Introduction
A desert is a region where water is extremely limited because precipitation is very low compared with the amount of moisture that can be lost through evaporation and plant transpiration. A commonly used introductory definition describes deserts as receiving about 25 centimeters (10 inches) or less of precipitation per year, although scientists also use measurements such as the aridity index to describe dryness more precisely.
Deserts are not necessarily hot, sandy, or completely lifeless. They can be rocky, mountainous, icy, salty, or covered with gravel. Antarctica qualifies as a polar desert because it receives extremely little precipitation, making the Antarctic Desert the largest desert on Earth. The Sahara is the world's largest hot desert.
Deserts occur on every continent and contain ecosystems specially adapted to water scarcity and extreme environmental conditions.
Learning Objectives
After studying this guide, you should be able to:
Explain what makes a region a desert.
Distinguish hot, cold, subtropical, coastal, rain-shadow, interior, and polar deserts.
Describe how atmospheric circulation and mountains can create desert climates.
Identify dunes, wadis, salt flats, alluvial fans, and other desert landforms.
Explain how desert plants and animals conserve water and control temperature.
Distinguish natural deserts from the process of desertification.
What Defines a Desert?
The defining characteristic of a desert is aridity, or extreme dryness.
Temperature alone does not determine whether a place is a desert.
A region can be:
Hot and dry.
Cold and dry.
Coastal and foggy but almost rainless.
Covered with ice yet receive very little precipitation.
This explains why both the Sahara and Antarctica are deserts despite having dramatically different temperatures.
Scientists often compare precipitation with potential evapotranspiration, the amount of water that could potentially be lost through evaporation and plant activity.
FAO describes drylands using an aridity index, calculated from precipitation relative to potential evapotranspiration.
Are All Deserts Sandy?
No.
The classic image of a desert as endless sand dunes represents only part of desert geography.
National Geographic estimates that dunes cover only about 10 percent of the world's deserts. Desert surfaces can instead consist of:
Bare rock
Gravel
Mountains
Salt flats
Dry lake beds
Ice
Volcanic terrain
NASA satellite imagery of the Sahara shows dunes alongside plateaus, volcanic features, mountains, and rocky desert surfaces.
Major Types of Deserts
Deserts can be classified according to why they are dry.
Subtropical Deserts
Subtropical deserts occur mainly near 20°–30° north and south latitude.
They form partly because of global atmospheric circulation.
Warm moist air rises near the equator, cools, and releases precipitation. Higher in the atmosphere, the drier air moves toward the subtropics and eventually descends.
Descending air warms and becomes even drier, making cloud formation and rainfall difficult.
The Sahara Desert is the most famous example.
Other subtropical deserts include parts of the Arabian and Australian desert regions.
Rain-Shadow Deserts
A rain shadow develops when mountains block moist air.
The process works approximately like this:
Moist air → rises over mountains → cools → precipitation falls → dry air descends on opposite side
The side facing incoming moist air is called the windward side.
The drier side is the leeward side.
The Great Basin Desert of the western United States provides a clear example. The Sierra Nevada removes much of the moisture from Pacific air before it reaches areas farther east.
Coastal Deserts
A coastal desert may form beside a cold ocean current.
Cold water cools the air above it and can create fog, yet atmospheric conditions may prevent substantial rainfall.
The Atacama Desert of western South America is a famous example of an extremely dry coastal desert.
The Namib Desert of southwestern Africa is another.
Some coastal desert organisms obtain important moisture from fog even when measurable rainfall is extremely scarce.
Interior Deserts
Some deserts occur deep inside continents.
Moist oceanic air may travel such a long distance that much of its moisture is lost before reaching the continental interior.
Distance from oceans can therefore contribute to aridity.
Central Asian desert environments illustrate this effect.
Polar Deserts
Polar deserts are extremely cold regions with very low precipitation.
Antarctica is the largest desert on Earth.
Although Antarctica stores enormous quantities of frozen water, very little liquid water is available to most terrestrial organisms, and precipitation is extremely limited.
Parts of the Arctic are also classified as polar desert.
Hot vs. Cold Deserts
Temperature provides another useful classification.
Hot Deserts
Examples include:
Sahara
Arabian Desert
Sonoran Desert
Mojave Desert
Daytime temperatures can become extremely high.
