Like other tropical cyclones, typhoons form when warm, calm ocean waters transfer warmth and moisture to the air above the surface. The air rises into cooler layers of the atmosphere, allowing the water to condense and the air to fall back down. This sets up a convective current that draws moisture and energy into the clouds and causes them to begin to spin.
The word "typhoon" is a regional term used in areas west of the International Dateline to describe a specific type of severe weather event that the National Oceanic and Atmospheric Administration (NOAA) calls a strong "tropical cyclone." Typhoons have different names in other areas of the world:
- Hurricane: east of the Dateline
- Tropical cyclone: southwest Indian Ocean
- Severe tropical cyclone: southwest Pacific Ocean, southeast Indian Ocean
- Very severe cyclonic storm: north Indian Ocean
Typhoons begin as tropical storms, much in the same way that hurricanes start out as lesser systems. If the storm continues to grow in strength and reaches wind speeds of 74 mph, it officially becomes a typhoon. The Hong Kong weather service further classifies typhoons by wind speed, much in the same way that the National Weather Service recognizes various categories of hurricanes.
Around one-third of the total tropical cyclone activity that occurs each year happens in the western Pacific, making it the most active area of the planet for tropical storms. In particular, the area just northeast of the Philippines is the most meteorologically active region. Unlike Atlantic hurricanes, which tend to occur in a narrow time frame every year, Pacific typhoons can happen year round.
The strongest typhoon as of 2014 was Typhoon Haiyan, which struck the Philippines on Nov. 8, 2013 with sustained wind speeds of 195 mph.