Overview
Previous Year UPSC-CSE Questions By the end you will be able to draft model answers for the following UPSC questions. Each question carries a collapsible framework showing how to approach it in the exam.
- UPSC Mains 2013 GS-IMajor hot deserts in the northern hemisphere are located between 20-30 degree N and on the western side of the continents. Why?
How to structure the answer in the exam
Introduction: Open with the subtropical high-pressure belt formed by the descending arm of the Hadley cell.
Body (sub-themes to develop):
- The 20-30 degree belt: sinking, warming, drying air of the subtropical high.
- The dry, stable trade winds blowing toward the equator.
- Western-side location: cold currents (Canary, Humboldt-type) cool and stabilise coastal air.
- Rain shadow and continentality add to the aridity.
- Examples: the Sahara, Arabian, Thar and the Atacama or Namib.
Conclusion: Conclude that latitude sets the belt while winds, currents and relief fix the exact, western location.
- UPSC Mains 2020 GS-IThe process of desertification does not have climate boundaries. Justify with examples.
How to structure the answer in the exam
Introduction: Distinguish natural aridity (the subtropical belt) from desertification, the human-driven degradation of land.
Body (sub-themes to develop):
- Desertification occurs in semi-arid and even sub-humid lands, not only at 20-30 degrees.
- Drivers: overgrazing, deforestation, poor irrigation and salinisation.
- Examples across climate zones, from the Sahel to parts of India and beyond.
Conclusion: Conclude that desertification is a land-management problem that crosses climatic boundaries.
The world's great hot deserts, such as the Sahara, Arabian, Thar, Kalahari and Australian, cluster in two belts around 20 to 30 degrees north and south of the equator, where the descending arm of the global atmospheric circulation keeps the air warm, dry and almost rainless.
Why deserts cluster at the same latitudes
The pattern of the world's hot deserts
Look at a world map and a striking pattern appears. The great hot deserts, the Sahara, Arabian, Thar, Kalahari, Atacama and Australian, sit in two belts around 20 to 30 degrees north and south of the equator, and many lie on the western sides of continents.
This is not a coincidence. The same band of latitude is dry across every continent because the cause is global, namely the way the atmosphere circulates between the equator and the poles. The rest of this article unpacks that circulation and the factors that sharpen it.
The master cause: the global circulation
The Hadley cell and the subtropical high
The master cause is the Hadley cell. Intense heating at the equator makes air rise, cool and pour down heavy rain, which is why the equatorial belt is so wet. This risen air then flows poleward high in the atmosphere.
Near 30 degrees the air sinks back to the surface. As it descends it is compressed and warms, so its clouds evaporate, creating a subtropical high-pressure belt of clear skies and very little rain. This sinking air is the engine of the desert belt.
These calm, dry latitudes are also called the horse latitudes. Their consistently warm, sunny, rainless conditions are the main reason the world's major hot deserts exist where they do, from the Sahara to the Australian desert.
The dry trade winds
From the subtropical highs, surface winds blow back toward the equator as the trade winds, north-easterly in the Northern Hemisphere and south-easterly in the Southern. Because they come from sinking, already-dried air, they are stable and bring little rain over land.
Where the trades cross a continent, they shed most of their moisture on the windward, eastern side and arrive parched in the interior and the west. This helps explain why so many deserts sit on the western margins, away from the moist easterlies.
Why the western margins are driest
Cold ocean currents and west-coast deserts
On the western coasts, a second factor deepens the dryness. Cold ocean currents, the Humboldt off South America and the Benguela off south-west Africa, chill the air just above the sea.
Cold air is stable and holds little moisture, so instead of rain it produces fog. This is why the Atacama and the Namib, beside cold currents, are among the driest places on Earth, the Atacama being the driest non-polar desert.
The rain-shadow effect
Relief adds further deserts through the rain-shadow effect. When moist air is forced up a mountain range, it cools and rains on the windward slope, then descends the far side.
Coming down, the air warms and dries, so the leeward side lies in a rain shadow. The Atacama is doubly shielded, hemmed by the Andes and a coastal range, and Patagonia is a classic rain-shadow desert behind the Andes.
Continentality and interior deserts
Distance from the sea matters too. Deep in a large continent, far from any moist ocean winds, the interior can be arid even outside the subtropical belt, a condition called continentality.
The cold Gobi of Central Asia is the classic example, dry because it lies far inland, far to the north and high on a plateau, beyond the easy reach of moist ocean winds. It shows that latitude sets the main desert belt, while geography fills in the rest.
The great deserts and the exam
The world's major deserts
Putting the factors together explains the world map of deserts. Most great hot deserts owe their existence first to the subtropical high, with cold currents, rain shadows or continentality deciding exactly where and how dry each one is.
| Desert | Continent | Main cause(s) |
|---|---|---|
| Sahara | Africa | Subtropical high; dry trade winds |
| Arabian and Thar | Asia | Subtropical high; continental interior |
| Atacama | South America | Cold Humboldt Current; rain shadow |
| Namib | Africa | Cold Benguela Current; subtropical high |
| Kalahari | Africa | Subtropical high |
| Great Victoria (Australian) | Australia | Subtropical high; offshore winds |
The list of causes at work is short, but it combines differently in each case:
- the subtropical high: the Sahara, Arabian, Thar and Australian deserts
- cold west-coast currents: the Atacama and the Namib
- the rain-shadow effect: Patagonia and parts of the Atacama
- continentality: the Gobi and the deserts of Central Asia
How this appears in the UPSC exam
This topic is a UPSC favourite because it links many ideas at once. The 2013 question on why northern-hemisphere hot deserts lie between 20 and 30 degrees north on the western side of continents is answered by exactly the factors above.