Cold Deserts
Examples include:
Antarctic Desert
Gobi Desert
Great Basin Desert
Cold deserts may experience freezing winters and snowfall.
The Great Basin is considered a cold desert, with much of its precipitation arriving as snow.
Major Deserts of the World
Desert | Region | Important Feature |
Antarctic Desert | Antarctica | Largest desert overall |
Sahara | North Africa | Largest hot desert |
Arabian Desert | Southwest Asia | Large subtropical desert |
Gobi | Mongolia and China | Cold continental desert |
Kalahari | Southern Africa | Dry region with grass and shrub ecosystems |
Atacama | Western South America | Extremely dry coastal desert |
Namib | Southwestern Africa | Coastal desert influenced by fog |
Great Victoria | Australia | Major Australian desert |
Sonoran | North America | Hot desert with distinctive cactus ecosystems |
Great Basin | North America | Cold rain-shadow desert |
Desert boundaries are ecological and climatic rather than simple political lines, so exact area measurements may differ between sources.
Desert Temperatures
Deserts can experience large temperature changes between day and night.
Dry air contains relatively little water vapor, while cloud cover is often limited.
During the day, strong sunlight can heat the surface rapidly.
At night, that heat may escape quickly.
As a result, some deserts experience very hot days followed by surprisingly cool nights.
Cold deserts may instead remain cool or freezing for long periods.
Desert Rainfall
Desert rainfall is usually:
Low
Irregular
Unpredictable
A location might receive much of its annual rainfall during a small number of storms.
Dry ground may absorb water slowly, especially when surfaces are hard or crusted.
This means deserts can experience dangerous flash floods even though their annual rainfall is low.
Wind in Deserts
Wind is an important force in dry environments because sparse vegetation often leaves soil and sediment exposed.
Wind can:
Remove fine particles.
Transport sand.
Build dunes.
Polish exposed rock.
Create dust storms.
The removal of loose particles by wind is called deflation.
Wind-driven particles can also wear away exposed surfaces through abrasion.
Sand Dunes
A dune is a mound or ridge of sand formed by wind.
Dunes develop where enough loose sand exists and wind loses enough energy to deposit it.
Several dune shapes occur.
Barchan Dunes
Crescent-shaped dunes that commonly form where sand supply is limited and winds come mainly from one direction.
Linear Dunes
Long narrow ridges of sand.
Star Dunes
Large dunes with several arms extending from a central high point.
They commonly form where wind direction varies.
Transverse Dunes
Ridges positioned roughly across the dominant wind direction.
NASA satellite imagery clearly shows the complex organization of linear and star dunes in major sand seas.
Erg, Reg, and Hamada
Not all desert surfaces have the same name.
Erg
An erg is a large area dominated by sand dunes, sometimes called a sand sea.
Reg
A reg is a desert surface dominated by gravel or small stones.
Hamada
A hamada is a rocky desert plateau or exposed bedrock surface.
These terms help explain why describing the entire Sahara as a giant field of dunes is inaccurate.
Wadis
A wadi is a dry valley or stream channel found in many arid regions.
It may remain dry for most of the year but carry large quantities of water after heavy rainfall.
Because sudden storms can send water rapidly through upstream drainage systems, a dry wadi can become dangerous very quickly.
Playas and Salt Flats
Water sometimes collects temporarily in low desert basins.
When the water evaporates, dissolved minerals remain behind.
Over repeated cycles, this can produce a playa or salt flat.
Salt flats can form enormous bright, nearly level surfaces.
Alluvial Fans
When a temporary desert stream leaves a steep mountain valley and reaches flatter ground, it slows down.
Sediment spreads outward in a fan-shaped deposit called an alluvial fan.
Several neighboring fans can merge into a larger apron of sediment along a mountain front.
Water may appear rare in deserts, but short-lived floods are powerful landscape-building forces.
Desert Soils
Desert soils generally contain less organic material than soils in wetter ecosystems because vegetation is sparse and decomposition patterns differ.
Some desert soils also contain high concentrations of:
Salts
Calcium carbonate
Other minerals
Evaporation can pull dissolved minerals toward the surface.
Soil characteristics vary greatly according to geology, climate, elevation, and drainage.
Desert Plants
Desert plants are often called xerophytes when they possess adaptations for very dry conditions.
