Why are the great deserts found near 30 degrees latitude and the rainforests at the equator?
Topic 4.5 Global Wind Patterns: explain how uneven solar heating and the Coriolis effect drive atmospheric circulation cells and global wind belts.
A focused answer to APES Topic 4.5, covering uneven solar heating, convection and the Hadley, Ferrel and polar cells, the Coriolis effect, the trade winds and westerlies, and why deserts and rainforests sit where they do, with a worked latitude-climate question.
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What this topic is asking
The College Board (Topic 4.5) wants you to explain how uneven solar heating and the Coriolis effect create global atmospheric circulation cells and wind belts, and how these explain the location of deserts and rainforests.
Uneven heating
The circulation cells
The Hadley cell explains the great climate bands: air rises at the equator, cools and drops heavy rain (tropical rainforests), then descends as dry air near 30 degrees, creating the world's major deserts (such as the Sahara). It then flows back toward the equator at the surface as the trade winds.
The Coriolis effect
The Coriolis effect bends the surface flows into the named wind belts: the trade winds (blowing toward the equator from the east) and the westerlies (blowing toward the poles from the west).
Why this matters
Global wind patterns set the climate map that the rest of the course depends on. They explain why terrestrial biomes (Topic 1.2) are arranged in latitude bands, why deserts and rainforests sit where they do, and how pollutants and weather systems move. Combined with the seasons (Topic 4.7) and geography (Topic 4.8), they account for regional climate, rainfall and the distribution of life.
Try this
Q1. Identify the circulation cell that operates between the equator and about 30 degrees latitude. [1 point]
- Cue. The Hadley cell.
Q2. Explain why the equator receives more solar energy per unit area than the poles. [2 points]
- Cue. At the equator sunlight strikes nearly vertically and is concentrated on a small area; at the poles it strikes at a low angle and spreads over a larger area, so each unit of surface receives less energy.
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2021 (style)4 marksSection II (FRQ). (a) Explain why the equator receives more solar energy per unit area than the poles. (b) Describe what happens to warm air that rises at the equator. (c) Explain why major deserts are commonly located near 30 degrees north and south. (d) Describe the Coriolis effect and its influence on wind direction.Show worked answer →
A 4-point FRQ on global wind patterns.
(a) Explain (1 point): sunlight strikes the equator nearly vertically, concentrating energy on a small area, while at the poles it strikes at a low angle and spreads over a larger area, so the equator is heated more.
(b) Describe (1 point): warm, moist air rises at the equator, cools, and releases its moisture as heavy rain (creating rainforests), then moves toward the poles aloft.
(c) Explain (1 point): that air descends near 30 degrees as dry air (having lost its moisture), warming and absorbing moisture, which produces the dry conditions of major deserts.
(d) Describe (1 point): the Coriolis effect is the apparent deflection of moving air caused by Earth's rotation, curving winds to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
Markers reward the angle-of-incidence reason, rising moist air giving equatorial rain, descending dry air giving deserts at 30 degrees, and the rotation-driven deflection for the Coriolis effect.
AP 2019 (style)1 marksSection I (multiple choice). The descending limb of the Hadley cell, near 30 degrees latitude, is associated with which climate condition? (A) Heavy rainfall and rainforests (B) Dry conditions and deserts (C) Constant snowfall (D) Strong ocean upwelling. Justify your choice.Show worked answer →
A 1-point MCQ on circulation cells. The answer is (B).
Near 30 degrees, air that rose at the equator and lost its moisture descends; sinking air warms and holds moisture rather than releasing it, producing dry conditions and the world's major deserts. Heavy rain and rainforests (A) occur where air rises at the equator; (C) and (D) are unrelated to this descending limb. The trap is associating any circulation with rain; descending air suppresses rainfall.
Related dot points
- Topic 4.4 Earth's Atmosphere: describe the composition of the atmosphere and the four main layers, and explain how temperature changes with altitude.
A focused answer to APES Topic 4.4, covering atmospheric composition, the four layers (troposphere, stratosphere, mesosphere, thermosphere), the temperature profile, the ozone layer, and the role of the atmosphere in weather and protection, with a worked composition calculation.
- Topic 4.7 Solar Radiation and Earth's Seasons: explain how the tilt of Earth's axis and its orbit produce variations in insolation that cause the seasons.
A focused answer to APES Topic 4.7, covering solar radiation (insolation), the 23.5 degree axial tilt, the solstices and equinoxes, the angle of incidence, why the tilt and not distance causes the seasons, and latitude effects, with a worked insolation question.
- Topic 4.8 Earth's Geography and Climate: explain how geographic features such as mountains and proximity to water shape regional climate, including rain shadows and El Nino and La Nina.
A focused answer to APES Topic 4.8, covering how mountains, latitude, ocean currents and proximity to water shape regional climate, the rain shadow effect, and the El Nino and La Nina (ENSO) cycle, with a worked rain-shadow question.
- Topic 1.2 Terrestrial Biomes: describe the global distribution of the major terrestrial biomes and explain how temperature and precipitation determine the type of biome found in a region.
A focused answer to APES Topic 1.2, covering the major terrestrial biomes, how temperature and precipitation define them, latitude and altitude patterns, and biome shifts under a changing climate, with a worked climograph question.
- Topic 1.7 The Hydrologic (Water) Cycle: describe the processes of the water cycle and explain how human activities alter the storage and movement of water.
A focused answer to APES Topic 1.7, covering evaporation, transpiration, condensation, precipitation, runoff, infiltration and groundwater, and how deforestation, paving and irrigation alter the cycle, with a worked water-budget calculation.
Sources & how we know this
- AP Environmental Science Course and Exam Description — College Board (2020)