AP® Environmental Science review sheet from Aim for Five (aimforfive.com/enviro/units/4/4-5)
Unit 4 · Topic 4.5
4.5 Global Wind Patterns
Global winds are driven by uneven heating: the equator gets the most direct sunlight, so warm air rises there, cools, drops rain, and sinks around 30° latitude. Earth's rotation deflects moving air (the Coriolis effect), creating the trade winds, westerlies and polar easterlies.
Key terms
- Hadley cell
- Coriolis effect
- trade winds
- westerlies
- polar easterlies
Uneven heating starts it all
Sunlight strikes the equator most directly, so the equator heats up most. Warm air is less dense, so it rises, leaving low pressure at the surface. Cooler, denser air sinks, creating high pressure. Air at the surface flows from high pressure toward low pressure, which is what we feel as wind.
Warm air can also hold more water vapor than cool air. This matters because when air rises and cools, it can't hold as much water, so the vapor condenses into clouds and rain.
Hadley cells and the three-cell model
At the equator, warm, moist air rises. As it rises, the air pressure around it drops, so it expands, and expanding air cools. The cooling makes water vapor condense, so the tropics get heavy rain, which supports tropical rainforests.
High up, the now-dry air flows toward the poles. Around 30° north and south, it sinks back down. Sinking air is compressed and warms, which lets it hold even more moisture, so it rarely produces rain. That's why many of the world's great deserts, like the Sahara, Arabian and Australian deserts, lie near 30° latitude. At the surface, the air flows back toward the equator, completing a loop called a Hadley cell.
Two more loops sit on each side of the equator. Between about 30° and 60° is the Ferrel cell, and between about 60° and the poles is the polar cell. Air rises again near 60°, bringing rain and storms, and sinks at the poles, which are cold, dry polar deserts.
The Coriolis effect
Because Earth spins, air moving across its surface appears to curve. In the Northern Hemisphere, moving air is deflected to the right; in the Southern Hemisphere, to the left. This is the Coriolis effect. Without it, surface winds would blow straight north or south.
The prevailing winds
Winds are named for the direction they come from, not where they're going. A west wind blows from west to east.
Where the trade winds from both hemispheres meet near the equator, there's a zone of rising air, clouds and heavy rain called the intertropical convergence zone (ITCZ). It shifts north and south with the seasons, which causes wet and dry seasons in places like the savanna.
| Wind belt | Latitude | Blows from | Notes |
|---|---|---|---|
| Trade winds | 0°–30° | Northeast (Northern Hemisphere); southeast (Southern Hemisphere) | Surface air returning to the equator, deflected by Coriolis; steady winds used by sailing ships |
| Westerlies | 30°–60° | West (southwest in the Northern Hemisphere) | Why weather in the U.S. generally moves from west to east |
| Polar easterlies | 60°–90° | East | Cold air flowing away from the poles |
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Explaining deserts at 30°
Explain why the Sahara Desert, at about 20–30° N, is very dry, while the Congo rainforest near the equator is very wet.
Show the solutionHide the solution
- Step 1: At the equator, intense sunlight heats the air, which rises, expands and cools. Cooling causes water vapor to condense, producing heavy rain over the Congo.
- Step 2: That rising air, having lost its moisture, flows toward the poles high in the atmosphere.
- Step 3: Around 30° N, it sinks. Sinking air is compressed and warms, so it can hold more moisture and clouds don't form.
- Step 4: Dry, sinking air over the Sahara means very little rain.
Answer: Rising air at the equator cools and drops rain on the Congo; the same dry air sinks and warms near 30° N, preventing rain over the Sahara.
- Example 2
Predicting wind direction
A ship sails at 15° N latitude in the Atlantic. From which direction do the prevailing winds blow, and why?
Show the solutionHide the solution
- Step 1: 15° N is in the Hadley cell, between 0° and 30°, where surface air flows toward the equator (from north to south).
- Step 2: In the Northern Hemisphere, the Coriolis effect deflects moving air to the right.
- Step 3: Air moving south, turned to the right, ends up moving toward the southwest, meaning it comes from the northeast.
- Step 4: These are the northeast trade winds.
Answer: From the northeast (the northeast trade winds), because southward-moving air is deflected to the right by the Coriolis effect.
Common mistakes
- Saying rising air warms up. Rising air expands and cools, which causes condensation and rain.
- Naming winds for where they go. A westerly blows from the west.
- Putting deserts at the equator. Deserts cluster near 30° latitude, where dry air sinks.
- Mixing up deflection directions. Coriolis deflects to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
On the exam
- Expect to explain why a region is wet or dry using rising or sinking air. Include the cooling or warming step, since that's what earns the point.
- Questions may ask how wind patterns affect ocean currents or the movement of air pollution; connect prevailing winds to the direction things travel.
Connected topics
Videos
Check yourself
4 questions on 4.5 Global Wind Patterns. Pick an answer to see if you got it, and why.
Many of the world's large deserts, such as the Sahara, lie near 30° N or 30° S latitude. Which statement best explains this pattern?
Surface air between the equator and 30° N flows toward the equator, yet the trade winds there blow from the northeast. Which statement best explains this?
Across most of the United States, storms and weather systems usually travel from west to east. Which global wind pattern best explains this?
Between the equator and 30° S, surface air flows toward the equator. Because of the Coriolis effect, from which direction do these trade winds blow?
0 of 4 answered