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.
Understanding How Deserts Form
The global circulation pattern begins with uneven heating. Sunlight is strongest near the equator, so the ground and ocean warm the air above them. Warm air expands and becomes less dense, causing it to rise.
As it rises high into the atmosphere, the surrounding pressure drops. The air expands further and cools. Cooling can bring water vapor to its condensation point, forming droplets, clouds, and heavy tropical rain.
Condensation releases stored heat into the air, which helps keep the rising motion going. Higher up, much of this air moves away from the equator toward the subtropics. By the time it sinks, it has already lost a large share of its moisture through rainfall.
Sinking air affects weather in a different way from rising air. It is compressed by greater pressure closer to the surface, which makes it warmer. Warmer air can contain more water vapor before it becomes saturated.
This means its relative humidity falls even when no water vapor is removed. The sinking motion also makes the atmosphere stable. A stable atmosphere resists the upward movement needed for tall clouds and thunderstorms.
This is why subtropical desert regions often have clear skies, intense sunshine, and large daily temperature changes. Bare ground heats rapidly during the day and loses heat rapidly after sunset.
Students should separate temperature from humidity here. Hot air is not automatically dry, but warming air with little added moisture has lower relative humidity.
Mountains can create a sharp contrast across a short distance. Moist winds meeting a mountain slope are forced upward. The rising air cools, and precipitation falls on the side facing the wind.
After crossing the high land, the air descends on the other side. It warms during descent, its relative humidity drops, and cloud formation becomes difficult. The dry region beyond the mountains is called a rain shadow.
The exact result depends on wind direction, mountain height, and how much moisture the air carried before it reached the range. In weather maps, students can trace prevailing winds from an ocean toward mountains to predict which slope is likely to be wetter. This pattern helps explain dry basins in western North America and parts of central Asia.
Cold currents produce a less obvious kind of dryness along some coasts. Water flowing from high latitudes cools the air just above the sea. Cool air reduces evaporation, so the atmosphere receives less water vapor.
It may form low fog when moist air contacts the cold surface, but fog is not the same as rain. The cooled air is often capped below warmer air higher up, preventing strong upward motion and deep rain clouds. Coastal deserts can therefore sit beside an ocean yet receive very little rainfall.
The Atacama shows this effect clearly. Its dryness is strengthened by a cold current, nearby mountains, and large-scale sinking air. Real climates usually have several causes working together, rather than one cause acting alone.
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.