almost all of it sits in the troposphere, because it comes from evaporation at the surface and thins out quickly with height. NCERT notes that in the troposphere, water vapour, ozone and other gases absorb much of the near-infrared part of incoming sunlight. The same water vapour also absorbs the long-wave (infrared) terrestrial radiation that the Earth gives off, and this is why the atmosphere is heated mainly from below.
- d: incorrect — This statement is wrong. Visible light covers only about 400–700 nm. Infrared lies beyond the red end, at wavelengths longer than 700 nm. It is invisible to the human eye and is a separate band of the electromagnetic spectrum.
How to eliminate
Two statements can be ruled out with basic science. Infrared means 'below red', so it lies outside the visible band by definition, and the last statement falls. 'Two-thirds' and '10 times' are large, oddly exact figures. Statements like these are often deliberate exaggerations. If you remember that sunlight is split roughly half infrared and slightly less than half visible, the two-thirds claim goes. The '10 times' claim is harder without data. The NCERT figure to remember is that surface insolation is about 320 W/m² in the tropics and about 70 W/m² at the poles, so the gap is about 4–5 times. What is left links two well-known facts: water vapour is a greenhouse gas that absorbs infrared, and it is concentrated near the surface. These facts also explain why air temperature falls with height in the troposphere.
Know more
How sunlight heats the Earth: short waves in, long waves out
The Sun's surface is about 6,000 °C, so it radiates mostly short waves: visible light plus near-infrared and ultraviolet. The atmosphere lets most visible light through. Ozone in the stratosphere absorbs the shortest UV waves. Water vapour and dust in the troposphere absorb part of the near-infrared. NASA estimates that about 23% of incoming solar energy is absorbed in the atmosphere by water vapour, dust and ozone.
The warmed ground then gives off long-wave (infrared) terrestrial radiation. Water vapour and CO₂ absorb this radiation easily. So the atmosphere is heated indirectly, from below, and the troposphere gets cooler with height (the normal lapse rate is about 6.5 °C per km).
| Band | Wavelength | Share of solar energy | Main absorber |
|---|---|---|---|
| Ultraviolet | < 400 nm | ~7% | Ozone (stratosphere) |
| Visible | 400–700 nm | ~43% | Mostly passes through to the surface |
| Infrared | > 700 nm | ~49% | Water vapour, CO₂ (troposphere) |
Why insolation varies with latitude: the angle of the Sun's rays, the length of the day, how transparent the atmosphere is, and the shape of the land (aspect). Near the poles, slanting rays spread the same energy over a larger area and pass through a thicker layer of air. The result is about 320 W/m² of insolation in the tropics against about 70 W/m² at the poles. Insolation is highest in the subtropical deserts, not at the equator, because the equator has more cloud.
Linked ideas UPSC tests: the heat budget (incoming energy equals outgoing energy, with about 35% reflected as albedo), the greenhouse effect, and why cloudy nights are warmer than clear ones (clouds and water vapour trap the outgoing long-wave radiation).
NCERT Concepts to Revise
- Insolation: Every statement here is about insolation: how it is distributed, what it is made of, and how the atmosphere absorbs it.
- Shortwave and longwave radiation: Telling the Sun's short waves apart from the Earth's long waves is the key to the infrared statements.
- Electromagnetic spectrum: One statement places infrared on the electromagnetic spectrum, so you need to know where the visible band ends.
- Terrestrial radiation: Terrestrial radiation is the long-wave infrared that water vapour in the lower atmosphere absorbs.
- Greenhouse gases: Water vapour is a greenhouse gas, and its ability to absorb infrared is what one statement tests.
- Factors affecting insolation: Latitude, angle of rays and day length explain the equator-to-pole insolation ratio that one statement claims.