Physical Geography I: Climate, Biomes, and the Global Circulation
Why do deserts sit at 30° latitude? Why is Iceland warmer than you'd expect at its latitude? Why does South Asia have a distinct wet season? Every one of these physical-geography 'why' questions comes down to a few atmospheric and oceanic patterns. Learn the mechanics once and you can answer them all.
Latitude first: the sun energy pattern
Earth's surface receives different amounts of solar energy at different latitudes — most at the equator (sun overhead), least at the poles (sun always at a low angle). This one physical fact drives most of what follows.
Global pressure belts
Warm equatorial air rises, cools, and descends around 30° north and south — creating subtropical high-pressure belts. This descending dry air is why most of the world's great deserts (Sahara, Kalahari, Arabian, Australian, Sonoran) sit around 30° latitude. It is not a coincidence.
Air also rises near 60° and descends near the poles, producing polar high pressure. The resulting pattern:
- 0°: equatorial low pressure — warm, wet, rainforests
- 30°: subtropical high pressure — descending dry air, deserts
- 60°: subpolar low pressure — mixing zone, temperate storms
- 90°: polar high pressure — cold dry air, tundra
Prevailing wind belts
Air flowing from high to low pressure creates the prevailing wind belts. The Coriolis effect deflects them — right in the Northern Hemisphere, left in the Southern:
- Trade winds (0°-30°) — blow east to west. Powered the historic Manila Galleon and helped sailing to the Americas.
- Prevailing westerlies (30°-60°) — blow west to east. Push maritime air onto west coasts of continents at middle latitudes — mild wet climates in Ireland, Norway, Pacific Northwest, southern Chile, New Zealand.
- Polar easterlies (60°-90°) — cold winds from the poles.
Ocean currents — climate moderators
Warm ocean currents (Gulf Stream, North Atlantic Drift, Kuroshio) push warm water toward higher latitudes on west sides of ocean basins, moderating coastal climates. Cold currents (California, Humboldt, Benguela) do the opposite — chilling coastlines even in tropical latitudes. Coastal deserts (Atacama in Chile, Namib in Namibia) sit downwind of cold currents.
The monsoon system
Continents heat and cool faster than oceans. In summer, the Asian landmass heats up faster than the Indian Ocean — creating low pressure over land and drawing in moist ocean air (the wet monsoon). In winter, the pattern reverses — dry continental air flows out toward the warmer ocean. This gives South and Southeast Asia a distinct wet season (June-September) and dry season (October-May). A billion people plan their agriculture around this pattern.
Rain shadows
Moist air pushed up a mountain range cools, condenses, and drops precipitation on the windward side. The air that descends the leeward side is dry and warms — creating a rain shadow. The Great Basin of the western US (leeward of the Sierra Nevada) and the Patagonian steppe (leeward of the Andes) are canonical examples.
Biomes follow climate
Combine temperature and precipitation and you get characteristic biomes:
- Tropical rainforest — warm, wet year-round. Amazon, Congo, Southeast Asia.
- Tropical savanna — warm, seasonal rainfall (wet/dry). Sub-Saharan Africa, Brazilian cerrado.
- Desert — arid. Sahara, Arabian, Australian, Sonoran.
- Mediterranean — mild wet winters, dry summers. Around Mediterranean Sea, California, central Chile, southwestern Cape of Africa, southern Australia. Olives, grapes, wheat.
- Temperate deciduous forest — four seasons, moderate precipitation. Eastern US, Western Europe, East Asia.
- Temperate grassland — mid-continental, cold winters, warm summers. US Great Plains, Ukrainian steppe, Argentine pampa.
- Boreal (taiga) — cold, coniferous. Canadian Shield, Siberia, Scandinavia.
- Tundra — permafrost, low-growing vegetation. Arctic margins.