Time-of-the-day dependence of long-term temperature changes at urban meteorological stations in Japan

Long-term changes of urban temperature was examined by using hourly or four-hourly data at 42 stations in Japan. The magnitude of temperature change (T') was defined by the difference between averages for two periods: one from 1891 to 1928 and the other from 1977 to 1994. At many stations, T' values are larger at nighttime than in the daytime, and are nearly constant between a few hours after sunset and sunrise. Moreover, nighttime T' values are nearly equal to the change of daily minimum temperature (T'(min)). However, some stations in the northern part of Japan show larger values of T'(min) than T' at each fixed time, with a difference of 1 degrees C or more at stations in inland of Hokkaido. This indicates a decrease in nocturnal temperature fluctuation, and is likely to be explained by a weakening of the surface inversion due to urban growth.