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Earth Science
How Deserts Form
Hadley cells, rain shadows, and cold currents
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Deserts form where an area loses more water to evaporation than it receives from precipitation. They are not always hot, but they are always dry, usually receiving less than 250 mm of precipitation per year. Understanding deserts helps explain global climate patterns, water scarcity, ecosystems, and why places like the Sahara, Atacama, and Mojave are so dry. Desert formation is controlled by air movement, mountains, ocean currents, distance from oceans, and solar heating.
Key Facts
- A desert is commonly defined as a region receiving less than 250 mm of precipitation per year.
- Hadley cells move air upward near the equator and downward near 30°N and 30°S, creating many subtropical deserts.
- Rising air cools and can form clouds, while sinking air warms and becomes drier.
- Relative humidity decreases when air warms if the amount of water vapor stays the same.
- Rain shadow deserts form when mountains force moist air upward, causing precipitation on the windward side and dry air on the leeward side.
- Cold ocean currents cool air near the coast, reducing evaporation and cloud growth, which helps form deserts such as the Atacama.
Vocabulary
- Desert
- A desert is a region that receives very little precipitation, usually less than 250 mm per year.
- Hadley cell
- A Hadley cell is a large atmospheric circulation pattern in which warm air rises near the equator, moves poleward, sinks near 30 degrees latitude, and flows back toward the equator.
- Rain shadow
- A rain shadow is a dry region on the leeward side of a mountain range where descending air has lost much of its moisture.
- Cold ocean current
- A cold ocean current is a flow of cool seawater that can reduce evaporation and help create dry coastal climates.
- Continental interior
- A continental interior is a land area far from oceans where air often contains less moisture and rainfall can be limited.
Common Mistakes to Avoid
- Thinking all deserts are hot, which is wrong because deserts are defined by dryness, not temperature. Cold deserts can exist in places such as Antarctica and parts of Central Asia.
- Forgetting that sinking air becomes warmer and drier, which is wrong because descending air compresses and its relative humidity drops. This is a major reason deserts occur near 30°N and 30°S.
- Assuming mountains make both sides wetter, which is wrong because mountains often create wet windward slopes and dry leeward rain shadows. The Mojave Desert is partly shaped by this effect.
- Blaming desert formation on one cause only, which is wrong because deserts often form from several processes working together. The Atacama is extremely dry because of cold currents, sinking air, and mountain barriers.
Practice Questions
- 1 A climate station records 18 mm, 9 mm, 12 mm, 5 mm, 0 mm, 2 mm, 1 mm, 0 mm, 3 mm, 8 mm, 15 mm, and 22 mm of precipitation in one year. What is the annual precipitation, and does it meet the common desert definition of less than 250 mm per year?
- 2 Two locations are compared: Location A is at 30°N and receives 180 mm of precipitation per year, while Location B is near the equator and receives 1600 mm per year. How much more precipitation does Location B receive, and which location is more likely affected by descending Hadley cell air?
- 3 Explain why the leeward side of a mountain range can become a desert even when the windward side has forests and frequent rainfall.