A strong answer starts with the Hadley cell and the subtropical high, then adds the trade winds, cold currents and rain shadow, and finishes with named examples. The key is to connect pressure, winds, currents and relief in one explanation.
Prelims MCQ practice
Each question below tests one specific concept on the topic. Click to reveal the answer and a full option-wise explanation.
Q1. With reference to the global atmospheric circulation, consider the following statements:
- In the Hadley cell, air rises near the equator and descends near 30 degrees latitude.
- The descending air forms a subtropical high-pressure belt.
- The descending air is moist and brings heavy rainfall.
Which of the statements given above is/are correct?
- 1 and 2 only
- 2 and 3 only
- 1 and 3 only
- 1, 2 and 3
Show answer and explanation
Answer: 1 and 2 only
Explanation.
Statement 1 is correct: in the Hadley cell air rises at the equator and sinks near 30 degrees. Statement 2 is correct: the sinking air forms a subtropical high. Statement 3 is wrong: the descending air is warm and dry, giving clear skies, not rain. Hence 1 and 2 only.
Q2. The main reason most hot deserts lie around 20 to 30 degrees latitude is:
- heavy convectional rainfall
- the descending air of the subtropical high-pressure belt
- permanent low pressure over these latitudes
- the melting of polar ice
Show answer and explanation
Answer: the descending air of the subtropical high-pressure belt
Explanation.
Hot deserts lie at 20-30 degrees because the descending arm of the Hadley cell creates a subtropical high of warm, dry, sinking air. Option (a) describes the wet equatorial belt; (c) is wrong because it is high, not low, pressure; (d) is irrelevant. Hence (b).
Q3. With reference to cold ocean currents and deserts, consider the following statements:
- The Humboldt Current is associated with the Atacama Desert.
- The Benguela Current is associated with the Namib Desert.
- Cold currents increase rainfall along the adjacent coast.
Which of the statements given above is/are correct?
- 1 and 2 only
- 2 and 3 only
- 1 and 3 only
- 1, 2 and 3
Show answer and explanation
Answer: 1 and 2 only
Explanation.
Statement 1 is correct: the Humboldt Current borders the Atacama. Statement 2 is correct: the Benguela Current borders the Namib. Statement 3 is wrong: cold currents stabilise the air and bring fog, reducing rainfall rather than increasing it. Hence 1 and 2 only.
Q4. The Atacama Desert is one of the driest places on Earth mainly because of:
- its high altitude alone
- the combined effect of a cold current and a rain shadow
- heavy monsoon winds
- its polar location
Show answer and explanation
Answer: the combined effect of a cold current and a rain shadow
Explanation.
The Atacama's extreme aridity comes from the cold Humboldt Current and the subtropical high, plus a two-sided rain shadow between the Andes and the coastal range. Options (a), (c) and (d) are incorrect. Hence (b).
Q5. With reference to the trade winds and the horse latitudes, consider the following statements:
- The trade winds blow from the subtropical highs toward the equator.
- The horse latitudes lie around 30 degrees north and south.
- The horse latitudes are belts of low pressure and heavy rain.
Which of the statements given above is/are correct?
- 1 and 2 only
- 2 and 3 only
- 1 and 3 only
- 1, 2 and 3
Show answer and explanation
Answer: 1 and 2 only
Explanation.
Statement 1 is correct: the trade winds blow from the subtropical highs toward the equator. Statement 2 is correct: the horse latitudes lie around 30 degrees. Statement 3 is wrong: the horse latitudes are high-pressure belts with calm winds and little rain. Hence 1 and 2 only.
Q6. Which one of the following deserts is primarily a result of continentality (distance from the sea)?
- Sahara Desert
- Atacama Desert
- Gobi Desert
- Namib Desert
Show answer and explanation
Answer: Gobi Desert
Explanation.
The Gobi, deep in Central Asia and far from moisture-bearing winds and shielded by high mountains, is the classic continental desert. The Sahara is mainly subtropical-high driven, while the Atacama and Namib are cold-current west-coast deserts. Hence the Gobi.
Sources and Further Reading
- Wikipedia: Hadley cell
- Wikipedia: Horse latitudes
- Wikipedia: Atacama Desert
- Wikipedia: Trade winds
- Wikipedia: Desert (interior and continental deserts)
- Wikipedia: Patagonian Desert
- Wikipedia: Gobi Desert
- NASA Earth Observatory: atmospheric circulation
- NOAA: global atmospheric circulation
- USGS: deserts and desertification
- NCERT: Geography (fundamentals of physical geography)
- India Meteorological Department (arid climate of India)
Editorial Disclaimer
This article explains why the world's hot deserts form at 20-30 degrees latitude, for UPSC preparation, drawing on standard climatology and earth-science sources. Mechanisms and examples reflect the cited authorities. Readers should consult the linked sources for fuller treatment.
