Why It Rains Where It Rains

Stand on the equator at noon and look up. The sun is nearly overhead, the ocean and land below are warm, and the air sitting on that warmth becomes buoyant and begins to rise. As it climbs, it cools. Cool air cannot hold as much water vapor as warm air, so the moisture it carried up from the sea condenses into droplets, builds towering clouds, and falls back as the heavy afternoon downpours that drench equatorial places like Singapore, the Congo Basin, and the Amazon. This is the simplest answer to the question of why it rains where it rains: warm, moist air goes up, and when air goes up, it eventually rains.

But air that rises somewhere has to come down somewhere else. The air that lifted off the equator spreads outward at high altitude, north and south, and by the time it has traveled roughly thirty degrees of latitude it has cooled, lost most of its moisture, and grown dense enough to sink back toward the surface. Sinking air does the opposite of rising air. As it descends it warms, and warming air can hold more water vapor, so instead of releasing moisture it absorbs it. The ground beneath that descending air dries out. This is not a coincidence of geography but a consequence of the loop: the Sahara, the Arabian Peninsula, the Kalahari, the Australian outback, and the deserts of the American Southwest all sit near thirty degrees north or south, directly under the descending branch of what meteorologists call a Hadley cell.

The pattern repeats. Around sixty degrees latitude, air rises again, which is why Britain, the Pacific Northwest, and southern Chile are damp and green. At the poles, cold dense air sinks, which is why Antarctica, despite its ice, is technically one of the driest places on Earth — almost no precipitation falls there in a given year. The planet is banded: wet, dry, wet, dry, from equator to pole, because of where the great convective loops of the atmosphere happen to rise and sink.

Latitude is only the first cut. Mountains carve the pattern further. When wind pushes moist air against a mountain range, the air has nowhere to go but up, and rising air rains. The windward side of the Cascades, the western slopes of the Andes in Colombia, the southwestern face of the Himalayas — all soak. On the far side, the now-dry air descends, warms, and parches the land. Eastern Washington, Patagonia, and the Tibetan Plateau are the rain shadows of those same mountains. A traveler can cross a single ridge and move from rainforest to near-desert in an afternoon.

Oceans matter too. Warm ocean currents feed moisture into the air above them, which is why the eastern coasts of continents at middle latitudes tend to be wetter than the western coasts at the same latitude. Cold currents do the reverse: the cold Humboldt Current along the coast of Peru chills the air above it, suppresses evaporation, and helps make the Atacama one of the driest deserts on Earth, even though it sits beside an ocean.

So the question "why does it rain here and not there" almost always resolves into the same smaller question: is the air above this place rising or sinking, and where did it get its moisture? Equatorial heat lifts air. Subtropical descent dries it. Mountains squeeze it. Ocean currents load or starve it. Rain is not distributed by accident across the globe; it is distributed by the architecture of moving air. Once you can see that architecture, a world map of deserts and rainforests stops looking like a patchwork and starts looking like a diagram.