Deserts: why so dry, and how life copes
Why does it hardly ever rain in some places? Find out how air, mountains and clear skies make deserts, why desert nights are cold, and run a fair test on how shape helps a plant save water.
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Scientists call a place arid when it gets less than 250 mm of rain a year. Some deserts go a whole year or more without any rain, and the driest parts of the Atacama Desert in Chile get less than 2 mm a year.[1][3]
Deserts cover between about a fifth and a third of Earth's land, depending on how you define them. They are on every continent, but only about 20% of desert areas are covered by sand. More than a billion people live in desert regions.[3][1]
Why are deserts dry? Many lie around 30° north and south of the equator. Air that rises near the equator has already lost most of its moisture as rain by the time it gets there, and high air pressure means few clouds. Other deserts lie in a rain shadow: moist air rises over mountains, cools and drops its rain on the mountains, so little is left for the land behind them.[4]
With few clouds, a lot of sunshine reaches the ground, so days are hot. At night, dry desert air lets about 90% of the day's heat escape back to the sky, compared with about half in moister places. Hot deserts average about 38 °C by day and about −4 °C at night.[4][2]
Desert plants save water in different ways. Cacti store water in their stems; some bushes grow only a few leaves or have huge root systems; and some small plants, called ephemerals, sprout, flower and make seeds within a few weeks after rain. Cacti grow wild in the Americas, while in Africa some Euphorbia plants look very similar.[2][6]
Desert animals avoid the heat and save water too. The kangaroo rat can survive without ever drinking: it gets water from the seeds it eats, sleeps in a burrow by day and does not sweat or pant. The Namib desert beetle collects drops of fog, and camels can go for weeks without water.[5][3]
Try it: Does shape help a plant save water? A fair test
You need
- Three identical paper towels
- Measuring spoon or syringe and a cup of water
- Kitchen scale (1 g or better)
- Three plates
- Sticky tape
- A warm, sunny spot
Safety
Use only water. Wipe up spills straight away. Keep the scale dry.
What to do
- Label three plates A, B and C. Wet each towel with exactly the same amount of water (for example 4 teaspoons).
- A: lay the towel open and flat (a thin leaf). B: fold it in half twice (a smaller leaf). C: roll it into a tight tube and tape it (a cactus stem).
- Weigh each plate with its towel and write down the masses.
- Put all three plates side by side in the same warm place.
- Weigh them again after 1 hour and after 3 hours.
- Work out how much water each towel lost. Which shape saved the most water?
What happens, and why?
Water evaporates from the surface that is open to the air. The flat towel has the most surface, so it loses water fastest; the rolled 'cactus' has the least surface for the amount of water inside, so it keeps its water longest. Cacti store water in thick stems and use it slowly, and desert bushes with few leaves lose less water. It is a fair test because only the shape changes: same towel, same water, same place, same time.[2]
Play: Desert survival tricks
Is each trick about saving water, avoiding the heat, or finding water? Sort them.
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Quiz
Other ages: Ages 6–7 · Ages 15–16