What Causes Wind? Pressure, Sunshine and a Spinning Earth

You cannot see wind, but you can see what it does: leaves skittering across the pavement, waves building on a lake, a kite pulling at its string. It can be a gentle breeze on a summer evening or a storm strong enough to tear roofs off houses. Yet it all comes from one simple idea: air moves to balance out differences in pressure.

Air has weight and pressure

The air around us may feel like nothing, but it has mass. The atmosphere presses down on every square metre of Earth’s surface with the weight of about 10 tonnes. This is air pressure, and it is not the same everywhere.

Where air is sinking and piling up, pressure at the surface is high. Where air is rising, pressure at the surface is low. Like water flowing downhill, air moves from high pressure towards low pressure. That movement is wind. The bigger the pressure difference over a given distance, the stronger the wind.

The Sun heats Earth unevenly

The ultimate engine of the wind is the Sun. It does not warm our planet evenly.

• The equator receives much more direct sunlight than the poles.
• Land heats up and cools down faster than water.
• Dark surfaces such as forests and fields absorb more heat than snow or ice.

Air above a warm surface heats up, expands, becomes lighter and rises. This leaves lower pressure behind at the surface. Cooler, denser air nearby sinks and flows in to replace it.

A small example: the sea breeze

You can feel this on almost any sunny beach. In the morning, land and sea have similar temperatures. As the day goes on, the sand and soil heat up much faster than the water.

The warm air over the land rises, and cooler air from over the sea flows in to take its place: a sea breeze. At night the land cools faster than the sea, the pattern flips, and a gentler land breeze blows out to sea. Mountains and valleys create similar daily winds, flowing uphill by day and downhill at night.

The spinning Earth bends the wind

If Earth did not rotate, air would flow straight from high to low pressure. But Earth spins, and that changes everything.

As air moves across the surface of a rotating planet, its path appears to curve. This is the Coriolis effect. In the Northern Hemisphere, moving air is deflected to the right; in the Southern Hemisphere, to the left.

The result is that air does not rush straight into a low-pressure area. Instead it spirals around it. In the Northern Hemisphere, winds circle anticlockwise around lows and clockwise around highs. This is why weather maps are full of swirling patterns, and why hurricanes, typhoons and cyclones all spin.

Global winds: a planet-sized machine

On a planetary scale, the same rules create belts of winds that circle the globe. Warm air rises near the equator, flows towards the poles high in the atmosphere, cools and sinks around 30° north and south. This loop is called the Hadley cell.

• Near the surface, air flowing back towards the equator is bent by the Coriolis effect into the steady trade winds, which once carried sailing ships across the oceans.
• Between about 30° and 60°, the prevailing winds blow from the west: the westerlies, which bring much of Europe’s weather from the Atlantic.
• Near the poles, cold, sinking air creates the polar easterlies.

High above, at about 9 to 12 kilometres, narrow rivers of fast-moving air called jet streams flow where warm and cold air masses meet. They can reach more than 300 km/h and steer storms across continents.

Local winds with names

Many regions have winds so familiar that they have their own names. In Turkey, the cold north-easterly poyraz and the warm, gusty south-westerly lodos shape life around the Sea of Marmara. In the Alps, the warm, dry Föhn blows down the mountainsides. Each is a local result of the same physics: pressure differences, terrain and the spinning Earth.

In short

• Wind is air moving from high to low pressure.
• Pressure differences come from the Sun heating Earth unevenly: equator vs poles, land vs sea.
• Sea breezes show the process on a small scale.
• The Coriolis effect bends winds to the right in the Northern Hemisphere, making them spiral around highs and lows.
• On a global scale this creates trade winds, westerlies and jet streams.