The Mainichi Shimbun answers some common questions readers may have about typhoons, which can cause significant damage in Japan, especially in autumn.
Question: Why do we often hear news about typhoons in autumn?
Answer: September is when typhoons approach the Japanese archipelago more often, tending to cause major damage. When a low-pressure system develops with a certain sustained wind speed, it is called a typhoon. Because the moisture that evaporates from warm seawater feeds the storm, typhoons tend to occur in summer through fall, when the seawater is warm. The higher the seawater temperatures are, the more likely it is that typhoons will become stronger.
Q: Is this why typhoons in September are powerful?
A: Yes, typhoons tend to become strong in September following the rising of seawater temperatures during the summer, and as a result they can cause significant damage when they make landfall. Since the Heisei era (1988-2019), there have been eight typhoons that killed 40 or more people each in Japan. All of them either made landfall or approached the Japanese archipelago between September and October. Because these typhoons can stimulate autumn rain fronts and cause downpours, we should be wary of them.
Q: Do typhoons occur most frequently in autumn?
A: According to the Japan Meteorological Agency, over the 30-year period until 2020, the month of August had the highest average number of typhoons at 5.7, followed by 5 in September and 3.7 in July. However, the average number of typhoons that made landfall in Japan was 1 in September, followed by 0.9 in August and 0.6 in July. The figure is highest in September due to the effects of high-pressure systems.
Q: How do high pressure systems affect the path of typhoons?
A: After a typhoon occurs in the tropical atmosphere over the Pacific Ocean, it moves along the edge of the high-pressure system, blown by eastward trade winds around the equator. Because Japan is often covered by a high-pressure system in the summer, typhoons are more likely to move toward the direction of the Korean Peninsula at that time of year. In the fall, however, the high-pressure system weakens to cover less of the country, increasing the chances of typhoons making landfall in Japan.
(Japanese original by Mayumi Nobuta, Science & Environment News Department)