Air temperature is sensitive to changes in local land use. In this study, we investigated the effect of dam lake water on the air temperature based on open climate data. Lake Sayama (and its surroundings), located in Tokorozawa City, 30 km from Tokyo, Japan, was undertaken as the case study. This artificial lake was empty for three years for seismic reinforcement. To remove the effects of time-series changes in the air temperature, nearby observation stations where the air temperature difference did not change from that of Tokorozawa during the analysis period were selected. Analysis of data revealed that the minimum air temperature increased, and the maximum air temperature slightly decreased during the period when the dam reservoir was full throughout the year. Moreover, the reduction in the diurnal range of air temperatures was 1.78°C in December and 0.46°C in August. The effect of the lake on the air temperature reflected the size of the lake and the climatic zone. These results highlight the need for maintaining a network of meteorological observation stations, whose data can prove useful for identifying local climate changes due to land use.