echoesof.science · Ages 10–11
Deserts: why so dry, and how life copes
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echoesof.science/school/deserts/ages-10-11/
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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.
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.
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.
Experiment: Does shape help a plant save water? A fair test
- 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?
| Towel | Mass at start (g) | After 1 hour (g) | After 3 hours (g) | Water lost (g) |
|---|---|---|---|---|
| A Flat leaf | Prediction | |||
| B Folded leaf | Prediction | |||
| C Rolled cactus | Prediction |
Questions
Give two different reasons why some places become deserts.
Describe one way a desert plant and one way a desert animal survive with little water.
In your fair test, what did you change, what did you measure and what did you keep the same?
Cairo gets about 28 mm of rain a year. The arid limit is 250 mm. Roughly how many times less rain does Cairo get than the limit?
Sources for every fact on this sheet: echoesof.science/school/deserts/sources/