Overview
Hot deserts form in two belts between about 20 and 30 degrees north and south of the equator because the air over these latitudes is sinking. Air that rises and rains at the equator flows poleward high in the atmosphere and descends near 30 degrees as part of the Hadley cell. As it sinks it warms and dries, creating the subtropical high pressure belt, where skies stay clear and rain rarely falls. On the western sides of continents, cold ocean currents make the air drier still, which is why the Sahara, the Atacama and the Namib reach right down to the coast.
Hot Deserts of the World: How They Form and Where They Lie
Why Hot Deserts Form at 20 to 30 Degrees Latitude
Major hot deserts in the northern hemisphere lie between 20 and 30 degrees north and on the western side of the continents because the air above them is sinking, and sinking air does not produce rain. The same belt is dry on every continent from Mexico to Australia, which shows that the cause is global rather than local. Four factors explain it; the first sets the belt and the rest decide how dry each place becomes:
- The subtropical high: Air that rose at the equator sinks near 30 degrees as part of the Hadley cell, warms as it descends and clears the sky of cloud.
- The trade winds: Winds blowing out of the subtropical high toward the equator are stable and dry, and a layer of warm air above them stops tall rain clouds from growing.
- Cold ocean currents: Along western coasts, cold currents cool the air from below, so it gives fog instead of rain.
- Relief and distance from the sea: Mountain barriers cast rain shadows, and the interiors of large continents lie beyond the reach of moist sea winds.
Major Hot Deserts of the World and Their Causes
The Sahara is the world's largest hot desert, covering about 9.2 million square kilometres of North Africa. East of it the belt runs through the Arabian Desert into the Thar of India and Pakistan. Most other deserts follow the same two belts, and many sit on western coasts beside a cold current.
| Desert | Where it lies | Main causes of aridity |
|---|---|---|
| Sahara | North Africa, across the Tropic of Cancer | Subtropical high; dry trade winds; cool Canary Current on the Atlantic coast; Atlas Mountains in the north |
| Arabian | Arabian Peninsula, Asia | Subtropical high; continental interior |
| Thar | North-west India and Pakistan | Subtropical high; scanty and variable monsoon rain |
| Sonoran and Mojave | South-west USA and north-west Mexico | Subtropical high; cold California Current; rain shadow of the Sierra Nevada |
| Atacama | Coast of northern Chile, South America | Cold Humboldt Current; South Pacific high; rain shadow on both sides |
| Namib | Coast of Namibia, southern Africa | Cold Benguela Current; subtropical high |
| Kalahari | Botswana, Namibia and South Africa | Subtropical high; interior location |
| Great Victoria | Western and South Australia | Subtropical high; continental interior |
Hadley Cell and the Subtropical High Pressure Belt
Hadley Cell: Rising Air at the Equator, Sinking Air Near 30 Degrees
The Hadley cell is the loop of air that circulates between the equator and about 30 degrees of latitude in each hemisphere. It is named after George Hadley, who proposed it in 1735 to explain the trade winds. The loop works in four steps:
- Rising: Strong heating at the equator makes air rise. As it rises it cools, its water vapour condenses, and heavy rain falls, which is why the equatorial belt carries rainforest.
- Flowing poleward: The risen air, now dry, spreads toward the poles high in the atmosphere.
- Sinking: Near 30 degrees the air descends. It is compressed as it sinks and so warms, and warmer air can hold more moisture, so any cloud evaporates.
- Returning: At the surface the air flows back toward the equator as the trade winds, closing the loop.
The sinking arm is what makes the deserts. Air reaches the surface near 30 degrees after losing its moisture over the equator, and it keeps warming on the way down. The result is a belt of clear skies, strong sunshine and very little rain.
Subtropical High Pressure Belt, Horse Latitudes and Trade Winds
Where the sinking air reaches the surface it forms the subtropical high pressure belt. These highs are semi-permanent and lie mainly over the oceans between about 20 and 40 degrees of latitude. Sinking air and a temperature inversion, a layer in which temperature rises with height, stop air from rising, so clouds cannot grow tall enough to rain. Fair, dry and hot weather goes with this high pressure band at about 30 degrees.
