Hurricanes require a warm ocean surface (at least 26.5 °C [79.7 °F]), low atmospheric pressure, and low wind shear. They begin as tropical depressions in warm waters off Africa’s coast, where unstable air allows a volume of moisture-laden warm air to rise and cool rapidly, forming thunderstorms. In addition, this region of storms must be at least 500 km (300 miles) north of the Equator so that the Coriolis force can facilitate its rotation.