Their main challenge is balancing two needs:
Obtain enough water
and
Avoid losing too much water
Plant Adaptations
Important desert plant adaptations include:
Small Leaves
Smaller leaves reduce surface area through which water can be lost.
Spines
In cacti, leaves have been highly modified into spines.
Spines can discourage herbivores and reduce exposure around the plant surface.
Waxy Coatings
Thick waxy surfaces reduce water loss.
Water Storage
Succulent plants store water in stems, leaves, or other tissues.
Wide Shallow Roots
Some plants spread roots near the surface so they can rapidly absorb brief rainfall.
Deep Roots
Other species develop long roots capable of reaching deeper moisture.
The National Park Service documents both wide shallow root systems and deep taproots among desert plants.
Cacti and Deserts
Cacti are strongly associated with deserts, but they are not found naturally in all of the world's deserts.
Cacti are native primarily to the Americas.
African and Asian deserts contain other plant groups that may look similar because they evolved comparable water-storage adaptations.
This is an example of convergent evolution, in which unrelated organisms develop similar traits under similar environmental pressures.
Desert Animals
Desert animals face challenges involving:
Heat
Cold
Water scarcity
Limited food
Exposure
Different species solve these problems in different ways.
Nocturnal Behavior
Many hot-desert animals are nocturnal, meaning they are most active at night.
Night activity reduces exposure to extreme daytime temperatures.
Other animals are most active near dawn and dusk.
The National Park Service identifies nocturnal behavior and burrowing as common adaptations to desert heat.
Burrowing
Underground environments often have more stable temperatures than exposed desert surfaces.
Animals may shelter in burrows during the hottest part of the day.
Burrows can also provide:
Protection from predators.
Moisture conservation.
Nesting areas.
Conserving Water
Desert animals may conserve water through efficient physiology.
Adaptations can include:
Highly concentrated urine.
Dry waste.
Reduced sweating.
Obtaining moisture from food.
Avoiding daytime heat.
Some small desert mammals can survive with very little direct drinking water.
Camouflage and Body Shape
Light coloration can reflect sunlight and provide camouflage.
Some desert animals also have large ears or long limbs that help release body heat.
Body form, behavior, and physiology usually work together rather than acting as isolated adaptations.
Oases
An oasis is an area within a desert where water is available near the surface.
Water may come from:
Springs
Groundwater
Rivers
Wells
Oases can support vegetation, agriculture, settlement, and travel routes.
Historically, many desert trade networks depended on reliable water sources separated by long distances.
People in Desert Environments
Deserts are not empty landscapes.
Human communities have lived in arid regions for thousands of years.
Traditional livelihoods include:
Pastoralism
Oasis farming
Trade
Hunting
Irrigated agriculture
Modern desert economies can also include:
Mining
Oil and gas production
Solar-energy generation
Tourism
Urban development
People living in deserts must manage water carefully because reliable supplies are limited.
Desert Agriculture
Agriculture in deserts is possible where water can be supplied.
Sources may include:
Rivers
Groundwater
Reservoirs
Irrigation systems
However, irrigation can create problems if poorly managed.
When irrigation water evaporates, dissolved salts may accumulate in the soil.
This process, called salinization, can reduce crop productivity.
Desertification
Desertification is not the natural expansion of an existing desert.
The United Nations Convention to Combat Desertification defines it as land degradation in arid, semi-arid, and dry sub-humid areas resulting from factors including climatic variations and human activities.
Possible contributing pressures include:
Overgrazing
Deforestation
Unsustainable cultivation
Poor irrigation
Soil erosion
Drought
Desertification reduces the biological and economic productivity of land.
Desert vs. Desertification
This difference is essential.
Desert: a naturally arid ecosystem or climatic region.
Desertification: degradation of productive dryland.
A healthy natural desert is not automatically an example of environmental degradation.
Many deserts contain highly specialized ecosystems that have existed for thousands or millions of years.
Deserts and Climate Change
Climate change can affect arid regions through changes in:
Temperature
Evaporation
Drought frequency
Rainfall timing
Extreme precipitation
These effects vary by region.
It is therefore too simple to say that climate change makes every desert uniformly larger. Some regions may become drier, while others experience different changes in seasonal precipitation.
Common Mistakes
Mistake 1: All Deserts Are Hot
False.
Antarctica is both a desert and the coldest continent.
Mistake 2: All Deserts Are Covered in Sand
False.