Sailors called these calm, dry latitudes around 30 degrees north and south the horse latitudes. The world's major hot deserts lie under this belt, which is also called the subtropical ridge. They run from the Sahara, the Arabian and Syrian deserts, and the Mojave and Sonoran in the north, to the Atacama, Namib, Kalahari and Australian deserts in the south.
From the subtropical highs, the trade winds blow toward the equator, from the north-east in the Northern Hemisphere and from the south-east in the Southern. Air sinking from the subtropical ridge above them forms a trade wind inversion, which keeps clouds shallow. Over the desert belt the trade winds therefore carry dust more readily than rain; Saharan dust carried on them sometimes reaches Florida.
Why Hot Deserts Lie on the Western Side of Continents
Cold Ocean Currents, Upwelling and Coastal Fog Deserts
The subtropical highs explain the latitude of the deserts; cold ocean currents explain why the driest of them sit on western coasts. In tropical and subtropical latitudes, the west coasts of continents are bordered by cool water, while warm currents run along the east coasts.
Two processes keep the western waters cold. The currents flow toward the equator from cooler latitudes; the Humboldt and California currents are of this kind. They are called eastern boundary currents because they run along the eastern edge of an ocean, which is the western coast of a continent. Winds blowing from the land toward the sea also drive warm surface water away from the coast, and cold water upwells from below to replace it.
Cold water chills the air just above it. Cool air lying under warmer air is stable and cannot rise, so it forms fog instead of rain. These coasts therefore have relatively low average temperatures, a narrow daily and yearly range, and frequent fog, but almost no rainfall. Five cold currents run along these desert coasts:
- Canary Current: Along Morocco and Western Sahara, beside the Atlantic edge of the Sahara.
- California Current: Along the Pacific coast of the USA and Baja California, beside the Sonoran Desert.
- Humboldt (Peru) Current: Along Peru and northern Chile, beside the Atacama.
- Benguela Current: Along Namibia and western South Africa, beside the Namib.
- West Australian Current: Along the west coast of Australia, whose dry interior holds the Great Victoria Desert.
The Atacama is the driest non-polar desert, where one station averaged only 5 millimetres of rain a year over 17 years. Off the Namib, fog forms where the cold Benguela Current meets warm air. This fog has wrecked more than a thousand ships on the Skeleton Coast, and it is also a vital source of water for desert life. The Namib has been arid or semi-arid for roughly 55 to 80 million years and may be the oldest desert in the world.
Why East Coasts at the Same Latitudes Are Wet
The eastern sides of continents at the same latitudes are green. Florida, southern China and eastern Australia lie at much the same latitudes as the Sahara and the Kalahari, yet none of them is a desert. Three differences explain the contrast:
- Warm currents: Warm currents flow along the east coasts in tropical and subtropical latitudes, giving warm and rainy climates.
- Onshore winds: The trade winds blow from the east, so on east coasts they arrive from the sea. Onshore winds also pile warm water against the coast, which raises its temperature further.
- The side of the high: East coasts lie on the western margins of the subtropical anticyclones, where moist tropical air flows in. Weak tropical lows and occasional cyclones add summer rain.
The result is the humid subtropical climate, found on the south-east side of every continent except Antarctica, generally between 25 and 40 degrees. In the middle latitudes the pattern reverses. There the westerlies blow from the west, and the west coasts of continents are bordered by warm water and have a mild marine climate.
Rain Shadow and Continentality: Deserts Beyond the Subtropical Belt
Some deserts owe more to relief than to latitude. When moist air meets a mountain range it is forced up, cools and drops its rain on the windward slope. It then descends the far side, warming and drying as it sinks, and leaves a rain shadow on the leeward side.
Four deserts show the effect clearly:
- Atacama: The Andes and the Chilean Coast Range block moisture from both the Pacific and the Atlantic, giving a two-sided rain shadow that the cold Humboldt Current and the South Pacific high reinforce.
- Mojave and Great Basin: Both lie in the rain shadow of the Sierra Nevada, which strips Pacific air of its moisture.
- Patagonian Desert: The Andes stop the moist westerlies from the Pacific, so the desert stays dry although about half of it lies within some 320 kilometres of the Atlantic. It is the largest desert in Argentina.
- Northern Sahara: The Atlas Mountains of Morocco, Algeria and Tunisia add a rain shadow on their leeward side.
