Wind is just moving air, but what gets it moving? The sun. It heats the ground unevenly — a sunny field warms up more than a shady forest or a cool lake. Where the ground is warm, the air above it heats up, puffs out and gets lighter, leaving a low-pressure patch. Where the ground is cool, the air is heavier and presses down harder, a high-pressure patch. Air always slides from the crowded high-pressure side toward the emptier low-pressure side, and that flow of air rushing across is what you feel as wind. Place warm and cool patches in the simulator and watch the arrows show which way the wind blows.
Most people think air simply decides to move and the wind comes from nowhere. In fact air only moves when something pushes it, and the push is always a pressure difference. The sun heats the ground unevenly, that crowds the air unequally, and wind is the air sliding to even it out.
What's actually happening
Wind arrives like it has a mind of its own (still one moment, gusting the next) and it is tempting to think the air simply decides to move. It does not. Air is lazy and obedient; it only moves when something pushes it, and the push always comes down to one thing being unequal: pressure. Make one patch of air more crowded than the patch next door and the crowd will spill toward the gap. That is the whole of wind. The interesting question is what does the crowding, and the answer overhead is the sun.
The sun does not heat the ground evenly. A bare sunlit field soaks up far more than a shaded wood, a dark road more than a pale lake, and that difference passes straight into the air above. Warm air expands, spreads out and grows lighter, so it presses down less — a patch of low pressure. Cooler air stays dense and heavy and presses down more — a patch of high pressure. Now you have two neighbouring columns of air at different pressures, and air, like anything else, flows from where there is more of it to where there is less. It accelerates down the pressure gradient from the high patch toward the low one, and that horizontal rush of air is exactly what you feel on your face. The simulator lets you build it directly: drop a warm patch beside a cool one and the arrows spring up, pointing from high pressure to low, the steeper the difference the faster they run.
At the scale of a field this is the whole story, but blow it up to the size of the planet and one more thing joins in. Because the Earth is spinning, anything moving across its surface gets nudged sideways, the Coriolis effect, so large winds do not fly straight from high to low pressure but spiral around them instead. That is why weather maps show winds circling the big H and L symbols rather than pointing at them, and why storms turn. But strip all that away and the engine underneath is beautifully simple: the sun heats the ground unevenly, that makes pressure differences in the air, and wind is just the air sliding downhill to even them out.
Wind is just air sliding downhill from high pressure to low, and the sun makes the hills by heating the ground unevenly.
- 1Drop a warm patch on one side of the map and a cool patch on the other, then watch the pressure colours and the wind arrows appear.
- 2Move the patches closer together and the arrows speed up — a steeper pressure gradient makes a stronger wind.
- 3Swap which side is warm and watch the whole wind field reverse, because the air always slides from high pressure toward low.
Common questions
The strength depends on how steep the pressure difference is. A bigger contrast between neighbouring high and low pressure patches makes the air accelerate faster, so a steeper pressure gradient gives a stronger wind.
Land heats faster than the sea, so air rises over the warm land and cooler sea air flows in to fill the gap. The pattern reverses at night when the land cools more quickly.
Because the Earth is spinning, the Coriolis effect nudges moving air sideways, so large-scale winds circle pressure centres rather than pointing straight at them. It is why storms spin and weather maps show winds curving around the H and L symbols.