UPSC Prelims 2022 Q81: High vs Low Clouds and Earth's Heat — Explained
Consider the following statements:
1. High clouds primarily reflect solar radiation and cool the surface of the Earth.
2. Low clouds have a high absorption of infrared radiation emanating from the Earth's surface and thus cause warming effect.
Which of the statements given above is/are correct?
- 1 only
- 2 only
- Both 1 and 2
- Neither 1 nor 2
Answer: (d) Neither 1 nor 2
Statement-wise explanation
- 1: incorrect — High clouds such as cirrus are thin and made of ice crystals. Sunlight passes through them easily, so they reflect little of it. They do absorb the infrared heat rising from the Earth and send part of it back down. NASA's Earth Observatory says their overall effect is to add to greenhouse warming, which is the opposite of cooling the surface.
- 2: incorrect — Low clouds such as stratocumulus are thick and bright. They reflect much of the incoming sunlight back to space, so less solar energy reaches the ground. They sit close to the surface and are nearly as warm as it, so they do little to change the infrared heat escaping to space. Their net effect is to cool the surface, not warm it.
How to eliminate
Option logic does little here. Each statement is a mechanism plus an effect, and each sounds plausible on its own. What helps is spotting the swap. The two statements give the textbook roles in reverse: reflecting sunlight and cooling is the job of low, thick clouds, while trapping infrared and warming is the job of high, thin clouds. Once you see that both statements are reversed, the answer is that neither is correct. If you have to work it out from first principles, ask how thick and how cold each cloud is. Thin ice clouds let sunlight through. Because they are very cold, they give off little heat to space, so they trap warmth. Thick water clouds near the ground block sunlight, but they are about as warm as the surface, so they trap little extra heat. One fact to remember: high, thin clouds warm; low, thick clouds cool.
Know more
Clouds act on two kinds of radiation
- Shortwave (incoming sunlight): clouds reflect part of it back to space. This is the albedo effect, and it cools the surface.
- Longwave (infrared given off by the Earth): clouds absorb it and send part of it back down. This is the greenhouse effect of clouds, and it warms the surface.
Whether a cloud warms or cools depends on which effect is stronger. The main factors are the cloud's height (which sets its temperature), its thickness, and whether it is made of water droplets or ice crystals.
| Feature | High clouds (cirrus, cirrostratus, cirrocumulus) | Low clouds (stratus, stratocumulus) |
|---|---|---|
| Made of | Ice crystals | Water droplets |
| Thickness | Thin, see-through | Thick, opaque |
| Effect on sunlight | Mostly let it through | Reflect a large share |
| Effect on Earth's infrared | Absorb it; very cold, so they give off little heat to space | Cloud top is about as warm as the surface, so little change |
| Net effect on the surface | Warming | Cooling |
Deep clouds such as cumulonimbus both reflect strongly and trap heat strongly, so the two effects roughly cancel.
The global balance. NASA's Earth Observatory notes that, averaged over the whole planet, cooling wins: the Earth's surface is cooler than it would be with no clouds. This links to the heat budget. The Earth reflects about 30% of incoming sunlight (its albedo), and clouds supply a large part of that.
Why it matters for climate change. Clouds are one of the biggest sources of uncertainty in climate models. If warming leaves fewer low clouds or more high clouds, clouds would add to the warming. This is called a positive cloud feedback.
NCERT Concepts to Revise
- Cloud classification by height: Both statements depend on which clouds count as high and which as low; this entry gives NCERT's grouping by height.
- Cirrus: Cirrus is the classic high cloud; how thin and icy it is decides how it treats sunlight and Earth's heat.
- Heat budget of the Earth: The question asks how clouds change the balance between incoming shortwave and outgoing longwave energy.
- Greenhouse gases: Statement 2 describes clouds absorbing infrared, the same way greenhouse gases trap heat.
- Insolation: Statement 1 is about clouds reflecting incoming solar radiation, which is insolation.