There has been no objecctive and reasonable criterion in determining the rainless periods when for a practical purpose it is desired that any particular days, which actually had a certain amount of precipitation during those periods, are still counted as rainless.
The author presents that a day may be considered as rainless when the daily amount of precipitation on that day is less than the value of the corresponding potential evapotranspiration which is to be obtained from the daily mean temperature.
From those rainless periods thus obtained for Fukuoka covering the last 30 years, each of which lasted for more than 10 days, the number of their occurrences according to the durations, and the season of the year for which they are liable to occur, are studied.
As to those rainless periods, it is not their lengths of duration but their “intensities” that are important.
Then, expressing the indices for such intensities of rainless periods by the water losses which occurred throught out the whole period, the number of occurrences according to the indices, and the season of the year in which these periods are apt to occur, are also researched.
