Hurricanes form over warm ocean waters and require specific conditions including warm temperatures, low pressure, high humidity, and the Coriolis effect. They can produce dangerous storm surges that are the most damaging part of a hurricane. Thunderstorms form from cumulus clouds and can produce tornadoes, hail, lightning and heavy rain. Flash floods occur when rainwater cannot drain quickly from an area, while river floods happen when a river overflows its banks due to heavy rainfall.