Distance from the sea, called continentality, makes the remaining deserts. The Gobi of Mongolia and northern China lies far inland, far to the north and on a plateau roughly 910 to 1,520 metres above sea level, which also makes it cold. The Taklamakan lies in the Tarim Basin of western China and covers about 3,37,000 square kilometres.
Hot Desert Climate (BWh): Temperature, Rainfall and Life
Hot Desert Climate Characteristics: Heat, Rainfall and Fog
In the Köppen classification a desert climate is one where evaporation far exceeds precipitation. The hot desert climate (BWh) is separated from the cold desert climate (BWk) by a mean annual temperature of 18 °C. BWh is the most extensive of all Köppen climate types, covering about 14.2 per cent of Earth's land area. Its main features are these:
- Scanty, erratic rain: Most deserts receive 25 to 200 millimetres a year. It comes in short, intense thundershowers that run off before they can build soil moisture, and the total varies greatly from year to year.
- Extreme heat: The world's absolute heat records, above 50 °C, are generally set in hot deserts. The official record is 56.7 °C at Death Valley in the USA, measured on 10 July 1913.
- Cold nights: Clear skies let heat escape quickly after sunset, so night temperatures can fall to freezing or below. Both the daily and the yearly range of temperature are large.
- Near-rainless extremes: Kufra in the Libyan Sahara records about 0.86 millimetres of rain a year.
- Coastal fog: Fog is common in coastal deserts bordering cold currents, even where rain almost never falls.
Desert Plants and People: Adaptations to Heat and Drought
Plants that survive long dry periods are called xerophytes. Their adaptations serve one purpose, to lose as little water as possible:
- Leaves: Hard, waxy leaves resist drying. Many desert plants have fewer and smaller leaves or none at all, and in cacti the leaves are reduced to spines.
- Water storage: Cacti store water in thick, rounded stems.
- Roots: Deep, widely spreading roots collect what little water the soil holds.
In the Sahara, life gathers at the oasis, an area with water and vegetation in the otherwise dry desert, where date palms grow.
The Sahara has long been home to nomadic tribes that herd sheep, goats and camels and trade for their livelihood. Ancient trade routes across it carried gold and salt to the Mediterranean world.
Hot and Cold Deserts: Differences and Types
Hot Desert vs Cold Desert: Key Differences and Examples
A desert is defined by dryness, not by heat. A cold desert is as dry as a hot one, but its winters are cold. In the Köppen scheme cold deserts have a mean annual temperature below 18 °C, and they lie mainly between 35 and 60 degrees of latitude, in the interiors of continents where moist sea winds do not reach and in areas often surrounded by mountains.
About one-third of Earth's land surface is desert. Deserts may be hot or cold, sandy or rocky, but they are always dry. A widely accepted system by Peveril Meigs (1953) grades dry lands by rainfall. Extremely arid land has at least 12 consecutive months without rain, arid land receives less than 250 millimetres a year, and semi-arid land receives 250 to 500 millimetres. Arid and extremely arid lands are deserts; semi-arid grasslands are usually called steppes.
| Basis | Hot deserts (BWh) | Cold deserts (BWk) |
|---|---|---|
| Mean annual temperature | Above 18 °C | Below 18 °C |
| Typical latitude | Core belt about 20 to 30 degrees, reaching about 15 to 35 at the edges, under the subtropical highs | About 35 to 60 degrees, in continental interiors |
| Main cause | Sinking air of the subtropical high, helped by cold currents | Distance from the sea, rain shadows and high altitude |
| Summers | Very hot | Hot or warm, but less hot than in hot deserts |
| Winters | Mild days, cold nights | Cold and dry |
| Examples | Sahara, Arabian, Thar, Kalahari, Great Victoria | Gobi, Taklamakan, Patagonian, Ladakh |
Deserts can also be grouped by what makes them dry. Five types cover the major deserts of the world:
- Subtropical (trade wind) deserts: Under the subtropical highs; the Sahara, Arabian, Thar, Kalahari and Australian deserts.
- Cold-current coastal deserts: On western coasts beside cold currents; the Atacama and the Namib.
- Rain-shadow deserts: Behind mountain barriers; Patagonia, the Mojave and the Great Basin.
- Interior (continental) deserts: Far from any sea; the Gobi and the Taklamakan.
