Underground temperature profiles from ground level to 4m below the surface were measured for about 1 year at two locations in Tokyo, Japan. The first site was Kyoka elementary school in Chuo-ku, a highly urbanized area. The second site was Tanashi city, a rural area. A one-dimensional thermal diffusion simulation was carried out for the underground temperature field in Kyoka, and the results were in good agreement with the measurements. Daily averages of the ground surface temperature and the air temperature directly above the surface showed a strong linear correlation. Annual average temperatures at 4m below the surface were 18.8 Celsius degrees in Kyoka and 16.6 Celsius degrees in Tanashi. The yearly fluctuations at both sites were small. In comparison with the past underground temperature in Tokyo of 15 Celsius degrees, the underground temperature at Kyoka and Tanashi were 3.8 Celsius degrees and 1.6 Celsius degrees higher, respectively.
From these results, the underground temperature fields are shown to record air temperature changes through fluctuations in the ground surface temperature, which was highly correlated with air temperature. Thus we can consider the underground temperature field to be a recorder of past changes in air temperature. Through analysis with air temperature fields, underground temperature fields could be important as both indexes and evidence for urban heat island phenomena