An institute for Japan Meteorological Agency says bands of heavy rain clouds have been generated from night till morning in most cases.

The Meteorological Research Institute examined 496 cases of bands of heavy rain clouds confirmed in Japan over the 14 years through last year.

The researchers say the Pacific side of the Kyushu and Shikoku regions, as well as the Kii Peninsula, observed many bands of rain clouds with more than 20 cases for each.

They say 251 cases were generated in the rainy season from June to July -- about half of the total.

As for a time frame, 388 cases, or 78 percent of the total, occurred between 6 p.m. and 9 a.m.

The researchers also say the clouds were observed especially around 11 p.m. and from 3 a.m. till 6 a.m.

In the torrential rain that caused severe damage to northern Kyushu in July 2012, the clouds were generated from 11 p.m. to 12 a.m.

The researchers say they are studying why many of the heavy rain clouds are generated from night till morning.

Kato Teruyuki, a senior official at the institute, says there have been cases in which people became aware of flooding when they woke up in the morning and could not evacuate because rain had not fallen at night. He points out that when heavy rain is forecast, it is important to frequently check weather information and notices from local governments and consider evacuation if necessary.