Wind & air pressure

You cannot see wind, but you feel it on your face, hear it in the trees and watch it lift kites into the sky. What is blowing there is nothing but moving air. But why does it move at all? The answer lies in two words you know from every weather report: high and low.

Warm air rises, cool air sinks

The sun warms the ground, and the ground warms the air above it. Warm air expands, becomes lighter and rises. Where air rises, less air presses down on the ground: a low pressure area forms. Cool air, on the other hand, is heavier and sinks. Where it piles up near the ground, more air presses down: a high pressure area. Air pressure is measured in hectopascals, hPa for short: at sea level it averages about 1013 hPa, a strong high reaches more than 1030 hPa, a deep low less than 990 hPa. And now comes the key step: air always flows from the high to the low to even out the difference. This balancing flow is exactly what wind is. The bigger the pressure difference, the stronger it blows.

The wind belts of the Earth

What happens on a small scale on a summer day happens on a grand scale across the whole Earth. At the equator the sun stands almost directly overhead and heats the air enormously: there it rises constantly, forming a band of low pressure. High up, the air flows towards the north and south, cools down and sinks again in the subtropics around 30 degrees latitude. Near the ground it flows back towards the equator. Because the Earth turns underneath the moving air, these flows do not run straight north or south but at a slant. This creates huge circulation loops that divide the Earth into wind belts: the reliable trade winds in the tropics, which blow towards the equator from the north east and south east, and the westerlies in our latitudes, which bring us the changeable weather from the Atlantic.

Wind as a driving force

Wind drives the weather: it pushes clouds and entire rain fronts across countries and seas. And people have used this force for thousands of years. Sailing ships followed the trade winds across the oceans long before engines existed, and the name of these winds goes back to an old word for a steady passage, because they carried ships across so reliably. Windmills ground grain, and today wind turbines generate electricity from the very same moving air.