concept

Geostrophic balance

The balance between the pressure-gradient force and the Coriolis force, and the reason large-scale wind blows ALONG the isobars instead of running straight from high pressure to low. This is the counterintuitive part: air does start moving toward the low, but the moment it moves the planet's spin deflects it sideways, and the deflection only stops when the two forces cancel — by which point the flow runs at right angles to where it set out. Buys Ballot's law states this in a form a sailor can use: in the Northern Hemisphere, stand with your back to the wind and the low pressure lies to your left.