- Polar deserts: Most of the interior of Antarctica, which receives very little precipitation.
Deserts of India: The Thar and Ladakh
Thar Desert and Ladakh: India's Hot and Cold Deserts
The Thar, or Great Indian Desert, lies in north-western India and Pakistan and is the easternmost extension of the Sahara-Arabian desert formation. It is bordered by the Aravalli Range to the east, the Indus plains to the west and the Rann of Kutch to the south. Its climate is tropical desert, with extremely hot summers and cooler winters.
Unlike the Sahara, the Thar receives monsoon rain in summer, but little of it. Most of its rain comes from the south-west monsoon, and the amount varies widely from year to year, with long spells of drought. Most of western Rajasthan receives less than 500 millimetres of rain a year, and extreme western Rajasthan has a hot desert climate. The Luni is the only major river system of the region.
Ladakh is India's cold desert. It is a high-altitude desert in the rain shadow of the Himalaya, which keeps the monsoon clouds from reaching it, and its main source of water is the winter snowfall on the mountains. Its height keeps it cold, with winter temperatures that can drop below −30 °C. The landscape is rugged, with rocky terrain and deep valleys.
Desertification: How It Differs from Natural Deserts
Desertification: Land Degradation Beyond the Desert Belt
Natural deserts develop over long periods of time through the circulation of the atmosphere. Desertification is different. The UN Convention to Combat Desertification (UNCCD) defines it as land degradation in arid, semi-arid and dry sub-humid regions, resulting from various factors including climatic variations and human activities. Because the definition includes dry sub-humid land, desertification is not confined to the 20 to 30 degree belts. The Convention was established in 1994 and has 197 Parties: 196 countries and the European Union. The United Nations marks the World Day to Combat Desertification and Drought on 17 June.
Its immediate cause is the loss of most of the vegetation, driven by these factors, alone or in combination:
- Drought and climatic shifts
- Tillage for agriculture
- Overgrazing by livestock
- Deforestation for fuel or construction materials
Drylands cover about 40 to 41 per cent of Earth's land area and are home to more than 2 billion people. The areas most affected are the Sahel in Africa, the Gobi region of Mongolia and China in Asia and parts of South America, and the effects include sand and dust storms, food insecurity and poverty. The Sahel lies south of the Sahara and the Gobi is a mid-latitude cold desert, so desertification clearly crosses the boundaries of the hot desert belt.
Previous Year UPSC‑CSE Questions
Each Prelims question is explained option by option, and each Mains question comes with an answer outline.
- UPSC Prelims 2021Consider the following statements:
- In the tropical zone, the western sections of the oceans are warmer than the eastern sections owing to the influence of trade winds.
- In the temperate zone, westerlies make the eastern sections of oceans warmer than the western sections.
Which of the statements given above is/are correct?
How to approach this Prelims question
Approach: Ask which way the prevailing wind pushes warm surface water in each zone. In the tropics the trade winds blow from the east and pile warm water in the western sections of the oceans, so statement 1 holds. In the temperate zone the westerlies blow from the west and pile warm water in the eastern sections, so statement 2 holds too. The two look opposite only because the winds blow in opposite directions.
Trap to watch: The two statements look opposite but both are true, because the trade winds and the westerlies blow in opposite directions.
Why each option is right or wrong:
- (a) 1 only: Wrong. It leaves out statement 2, which is also true.
- (b) 2 only: Wrong. It leaves out statement 1, which is also true.
- (c) Both 1 and 2: Correct. Trade winds warm the western tropical oceans; westerlies warm the eastern temperate oceans.
- (d) Neither 1 nor 2: Wrong. Both statements are true.
Answer: (c) Both 1 and 2. The trade winds warm the western tropical oceans and the westerlies warm the eastern temperate oceans.
- UPSC Prelims 2018Which of the following leaf modifications occur(s) in the desert areas to inhibit water loss?
- Hard and waxy leaves
- Tiny leaves
- Thorns instead of leaves
Select the correct answer using the code given below:
How to approach this Prelims question
Approach: This repeats Prelims 2013 (the next question) with slightly changed items and options. Hard and waxy leaves, tiny leaves and thorns instead of leaves all reduce water loss, so every listed modification counts.
