1. Why Typhoons Need Warm Tropical Seas, but Warm Water Alone Is Not Enough
Typhoons are born over tropical seas where the sea surface temperature is high [2]. As a rough guide, a temperature of 26 degrees Celsius or higher is often mentioned. However, some sources write 26 to 27 degrees Celsius, so it is not a strictly fixed number [5]. If temperature were the only factor, the sea near the equator is warm, so typhoons should easily form right on the equator. Yet, an explanation from Nagoya University states that typhoons do not form on the equator [5].
The reason given is the Coriolis force. In the Northern Hemisphere, this force acts to bend the wind to the right of its direction of travel. This force is stronger at higher latitudes but becomes zero at the equator [5]. Therefore, a typhoon is born in latitudes where there is warm sea water and where the force that bends the wind is also at work.
2. How Clusters of Thunderclouds Turn into a Swirl and Grow Using Heat from Water Vapor
Over warm seas, rising air currents occur easily. Many cumulonimbus clouds form in these rising air currents. When many cumulonimbus clouds gather and begin to form a swirl, they grow into a typhoon [2].
The energy that grows the typhoon comes from the heat released when water vapor supplied from the sea surface condenses into cloud droplets [1]. The Japan Weather Association also explains that this heat warms the surrounding air, strengthens the rising air currents, and causes the air pressure to drop [6]. You can imagine a typhoon as growing by receiving heat from the sea in the form of water vapor.
3. How Surrounding Winds Carry a Typhoon Along
A typhoon moves by riding on the flow of surrounding winds, much like a leaf floating on a river is carried by the water flow. The winds that carry the typhoon change depending on the location [1].
The way a typhoon moves happens in three stages. At low latitudes where easterly winds blow, it moves west. Next, it moves north, circling around the Pacific High. When it reaches middle to high latitudes, it rides on the strong upper-level westerly winds and heads northeast at high speed [1]. In this way, the path of a typhoon curves from the seas south of Japan to the west, then north, and finally northeast.
4. How Typhoon Paths Change from Season to Season
In early spring, typhoons born at low latitudes often move west toward the Philippines. In summer, typhoons are born at higher latitudes. Many of these circle around the Pacific High and move north toward Japan [3].
The Japan Meteorological Agency explains that in August, the upper-level winds that carry typhoons are still weak, so they often take unstable paths. From September onward, they often pass near Japan, drawing a parabolic curve from the southern seas [3]. According to the average number of typhoons from 1991 to 2020, the most typhoons form in August with 5.7, followed by September with 5.0, and July with 3.7 [7].
5. Why Typhoons Weaken Near Japan and How to Read Forecast Circles
When a typhoon reaches the area near Japan, where sea surface temperatures are lower than in the tropics, the supply of water vapor from the sea decreases, and its strength may weaken. It may also change into an extratropical cyclone or a tropical depression [2].
The forecast circle in weather reports shows the area where there is a 70% probability that the center of the typhoon will be at the forecast time. The Japan Meteorological Agency warns that the center of the typhoon does not necessarily travel along the line connecting the centers of the forecast circles [4].
6. How to Look at Past Typhoon Paths Yourself
If you want to learn more, you can visit the Japan Meteorological Agency’s page on typhoon tracks. Look for maps of past typhoon paths. Choose one typhoon from spring and one from around September. Trace their paths with your finger and compare them. See if they head west or curve near Japan. This makes the idea that "typhoons are carried by winds" visible to your eyes. The official pages on "Typhoon Tracks" and "Types and Expressions of Typhoon Information" explain the concepts used in this article.