Climate Systems: Why the World Has Different Weather Zones

London and Ulaanbaatar (the capital of Mongolia) sit at almost exactly the same latitude — roughly 51–48°N. Yet London rarely dips below −5°C in winter, while Ulaanbaatar regularly hits −30°C. How can two cities so close on a globe feel worlds apart? Think about what could make such a massive difference. Jot down two ideas before reading on.

**What is a climate system?** Weather is what happens outside today. Climate is the long-term pattern of weather in a place — usually measured over 30 years. A climate system is the set of forces that produce and maintain those patterns across the whole planet. **The engine: solar energy and latitude** The Sun is the engine of every climate system. Because Earth is a sphere, sunlight strikes the equator almost straight on, concentrating its energy. Near the poles, the same amount of sunlight spreads over a much wider area, so it heats the surface far less. This is why equatorial regions are hot and polar regions are cold. Latitude — your distance north or south of the equator — is the single biggest control on climate. **Atmospheric circulation: the Hadley, Ferrel, and Polar cells** Hot air at the equator rises, cools, and sinks back down around 30° north and south — creating the subtropical high-pressure belts. This circulation loop is called a Hadley cell. Between 30° and 60° latitude, the Ferrel cell moves air poleward. Beyond 60°, the Polar cell pushes cold air back toward the mid-latitudes. These cells drive the world's prevailing winds: the trade winds in the tropics, the westerlies in the mid-latitudes, and the polar easterlies near the poles. **The major climate zones** Geographers group climates into five main zones using the Köppen classification system: 1. **Tropical** (near the equator) — hot and wet all year; home to rainforests. 2. **Arid/Desert** (near 30° N and S) — very dry, hot days, cold nights. 3. **Temperate** (30°–60° N and S) — mild seasons, moderate rainfall. 4. **Continental** (inland, mid-latitudes) — extreme seasonal temperatures. 5. **Polar** (above 60° N and S) — permanently cold. **Ocean currents and the maritime effect** Oceans store and transport heat. Warm ocean currents — like the North Atlantic Current — carry tropical heat toward Europe, making coastal climates milder than inland ones at the same latitude. Ulaanbaatar is deep in a vast continent, cut off from the ocean's moderating effect. London sits on the Atlantic fringe and receives warm, moist air all year. This explains our opening puzzle. **Prevailing winds and orographic rainfall** When moist ocean winds hit a mountain range, they are forced upward. The air cools and the moisture falls as rain on the windward side. The sheltered leeward side receives far less rain — this dry area is called a rain shadow. The Scottish Highlands and the Atacama Desert in South America are both products of this effect.