Wind is air moving from where there is more of it to where there is less. That is nearly the whole answer, and it would be the whole answer if the planet were not spinning.
Uneven heating makes uneven pressure
The Sun does not warm the Earth evenly. The tropics receive sunlight close to head-on and the poles receive it at a glancing angle spread over more surface. Land heats faster than sea. A clear field heats differently from a forest.
Warm air expands, becomes less dense, and rises, which lowers the pressure at the surface beneath it. Cool air sinks, which raises it. Within hours you have regions of higher and lower pressure sitting next to each other.
Air, being a fluid, does not tolerate that. It accelerates from the high towards the low. The force driving it is called the pressure gradient force, and its strength depends on how sharply pressure changes over distance, which is exactly what the spacing of isobars on a weather map shows you.
The sea breeze, which is the whole thing in miniature
Stand on a beach on a sunny afternoon and you can watch this happen at a scale you can hold in your head.
Water has a large heat capacity and mixes heat down through its depth, so the sea warms slowly. Land has neither property, so its surface warms quickly. By early afternoon the air over the land is distinctly warmer, it rises, pressure at the surface falls, and air flows in from the sea to replace it. That is the onshore breeze, and it strengthens as the temperature difference grows.
At night it can run backwards. Land loses heat faster than water, so the sea is now the warmer surface and the breeze blows offshore.
Then the planet turns underneath it
If that were the end of it, wind would blow straight from high pressure to low, and weather maps would be simple. They are not, because the Earth is rotating and the air is not attached to it.
A parcel of air moving across the surface travels in a fairly straight line through space while the ground beneath it turns. To anyone standing on that ground, the air appears to curve. This apparent deflection is the Coriolis effect: to the right in the northern hemisphere, to the left in the southern.
It is not a force in the ordinary sense. It is what a straight line looks like when you are watching from a rotating platform. It is also zero at the equator and strongest near the poles, and it grows with the speed of the air, so it matters most for fast-moving air over long distances.
Which is why wind follows the isobars
Now put the two together. Air starts moving directly from high towards low. As it picks up speed the deflection grows, curving it sideways. It keeps turning until the deflection is pulling exactly opposite to the pressure gradient force.
At that point the two are in balance and the air stops turning. It is now travelling at right angles to the pressure gradient, which means it is running parallel to the isobars rather than across them. Meteorologists call this the geostrophic wind, and it is a good description of the wind well above the surface.
This is the counterintuitive part. Air is not flowing from high pressure to low pressure. It is circulating around the low, held in orbit by the balance between the two effects. It is why weather systems on a map are rotating spirals rather than arrows pointing inward.
Near the ground, friction spoils the balance
Close to the surface, air is dragged on by ground, trees and buildings. That slows it, and because the Coriolis deflection depends on speed, slowing the air weakens the deflection.
The balance tips back towards the pressure gradient, and the wind crosses the isobars at an angle instead of running along them, angled inward towards the low. So surface air spirals into a low-pressure system rather than circling it.
That inward spiral has to go somewhere, and the only direction available is up. Rising air cools, water vapour condenses, and cloud forms. This is why low pressure is associated with unsettled weather, and why high pressure, where air is sinking and spreading outward, brings clear skies.
It is also the basis of an old sailor’s rule: stand with your back to the wind in the northern hemisphere, and low pressure is on your left.
Reading a forecast map
Three things follow directly. Tightly packed isobars mean strong wind. Wind runs roughly along the isobars, not across them. And a low on the map means air converging and rising, which means cloud and rain.
More atmosphere
For what the same atmosphere does to sunlight, why the sky is blue and not violet. For a large-scale rhythm driven by something outside the Earth entirely, why the tide comes in twice a day.
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