Severe Weather in Japan

This post is about tornadoes and hurricanes in Japan and how they compare to their counterparts in the United States.

First, a bit about Japan's location. Japan is located in the Northern Hemisphere. Its minimum latitude is 24°N and its maximum latitude is 46°N. Its minimum longitude is 123°E and its maximum longitude is 146°E. The nearest ocean is the Pacific Ocean.

Tornadoes

How do tornadoes form? Due to vertical wind shear, a rotating body of air forms at the ground. The air at the bottom is slower than the air higher up. A warm updraft from the ground lifts the spinning air into a loop, and once it is vertical it is considered a mesocyclone. Funnel clouds form from the "wall cloud" in the lower part of the spinning cloud. When the funnel cloud touches ground, it is called a tornado.

In the United States, tornadoes usually travel in a northeast direction. This is due to the fact that tornadoes move in the same direction and with the same speed as the thunderstorms that produce them, and most thunderstorms move from the southwest to the northeast. On average, there are 1,274 tornadoes observed in the US per year (Source).

Tornadoes in Japan also tend to travel in a northeast direction. I can't find a concrete explanation for this, but apparently tornadoes in the northern hemisphere tend to move in that direction. Tornadoes occur far less often in Japan than the US, with an average of 15 per year (Source).

Graphs showing annual tornado counts over the last few decades indicate that tornado occurrences have increased over time. However, you should keep in mind that weather technology has advanced, in turn advancing our ability to detect storms. There aren't necessarily more tornadoes occurring—just more tornadoes being observed.

Global map of tornado occurrence. (Source)

Hurricanes

There are three main requirements for hurricane formation. The first is warm ocean temperatures (>80°F or 27°C), because you need lots of evaporation. The second is a deep warm ocean layer (200m in depth), so that there is lots of warm water even after winds churn up the water. The third is coriolis, to initiate the spinning of the hurricane.

There are different terms used for hurricanes in each of the main hurricane formation regions. Hurricane is the name in the Atlantic and East Pacific. In the Indian Ocean near Australia, they are called cyclones. Off the coast of China and Indonesia, they are called typhoons.

Formation regions of hurricanes. (Source)

In the United States, hurricanes generally travel north to northeast. If they travel over 25 or 30 degrees latitude (around the top of Florida), they lose the influence of the trade winds pushing them over the Atlantic Ocean and will usually be influenced by local winds. These tend to push them north-northeast, along the east coast or off of it. If they do not reach 25 or 30 degrees latitude, the trade winds will continue to push the storm westward into the Gulf of Mexico, and it will land in Texas or Mexico. About 1.75 hurricanes hit the US per year (Source).

In Japan, hurricanes (or typhoons) tend to travel in the same way, north to northeast. This is because they usually encounter southwesterly winds, which blow northeastward, around a low-pressure system passing over China or Siberia. About 2.7 hurricanes hit Japan per year (Source).


In conclusion...
Tornadoes clearly occur far more often in the US than in Japan, but tornadoes occur more often in the US than anywhere else in the world anyway. This is offset by the fact that Japan has a slightly higher rate of hurricane occurrence than the US. Tornadoes in the US occur throughout the year, just in different places around the country depending on the time of year. Tornadoes in Japan tend to occur more frequently in the summer and fall, and rarely in the spring. Their hurricane seasons are almost identical, however: For the US, it's between June and November, and for Japan it's between July and November. Even though the US and Japan are on different parts of the globe, their tornadoes and hurricanes still move in very similar directions, which may be at least partially explained by the fact that they are both located in the northern hemisphere.