What Is Air Pressure?
Air has weight. Air pressure is how hard all that air is pushing down on a place — and it's never quite the same everywhere.
High Pressure
Air molecules crowd close together and sink toward the ground. High pressure usually means calm, clear, sunny weather.
Low Pressure
Air molecules spread apart and rise upward, cooling as they climb. Low pressure usually means clouds, wind, and storms.
💨 So Where Does Wind Come From?
The atmosphere is always trying to even itself out. Wind is simply air flowing from a high-pressure area toward a nearby low-pressure area, working to balance the difference. The bigger that pressure difference is, the faster the air rushes to close the gap — which means stronger wind.
What Causes Pressure to Differ?
If the whole atmosphere were the same everywhere, there'd be no wind at all. Two big things keep pressure — and wind — constantly changing.
Uneven Heating
The equator gets more direct sunlight than the poles, and land heats up faster than water. Warm air is less dense, so it expands and rises, leaving lower pressure behind. Cool air is denser and sinks, building higher pressure. Because heating is never even, pressure is always shifting from place to place — which is what keeps wind blowing.
Elevation
At sea level, the entire weight of the atmosphere presses down, so pressure is highest there. Climb a mountain and there's less air above you pushing down, so pressure drops the higher you go — part of why it's harder to catch your breath, and why your ears might pop, at high elevations.
How Is Air Pressure Measured?
Meteorologists use a tool called a barometer — a name built from Greek words meaning "weight measure."
📏 The Barometer
The first barometer was invented in 1643 by Evangelista Torricelli. It used a glass tube filled with mercury: air pressure pushed the mercury up the tube, and a taller column meant higher pressure. Most barometers today are aneroid barometers, which use a small sealed metal capsule that flexes as pressure changes, moving a needle on a dial — no liquid required.
Air pressure is usually reported in inches of mercury (in Hg) or millibars (mb). Average pressure at sea level is about 29.92 in Hg, or 1013 millibars. Meteorologists collect readings like these from stations all over the world to draw the H's and L's you see on weather maps.
Pressure Difference & Wind Strength
Drag the slider to change how different the pressure is between these two areas, and watch what happens to the wind.
PRESSURE
PRESSURE
Sea Breeze vs. Land Breeze
Land and water heat up and cool down at different speeds — which creates a pressure difference that flips the wind direction at the coast every day.
Daytime: Sea Breeze
Land heats up much faster than water in the sun. The warm air over the land rises, creating lower pressure at the surface. Cooler air over the water rushes in to take its place — and that incoming wind, blowing from sea to land, is called a sea breeze.
Did You Know?
A few surprising facts about air pressure and wind.
The fastest wind gust ever recorded at Earth's surface was about 253 miles per hour, measured on Barrow Island, Australia during a tropical cyclone in 1996.
Your ears might "pop" in an elevator, an airplane, or driving up a mountain — that's air pressure changing around you as your elevation changes.
Weather maps mark centers of pressure with the letters H and L, helping forecasters track where wind and storms are headed.
Because Earth is constantly spinning, wind doesn't travel in a perfectly straight line — it curves! This is called the Coriolis effect, and it's part of why hurricanes spin the way they do.
Test Your Knowledge
Answer these 4 questions. Get 3 or more correct to master this lesson!