What is a hot desert and where are they?
A desert is a place that gets very little rain: less than about 250 mm a year. A hot desert is also hot for most of the year.
Most hot deserts lie in two belts, about 15° to 30° north and south of the equator, near the Tropics of Cancer and Capricorn. Big examples: the Sahara and Arabian deserts, the Thar in India and Pakistan, the Kalahari and Namib in southern Africa, the Atacama in South America and the deserts of central Australia.
We can see a desert as a system. Energy comes in (strong sunshine, wind, rare runoff after storms). Sediment (sand, gravel, dust) moves through it. Landforms such as dunes, rocky plateaus and dry river beds (wadis) are the result.
Why are hot deserts so dry?
- Sinking air: warm wet air rises at the equator and rains there. That air, now dry, sinks near 30°. Sinking air warms up and stops clouds forming. This high pressure belt makes most hot deserts.
- Cold ocean currents: cold water along some coasts (for example the Atacama and Namib) cools the air above it, so it cannot rise and rain. You get fog but almost no rain.
- Rain shadow: wet winds drop their rain on one side of a mountain range; the far side stays dry.
- Continentality: places far from the sea get little moisture, because winds have lost it on the way.
Desert climate, soils, vegetation and the aridity index
Climate
Days are very hot, often over 40 °C in summer. Clear skies let heat escape at night, so temperatures can drop near 0–5 °C. The diurnal range (day minus night temperature) can be 30 °C or more. Rain is rare and unreliable, often in short, heavy storms.
Aridity index
The aridity index (AI) compares water in and water that could be lost: AI = P ÷ PET, where P is yearly rainfall and PET is potential evapotranspiration (how much water could evaporate and be given off by plants if water were always there). Under 0.05 is hyper-arid, 0.05–0.20 arid, 0.20–0.50 semi-arid.
Soils
Desert soils are thin, sandy or stony, and dry. They have little humus (rotted plant matter) because few plants grow. Water drawn up and evaporated can leave a salty crust on top.
Vegetation
Plants are scattered with bare ground between them: cacti, thorny bushes, tough grasses, date palms near an oasis (a place where groundwater reaches the surface).
How plants and animals adapt
- Storing water: cacti and baobabs have thick, fleshy stems.
- Less water loss: spines or tiny leaves, thick waxy skin, pores that open only at night.
- Roots: very deep tap roots reach groundwater (some acacias over 30 m); or wide shallow roots catch every drop of rain.
- Waiting: seeds of many flowers lie still for years and sprout fast after rain.
- Camel: hump stores fat, not water; can lose a lot of body water safely; wide feet do not sink in sand; long eyelashes and closable nostrils block sand.
- Small animals: fennec foxes, gerbils and many insects stay in cool burrows by day and come out at night (they are nocturnal). Big ears of the fennec fox give off heat.
Development opportunities and challenges
Opportunities
- Energy: strong sunshine all year for solar power (Bhadla in Rajasthan, Noor in Morocco); oil and gas in some deserts.
- Minerals: copper, gold, phosphates, salt.
- Farming: with irrigation from canals, wells or rivers (the Nile valley, the Indira Gandhi Canal).
- Tourism: camel safaris, dunes, forts and festivals (Jaisalmer).
Challenges
- Extreme heat makes outdoor work hard and harms machines.
- Water supply: little rain, high evaporation, groundwater running low.
- Remoteness: long distances, few roads, costly to build and to bring goods.
- Flash floods: rare storms on hard ground cause sudden floods in wadis.
Case study idea
Pick one desert (for example the Thar or the Sahara) and note: location, climate figures, one opportunity, one challenge and how people respond.
Desertification: causes, impacts and solutions
Desertification is when land on the edge of a desert slowly loses its plants and good soil and becomes like desert. It is not sand dunes marching; it is the land getting worn out.
Deserts change over time
About 10,000–5,000 years ago the Sahara was much wetter (the "Green Sahara") with lakes, grass and animals. Shifts in Earth's orbit changed monsoon rains and it dried. So deserts grow and shrink naturally.
Causes
- Climate: longer droughts and higher temperatures with climate change.
- People: overgrazing, cutting trees for fuel, over-cultivation, poor irrigation that leaves salt, and fast population growth.
Areas at risk and impacts
The Sahel (south of the Sahara), parts of western India, central Asia and northern China are at risk. Impacts: lower crop yields, hunger, dust storms, people moving to cities.
Solutions
Tree belts and "green walls", stone lines (magic stones) to hold rainwater, rotating where animals graze, drip irrigation, solar cookers to save trees, and helping local people choose their own future.
Local example
In parts of the Sahel, farmers protect tree stumps so they regrow (farmer-managed natural regeneration), bringing back trees on millions of hectares.
Skills: measuring and handling desert data
- Draw a climate graph: bars for monthly rain, a line for temperature.
- Work out the aridity index and the diurnal range.
- Use satellite images over years to map plant cover or dune movement.
- Sketch a landform and label it; measure dune height or slope angle.
- Test data: find the mean, range and look for a trend; ask if your sample is big enough.
Try it at home
Put two cups of the same damp soil in the sun: one bare, one covered with dry leaves. Weigh or feel them after 3 hours. The bare soil dries much faster. This is why plant cover protects land from desertification. Then use the slider in the 3D (last step) to see the same idea.
Key formulas and definitions
- Desert: rainfall < 250 mm per year
- Aridity index AI = P ÷ PET
- Hyper-arid < 0.05; arid 0.05–0.20; semi-arid 0.20–0.50
- Diurnal range = maximum − minimum temperature in one day
- Hot desert belts ≈ 15°–30° N and S
- Desertification = land degradation in drylands
Worked examples
1. A desert town has a day maximum of 44 °C and a night minimum of 9 °C. What is the diurnal range?
Diurnal range = 44 − 9 = 35 °C. This big swing happens because clear skies let heat escape at night.
2. A place gets 150 mm of rain a year and its PET is 1,500 mm. Find the aridity index and the zone.
AI = 150 ÷ 1,500 = 0.10. This lies between 0.05 and 0.20, so the place is arid.
3. Explain why the Atacama coast is one of the driest places on Earth even though it is next to the ocean.
A cold ocean current runs along the coast. It cools the air above it, so the air is stable and does not rise to form rain clouds. Also it lies in the sinking-air belt and in the rain shadow of the Andes. So you get fog, but almost no rain.
Common mistakes
- Thinking all deserts are hot and sandy. Many are rocky or stony, and some (like Antarctica) are cold. A desert is defined by low rain.
- Saying a camel's hump stores water. It stores fat, which gives energy.
- Calculating the aridity index as PET ÷ P. It is P ÷ PET, so smaller numbers mean drier.
- Thinking desertification is just sand dunes moving. It is fertile land getting worn out by climate and human use.