The number of days when it either rains heavily or not at all in Japan has increased during the past 100 years, while the number of moderately rainy days has decreased, according to the Meteorological Research Institute.

Global warming is expected to accelerate this trend by the end of this century, increasing fears of floods and landslides caused by torrential rain, according to the governmental institute in Tsukuba, Ibaraki Prefecture.

Institute researchers found the average number of days when rainfall was at least 100 mm was 1.11 annually from 2001 to 2004, up 25 percent from the average of 0.89 in the 1901-1904 period, according to calculations made using data taken at 51 observation points.

There were on average 255 days out of the year with no rain -- rainfall of less than 1 mm -- in the 2001-2004 period, up from 241 in 1901-1904.

In contrast, the number of days with moderate rainfall of between 1 to 5 mm was 42, down from 49 a century ago.

One reason for the increase in days of heavy rain is global warming, which has raised the amount of moisture in the air, making it easier for thunderclouds to form, an institute official said, adding that researchers have yet to find an explanation behind the increase in no-rain days.

The institute predicts the number of days with 100 mm or more of rain will increase in most parts of Japan -- by three days annually in some areas -- in 2081-2100. The number of days without rain is expected to rise from between one and eight.