Air pressure is the force of air molecules pushing on everything around them due to gravity. At sea level, the atmosphere presses on every surface with a force of about 14.7 pounds per square inch (101.3 kPa) — we do not feel it because our bodies push back equally. Warm air is lighter and rises, creating areas of low pressure. Cool air is heavier and sinks, creating areas of high pressure. Wind is air flowing from high-pressure areas to low-pressure areas to equalize the difference. The greater the pressure difference, the stronger the wind.
Place a ruler hanging over the edge of a table with a full sheet of newspaper over the part on the table — try to flip the ruler by pressing the overhanging end. The newspaper is surprisingly hard to lift because air pressure pushes down on its entire surface. Use a candle under a pinwheel or near a gap under a door to demonstrate convection currents. Weather maps showing H (high) and L (low) pressure systems with wind arrows make the concept visual and practical.
You might not realize it, but right now the atmosphere is pressing on every square centimeter of your body. At sea level, that pressure is about 101,325 pascals — roughly the weight of a one-kilogram object on every square centimeter. We do not feel it because the pressure pushes equally from all directions — up, down, sideways — and our bodies have adapted to push back with the same force. But this invisible pressure is the driving force behind all wind and weather.
Air pressure is simply the weight of all the air molecules above a given point, pulled downward by gravity. At sea level, you have the full column of atmosphere — over 100 kilometers of air — sitting above you. Climb a mountain, and there is less air above you, so the pressure drops. This is why your ears pop on an airplane and why climbers at high altitudes struggle to breathe — there are fewer air molecules in each breath.
The key to understanding wind is uneven heating. The Sun does not heat Earth's surface evenly — land heats faster than water, dark surfaces absorb more heat than light surfaces, and the equator receives more solar energy than the poles. Where the ground is warm, it heats the air above it. This warm air expands, becomes less dense (lighter), and rises. As it rises, it leaves behind a zone of low pressure — there are fewer air molecules pressing down at the surface. Meanwhile, where the ground is cool, the air above it is denser (heavier) and sinks, creating a zone of high pressure — more molecules pressing down.
Wind is simply air flowing from high pressure to low pressure, trying to equalize the difference. Think of letting air out of a balloon — the high-pressure air inside rushes toward the lower-pressure air outside. The atmosphere works the same way, just on an enormous scale. The greater the pressure difference between two locations, the faster the air flows and the stronger the wind. This is why weather maps show pressure systems — the spacing of the pressure lines (isobars) tells you how strong the wind will be. Closely packed lines mean steep pressure gradients and strong winds; widely spaced lines mean gentle breezes.
This simple principle — air heated unevenly creates pressure differences, and pressure differences create wind — is the engine that drives everything from a gentle afternoon sea breeze to the planet's great wind belts and violent hurricanes.
No topics depend on this one yet.