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There is no single definition of a desert. In Meigs' classification, extremely arid land has at least 12 months in a row without rain, arid land less than 250 mm a year and semiarid land 250–500 mm (usually grassland steppe). Using different definitions, the USGS says about one-third of Earth's land is desert and National Geographic more than one-fifth: the same evidence can give different numbers when the categories differ.

Many deserts lie near 30° north and south, where air that rose near the equator has already lost much of its moisture and high pressure keeps skies clear. In such places at least 90% of the Sun's rays reach the surface, while humid lands receive only about 40% of the possible solar radiation. Rain-shadow deserts form downwind of mountains that remove moisture from rising air.

Desert water budgets are strongly negative: evaporation in American deserts can be 70–160 inches a year (about 1,800–4,100 mm), far more than the rain that falls. Rainfall depth converts directly to volume: 1 mm on 1 m² is 1 litre, so Cairo's 28 mm a year is about 28 litres per square metre, and the driest Atacama gets less than 2 litres.

Experiment: Surface area and the rate of water loss

1. Add exactly 10.0 ml of water to each towel on its own dish.
2. A: lay flat. B: roll into a tight cylinder and tape it. C: roll into a cylinder and wrap the curved side in cling film, leaving only the two ends open.
3. Estimate the exposed surface area of each (A: both faces of the sheet; B: curved surface plus ends; C: the two ends only) with a ruler.
4. Record the mass of each dish and towel at t = 0, then every 15 minutes for 90 minutes. Keep all three in the same place, away from heaters.
5. Plot mass against time and find the rate of water loss (g/h) from the gradient.
6. Calculate the rate per 100 cm² of exposed surface and estimate the uncertainty from the balance and your area measurement.

Towel	Exposed area (cm²)	Mass at 0 min (g)	Mass at 90 min (g)	Rate (g/h)	Rate per 100 cm² (g/h)
A Flat	Prediction
B Rolled	Prediction
C Rolled and wrapped	Prediction

Questions

1. Convert the National Park Service evaporation range of 70–160 inches a year into mm (1 inch = 25.4 mm). How many times greater is the top of the range than the 250 mm arid limit?

2. A wet towel falls from 24.0 g to 15.6 g in 90 minutes. Its exposed area is 300 cm². Calculate the rate of water loss in g/h and in g/h per 100 cm².

3. Use an energy budget to explain why a desert can be 38 °C by day and below freezing at night.

4. One source says a third of Earth's land is desert, another more than a fifth. How should a scientist report this?

Sources for every fact on this sheet: echoesof.science/school/deserts/sources/

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