Long-term changes in diurnal variation patterns of precipitation frequency were analyzed using visual data (ww) for 42 years (1961 to 2002) in Japan. The precipitation frequency relative to the daily mean was found to have increased in the nighttime and decreased in the daytime at a rate of the order of 0.01 per decade. This change was observed for all seasons and regions. A supplementary analysis based on hourly automated data on the AMeDAS network (1979 to 2002) confirmed the relative decreasing trend of daytime precipitation frequency, apart from some differences by regions and precipitation intensity. Changes in diurnal variation patterns were also found for vapor pressure and temperature, with a relative increase in nighttime values, although little change was detected for the diurnal variation patterns of relative humidity and cloud amount.