6.4 How Winds Form

Some days are calm; other days the wind howls hard enough to bend trees and slam doors. What sets the air in motion? The answer is the idea we’ve been building toward: differences in air pressure.

Activity 6.5 — Air finds the low ground Join an inflated balloon to an uninflated one through a straw. Air rushes from the inflated balloon (high pressure) into the empty one (low pressure) — until both reach the same size, when the flow stops. The air moved on its own, from where pressure was high to where it was low, and stopped once the pressures matched.

The Golden Rule of Wind Air moves from a region of high pressure to a region of low pressure. That moving air is wind. And the bigger the pressure difference, the faster the wind blows. (It’s the same reason air rushes out of a punctured tyre — from high pressure inside to low pressure outside.)

Set the two pressures below and see which way the wind blows — and how strong it is.

Which way does the wind blow?
Wind blows from HIGH to LOW pressure; a bigger gap means a stronger wind.
Region A: 1010 hPa
Region B: 1000 hPa

Land breeze and sea breeze

The Coast Breathes In and Out By day, land heats up faster than the sea. The warm air over the land rises, leaving low pressure there, so cooler air blows in from the sea — a sea breeze. By night, the sea stays warmer than the land, so the low-pressure area forms over the sea, and the wind blows from the land — a land breeze. Both are simply air flowing from high to low pressure, driven by uneven heating.

When winds move fast

Activity 6.6 — Balloons that move together Hang two inflated balloons a few centimetres apart and blow hard into the gap between them. Instead of flying apart, they swing toward each other! Blowing creates fast-moving air in the gap, which lowers the pressure there. The higher pressure on the outsides then pushes the balloons together.

Fast Wind = Low Pressure High-speed winds are accompanied by lower air pressure. When a fast wind blows over a roof, the pressure above the roof drops below the pressure inside the house — and if that difference is large and the roof is weak, the roof can be lifted off. This is why, in a storm, it is safer to keep doors and windows open: letting air flow through reduces the pressure difference, helping the roof stay put.