Several atmospheric factors, such as warm ocean water and moist mid-atmosphere, must meet in order for large amounts of heat energy to move northward in the tropics' higher latitudes, forming typhoons.
Tropical cyclones with maximum sustained surface winds of less than 39 mph are referred to as tropical depressions. Tropical storms occur when the wind speed exceeds 39 mph and if the wind speed reaches 74 mph the tropical storm is referred to as a typhoon.
The northwest Pacific basin has tropical cyclones occurring regularly all year round. Though there is no official typhoon season, the peak typhoon activity is in late August to early September and typhoons are least active in February and the first half of March.
Typhoons work as engines that obtain heat from the warm, humid air over the tropical ocean and release this heat, through condensation of water vapor, into water droplets causing severe thunderstorms.
Typhoon Nancy hit the northwest Pacific Ocean on Sept. 12, 1961 with an estimated maximum sustained speed of 213 mph.