Air Pressure and Wind

The ocean of air you live in

You live at the bottom of a deep ocean — not of water, but of air. The atmosphere may feel like nothing, but air is made of real molecules that have weight. The whole column of air stretching from your head up to space presses down on you. We call that push air pressure.

At sea level, this push is surprisingly strong: about 10 tonnes of force on every square metre. You do not get crushed because air pushes on you equally from all sides, and your body pushes back just as hard. But when air pressure is uneven from place to place, something important happens — the air starts to move, and that movement is wind.

To understand how the atmosphere is built up in layers above you, see The Layers of the Atmosphere. Here we focus on what that air does near the ground.

High pressure and low pressure

Air pressure is not the same everywhere. In some places the air is pressing down harder (high pressure); in others it is pressing down less (low pressure). Why the difference?

It comes back to the Sun. The Sun heats the ground unevenly. Where the surface is warm, the air above it heats up, expands and becomes lighter, so it rises. With air rising away, less air is left pressing down at the surface — this creates low pressure.

Where the surface is cool, the air is denser and sinks. More air piles up and presses down — this creates high pressure.

So warm spots tend to make low pressure, and cool spots tend to make high pressure. This uneven heating is the original cause of almost all wind on Earth.

Wind: air flowing downhill in pressure

Here is the key idea. Air, like water, always tries to flow from where there is more to where there is less. Air flows from high pressure toward low pressure, trying to even things out. That flowing air is wind.

Think of blowing up a balloon and letting go: the high-pressure air inside rushes out into the lower-pressure room. The same thing happens across the whole planet, just more gently.

The bigger the pressure difference over a given distance, the harder the air rushes, and the stronger the wind. When forecasters draw lines of equal pressure (called isobars) on a weather map, tightly packed lines mean a steep pressure "slope" and strong winds.

Why the wind curves: the Coriolis effect

You might expect wind to blow in a straight line from high to low pressure. But the Earth is spinning, and this bends the path of moving air. The deflection is to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This is the Coriolis effect.

Because of it, winds spiral. Around a low-pressure centre, air spirals inward and upward, cooling and forming clouds and rain. Around a high-pressure centre, air spirals outward as it sinks, giving calm, clear skies.

What pressure tells us about the weather

This is why air pressure is one of a forecaster's most useful clues:

• Low pressure brings rising air, which cools and condenses into clouds. Expect clouds, wind, and often rain or storms.
• High pressure brings sinking air, which warms and dries. Expect calm, settled, often clear weather — sunny in summer, but frosty and foggy on still winter nights.

A falling barometer often warns that low pressure and unsettled weather are on the way. A rising barometer suggests improving, calmer conditions. People used home barometers to predict weather long before the internet. To see how warm and cold air masses meet and battle, read about Climate and Weather.

Activity: make a simple barometer

Try this weather-watch experiment to measure pressure changes at home.

1. Stretch a piece of balloon tightly over the open top of a glass jar and hold it firm with a rubber band, sealing the air inside.
2. Tape a drinking straw flat across the top of the stretched balloon, so most of the straw points out to the side like a needle.
3. Stand the jar next to a wall and tape a small card behind the straw's tip. Mark where the straw points.
4. Watch over several days. When outside air pressure rises, it presses the balloon down and the straw tip lifts. When pressure falls, the trapped air pushes the balloon up and the straw tip drops.
5. Each day, mark the straw's position and note the weather.

After a week, look for a pattern: does the straw dropping (falling pressure) come before cloudy, rainy days? You have built a real instrument and used it to forecast the weather, just like meteorologists do.