Trap to watch: Students who doubt that hard, waxy leaves count as a modification pick (a); a waxy coating is a change in the leaf that cuts water loss.
Why each option is right or wrong:
- (a) 2 and 3 only: Wrong. It drops hard and waxy leaves.
- (b) 2 only: Wrong. It drops waxy leaves and thorns.
- (c) 3 only: Wrong. It keeps only thorns.
- (d) 1, 2 and 3: Correct. All three are desert adaptations.
Answer: (d) 1, 2 and 3. Waxy leaves, tiny leaves and spines in place of leaves all cut water loss.
- UPSC Prelims 2013Which of the following leaf modifications occurs/occur in desert areas to inhibit water loss?
- Hard and waxy leaves
- Tiny leaves or no leaves
- Thorns instead of leaves
Select the correct answer using the codes given below.
How to approach this Prelims question
Approach: Check each leaf feature against the aim of cutting water loss. Hard, waxy leaves resist drying; leaves that are tiny, few or absent lose less water; and in cacti the leaves are reduced to spines, which are thorns in place of leaves. All three are genuine desert adaptations.
Trap to watch: Students who count spines as stems rather than leaves pick (a); in cacti the spines are reduced leaves.
Why each option is right or wrong:
- (a) 1 and 2 only: Wrong. It drops thorns, a separate adaptation seen in cacti.
- (b) 2 only: Wrong. It drops waxy leaves and thorns, both real adaptations.
- (c) 1 and 3 only: Wrong. It drops tiny or absent leaves.
- (d) 1, 2 and 3: Correct. All three features cut water loss.
Answer: (d) 1, 2 and 3. Waxy leaves, tiny or absent leaves and spines all cut water loss.
- UPSC Prelims 2011What could be the main reason/reasons for the formation of African and Eurasian desert belt?
- It is located in the sub-tropical high pressure cells.
- It is under the influence of warm ocean currents.
Which of the statements given above is/are correct in this context?
How to approach this Prelims question
Approach: Ask what keeps the African and Eurasian belt dry. It lies under the subtropical high-pressure cells, where sinking air suppresses rain, so statement 1 holds. The currents beside it are cold, such as the Canary Current off north-west Africa; warm currents bring rain to east coasts, so statement 2 is false.
Trap to watch: Deserts lie beside cold currents, not warm ones; warm currents bring rain to east coasts.
Why each option is right or wrong:
- (a) 1 only: Correct. The desert belt lies under the subtropical high-pressure cells.
- (b) 2 only: Wrong. Deserts lie beside cold currents, not warm ones.
- (c) Both 1 and 2: Wrong. It includes the false statement 2.
- (d) Neither 1 nor 2: Wrong. Statement 1 is the main reason.
Answer: (a) 1 only. The belt lies under the subtropical highs, and the currents beside it are cold, not warm.
- UPSC Prelims 2009Consider the following statements :
- In the world, the tropical deserts occur along the western margins of continents within the trade wind belt.
- In India, the East Himalayan region gets high rainfall from north-east winds.
Which of the statements given above is/are correct ?
How to approach this Prelims question
Approach: Test each statement against the cause of the rain in that region. Tropical deserts occur on the western margins of continents within the trade wind belt, where cold currents and dry, stable air meet, so statement 1 holds. The eastern Himalaya is watered by the Bay of Bengal branch of the south-west monsoon moving up the Brahmaputra valley, not by north-east winds, so statement 2 is false.
Trap to watch: The eastern Himalaya is watered by the Bay of Bengal branch of the south-west monsoon, not by north-east winds.
Why each option is right or wrong:
- (a) 1 only: Correct. Tropical deserts lie on the western margins of continents in the trade wind belt.
- (b) 2 only: Wrong. The eastern Himalaya's rain comes from the south-west monsoon, not north-east winds.
- (c) Both 1 and 2: Wrong. It includes the false statement 2.
- (d) Neither 1 nor 2: Wrong. Statement 1 is true.
Answer: (a) 1 only. The eastern Himalaya's rain comes from the Bay of Bengal branch of the south-west monsoon, so only statement 1 holds.
- UPSC Prelims 2007Consider the following statements:
- Either of the two belts over the ocean at about 30°–35° N and S latitudes is known as Horse Latitude.
