Primary circulation of a hurricane with the force balance. The pressure gradient force (green line in this graphic) is directed to the left of the wind towards the low pressure center in the eye. The Coriolis force (purple line) and centrifugal force (orange line) are directed to the right of the wind away from the eye. The balance of these three forces is called gradient wind balance. Close to the earth's surface, a fourth force, friction (red line), opposes the gradient wind. Friction slows down the wind, which reduces the Coriolis force and the centrifugal force but does not affect the pressure gradient force. As a result, friction causes the wind near the earth's surface to be angled inward towards the eye of the hurricane, contributing to the hurricane's secondary circulation.