Tropical oceans spawn approximately 80 tropical storms annually, and about two-thirds are severe (category 1 or higher on the Saffir-Simpson scale of intensity). Almost 90 percent of these storms form within 20° north or south of the Equator. Poleward of those latitudes, sea surface temperatures are too cool to allow tropical cyclones to form, and mature storms moving that far north or south will begin to dissipate.
