Tropical Cyclones are spiralling storm systems, with low-pressure centres. They form over warm waters, near the Equator. A tropical cyclone needs two things to form: a thunderstorm and warm water. The warm, tropical ocean is evaporated by the storm, which then condenses to form clouds. The heat that is released when the water evaporates, combines with the rotation of the Earth, which gets the cyclone spinning. The cyclone's low-pressure centre, or the 'eye', is calm. However this is just an illusion, because the cyclone's wall is far from safe. The 'eye' is normally about 40 km in diameter, however they can vary from as small as 10 km, to well over 100 km.