Abstract
Rapid decrease in relative humidity at urban climatological stations in Japan during 1950's and 1960's abruptly ended around 1970. Since then the annual mean of relative humidity has remained at almost the same level excepting interannual variation. The recent trends are even suggestive of its increase for data at stations in big cities such as Tokyo and Osaka. It is important to note that apparent decreasing trends prevail recent years at stations in rural areas instead. The differences between urban climatological stations with near-by rural stations in both relative humidity and vapor pressure are decreasing recent 10∼20 years. It is shown that during the rapid drying period both the effects of water vapor pressure decrease and temperature rise worked effectively. The recent decrease in urban-rural difference due not only to drying of rural areas noted above but also to increase in the water vapour contents in the urban atmosphere, latter by human activities. At the present urban-rural difference in relative humidity at Tokyo and Osaka are largely attributable to urban warming. It is also demonstrated that urban drying reduces discomfort from hot and humid climate in summer at large cities west of Tokyo.