- Horse Latitudes are low-pressure belts.
Which of the statements given above is/are correct?
How to approach this Prelims question
Approach: Recall what lies under the sinking arm of the Hadley cell near 30 degrees. Sinking air forms the subtropical high pressure belt, and the belts over the ocean at about 30 to 35 degrees north and south are the horse latitudes, so statement 1 holds. Because the air sinks there, they are high-pressure belts, so statement 2 is false.
Trap to watch: Statement 2 calls the horse latitudes low-pressure belts; sinking air makes them high-pressure belts.
Why each option is right or wrong:
- (a) 1 only: Correct. The horse latitudes are the oceanic belts at about 30 to 35 degrees north and south.
- (b) 2 only: Wrong. Statement 2 is false, because sinking air makes them high-pressure belts, not low-pressure ones.
- (c) Both 1 and 2: Wrong. It includes the false statement 2.
- (d) Neither 1 nor 2: Wrong. Statement 1 is true.
Answer: (a) 1 only. The horse latitudes are high-pressure belts, because the air sinks there.
- UPSC Mains 2020 GS‑IThe process of desertification does not have climatic boundaries. Justify with examples.
Answer outline
The outline sets out what the question asks, the main arguments, relevant examples and the direction of the conclusion; it is not a full model answer.
Introduction: The UNCCD defines desertification as land degradation in arid, semi-arid and dry sub-humid regions resulting from climatic variations and human activities.
Body (sub-themes to develop):
- Natural deserts and desertification: Natural deserts are set by the global circulation; desertification is degradation of land that need not be desert.
- Causes: Loss of vegetation through drought, tillage, overgrazing and deforestation.
- Examples beyond the hot desert belt: The Sahel south of the Sahara, the Gobi region of Mongolia and China, and parts of South America.
- Scale: Drylands cover about 40 to 41 per cent of the land and hold more than 2 billion people.
Conclusion: Desertification follows land use and rainfall variability, not a latitude belt, so it has no climatic boundary.
- 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?
Answer outline
Introduction: Air rising at the equator sinks near 30 degrees in the Hadley cell and forms the subtropical high-pressure belt.
Body (sub-themes to develop):
- Dry belt: Sinking air warms as it descends and the trade wind inversion stops clouds from growing.
- Trade winds: They blow out of the high toward the equator and are dry and stable over land.
- Western side: Cold currents such as the Canary and California, with offshore winds and upwelling, give fog instead of rain.
- Contrast: East coasts at the same latitudes have warm currents and onshore winds, so they are wet.
- Examples: The Sahara, Arabian, Thar and the Sonoran and Mojave deserts.
Conclusion: The latitude comes from the global circulation, while currents and winds decide the western location.
Sources
- Wikipedia: Hadley cell
- Wikipedia: Horse latitudes
- Wikipedia: Trade winds
- Wikipedia: Desert climate
- Wikipedia: Sahara
- Wikipedia: Thar Desert
- Wikipedia: Atacama Desert
- Wikipedia: Namib
- Wikipedia: Canary Current
- Wikipedia: Humboldt Current
- Wikipedia: Benguela Current
- Wikipedia: California Current
- Wikipedia: West Australian Current
- Wikipedia: Humid subtropical climate
- Wikipedia: Rain shadow
- Wikipedia: Patagonian Desert
- Wikipedia: Gobi Desert
- Wikipedia: Taklamakan Desert
- Wikipedia: Highest temperature recorded on Earth
- Wikipedia: Xerophyte
- Wikipedia: Desertification
- Wikipedia: Ladakh
- NOAA JetStream: Global Atmospheric Circulations
- USGS: What Is a Desert? (Deserts: Geology and Resources)
- UNCCD: The Convention to Combat Desertification
- United Nations: World Day to Combat Desertification and Drought
- NCERT: Fundamentals of Physical Geography (Class XI), climate and ocean currents
- NCERT: India Physical Environment (Class XI), climate
- NCERT: Exploring Society: India and Beyond (Grade 7), Geographical Diversity of India
- NCERT: Exploring Society: India and Beyond (Grade 8), World Geography: Some Glimpses
Editorial Disclaimer
Desert extents and rainfall figures are rounded from the cited sources, and the mechanisms follow standard climatology texts.
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