Sand dunes make up only a minority of global desert surfaces. Many deserts are rocky, gravelly, icy, or mountainous.
Mistake 3: The Sahara Is the World's Largest Desert
Only if the question asks for the largest hot desert.
The Antarctic Desert is larger overall.
Mistake 4: Deserts Never Flood
False.
Intense storms can produce severe flash floods in dry channels and valleys.
Mistake 5: Desertification Means Sand Dunes Spreading Everywhere
False.
Desertification means land degradation in drylands and can occur without moving dunes.
Mistake 6: Deserts Have Almost No Life
False.
Desert ecosystems contain highly adapted plants, animals, microorganisms, and human communities.
Memory Tips
Remember:
Desert = dry, not necessarily hot
For major desert formation:
Subtropical → sinking dry air
Rain shadow → mountains block moisture
Coastal → cold current reduces rainfall
Interior → far from ocean moisture
Polar → cold + extremely low precipitation
For landforms:
Erg = sand
Reg = gravel
Hamada = rock
Wadi = temporary water channel
Playa = dry lake/salt basin
For organisms:
Avoid heat → conserve water → capture water quickly
Summary
Deserts are regions defined mainly by extreme dryness rather than high temperature. A commonly used threshold is about 25 centimeters of precipitation per year, although scientific measures of aridity also compare precipitation with potential water loss.
Deserts form for several reasons. Subtropical atmospheric circulation creates major hot deserts, mountains produce rain shadows, cold currents help create coastal deserts, continental interiors can become very dry, and polar regions receive so little precipitation that they qualify as deserts.
Desert landscapes include much more than sand dunes. They contain rock plateaus, gravel plains, wadis, salt flats, alluvial fans, mountains, and ice.
Plants and animals survive through specialized adaptations involving water storage, reduced water loss, nocturnal behavior, burrowing, efficient kidneys, and specialized roots.
Humans have also occupied desert environments for thousands of years using trade routes, pastoralism, irrigation, groundwater, and oasis agriculture.
Natural deserts should not be confused with desertification. Desertification is the degradation of productive drylands through interacting climatic and human pressures.
FAQ
1. What makes an area a desert?
A desert has extremely low available moisture and usually receives very little annual precipitation.
2. Are all deserts hot?
No. Cold deserts include Antarctica, the Gobi, and the Great Basin.
3. What is the largest desert on Earth?
The Antarctic Desert.
4. What is the largest hot desert?
The Sahara Desert.
5. What is a rain-shadow desert?
It is a dry region formed on the leeward side of mountains after rising air loses much of its moisture on the windward side.
6. What is a sand dune?
A dune is a mound or ridge of sand deposited and shaped mainly by wind.
7. What is a wadi?
A wadi is a valley or stream channel that is usually dry but may carry water after heavy rainfall.
8. How do desert plants conserve water?
Adaptations can include small leaves, spines, waxy surfaces, water-storage tissues, and specialized root systems.
9. Why are many desert animals nocturnal?
Night activity allows animals to avoid the most intense daytime heat and reduce water loss.
10. What is desertification?
Desertification is land degradation in arid, semi-arid, and dry sub-humid regions caused by interacting climatic variations and human activities.
Key Takeaways
Deserts are defined mainly by aridity and low precipitation, not by heat or sand.
Antarctica is the world's largest desert, while the Sahara is the largest hot desert.
Subtropical circulation, rain shadows, cold ocean currents, continental location, and polar climates can all produce deserts.
Desert organisms survive through adaptations that conserve water, capture scarce moisture, and reduce exposure to temperature extremes.
Desertification is degradation of dryland ecosystems and should not be confused with healthy natural deserts.
References
U.S. National Park Service. Deserts — Great Basin National Park.
U.S. National Park Service. Desert Ecosystem — Grand Canyon-Parashant National Monument.
U.S. National Park Service. Plant Adaptations.
NASA Earth Observatory. The Sahara's Diverse Landscape.
NASA Visible Earth. Linear and Star Dunes.
National Geographic Society. Desert.
National Geographic Society. Not All Deserts Are Sandy.
Food and Agriculture Organization of the United Nations. Dryland Forests and Agrosilvopastoral Systems — Background.
United Nations Convention to Combat Desertification. UNCCD FAQ — What Is Desertification?
United Nations Convention to Combat Desertification. Article 1: Use of Terms.