What is a typhoon?

A typhoon, tai-fu (台風) in Japanese, is the same weather phenomenon as a hurricane: a tropical cyclone. A tropical cyclone is a generic term for thunderstorms and a system of clouds that originate over tropical or subtropical waters. The weakest tropical cyclones are called “tropical depressions,” and they become “tropical storms” when their maximum sustained winds reach 39 miles (approx. 63 kilometers) per hour (i.e., 17 meters/s). Once their maximum sustained winds reach 74m/h (119k/h), they become either “hurricanes” or “typhoons,” depending on the location where the storms occur.

A typhoon is a storm that occurs between the 180th meridian and 100 east longitude in the Northern Hemisphere (i.e., Northwest Pacific), and one that occurs in the North Atlantic, central North Pacific and eastern North Pacific is called a “hurricane.” As a side note, one that occurs in the South Pacific and the Indian Ocean is called a “tropical cyclone.” Therefore, a hurricane would become a typhoon when it crosses the International Date Line at 180 longitudes.

A typhoon and a tai-fu are different

Strictly speaking, a typhoon and the Japanese word “tai-fu (台風)” differ. In international standards, a storm with a maximum wind speed of more than 64 knots is called a “typhoon.” In contrast, that with a maximum wind speed of more than 34 knots or 17m/s is classified as a “tai-fu” in Japan. Thus, it can be confusing for non-Japanese when the Japanese weather forecast reports tai-fu.

When is the typhoon season in Japan?

The typhoon season ranges from May to October, with August and September being the peak season. According to Japan Meteorological Agency, on average approximately 25 typhoons occur every year, many of these approach Japan without making landfall, and approximately three out of 25 typhoons strike Japan every year. Some may think fewer typhoons hit Japan than expected. However, the scale and intensity of typhoons have been increasing over decades.

Climate change causing less frequent but more intense typhoons

A recent study suggests that the number of typhoons has declined by 13% since pre-industrial times because factors like air circulation and sea surface temperature that are required to form tropical cyclones have been changed due to human activity. Changes in conditions of these factors create less favorable conditions to form typhoons.

However, once a typhoon is generated, it gives a typhoon much more energy, which causes a more intense, harsher and slower typhoon. As a result, some may say that a typhoon, or tai-fu, has now been a human-caused disaster rather than a natural disaster.