Why the heavy rain stayed over Chiba in August

Very heavy rain fell on Chiba, an area east of Tokyo in Japan, in August 2026. On September 10, the Meteorological Research Institute^*1 explained why the rain stayed in one place for so long. The institute is part of the Japan Meteorological Agency, the government's weather office. It said these are its first results.

The rain came from a long line of storm clouds. Weather scientists call this kind of line a squall line^*2. A squall line has very big thunderclouds^*3 and brings very heavy rain. It usually moves fast, so the rain in one place does not last long. But this time, the line stopped near the Boso Peninsula, the part of Chiba that reaches out into the sea. It stayed there for many hours. The institute said this is rare.

Why did the line stop? A layer^*4 of cold air was pushing the clouds to the east. At the same time, wind was blowing in from the Pacific Ocean toward the west. The two pushes were about equal, so the clouds moved much more slowly.

The rain was strongest in the north part of the line. There, the institute found that the air inside the thunderclouds began to spin. The spinning air made the air rise faster in that spot. Rising air helps raindrops form, so the rain became stronger. The institute thinks this was one reason for the very heavy rain.

1. *1 institute: a place where scientists study something
2. *2 squall line: a long line of storm clouds that moves together and brings heavy rain
3. *3 thunderclouds: very tall, dark clouds that bring thunder and heavy rain
4. *4 layer: a flat sheet of something lying over or under another thing