Draw two stripes on a globe, at 30 degrees north and 30 degrees south. You will have traced almost every major desert on the planet. The Sahara in North Africa, the Arabian Peninsula, the Mojave and Sonoran of the American Southwest, the Taklamakan in western China — all on the 30°N band. The Atacama in Chile, the Kalahari and Namib in southern Africa, the Australian Outback — all on the 30°S band.

This isn't coincidence. It's the shape of Earth's atmosphere.

The sun heats the equator hardest. Warm wet air rises, drops its moisture as rain, and spreads toward the poles. By the time it reaches 30° latitude, it has cooled enough to sink. Sinking air is dry air — it has already shed its water somewhere else. When dry air sinks, it compresses and heats up, further drying out whatever is underneath.

The result: a planetary-scale dehumidifier, parked permanently at 30° north and south. Anywhere under those zones gets consistent, predictable, relentless dryness. The circulation is called the Hadley Cell, first proposed by George Hadley in 1735. It's what creates the trade winds sailors depended on, the monsoons of South Asia, and — as a side effect — the desert belts.

There are exceptions. Cold ocean currents create desert conditions where they shouldn't otherwise exist — the Atacama is one of the driest places on Earth partly because the Humboldt Current offshore chills the air. The Gobi is far inland, on the 'wrong' side of the Himalayas — which block rain. But the 30° rule holds for the majority.

Look at any world map. The planet's geography of dryness is dictated by physics you can't see.