What is Buys Ballot's law?

The law states how the location of pressure systems can be deduced from the wind direction in the Northern Hemisphere.

"If one stands with his back to the wind, atmospheric pressure is higher to his right and lower to his left" (restated: a high pressure system may be to your right, or a low pressure system to your left).

This is because in the Northern Hemisphere, the rotation of high pressure systems is clockwise and in low pressure systems, it is counterclockwise.