Results of experimental studies conducted in Teaneck, New Jersey and Harrisburg, Pennsylvania to determine seasonal variations of subsurface ground temperature for one year are presented in this paper. The experimental set up for automated data collection to a depth of 17 feet (5.18 m) is described. The results indicate that the ground temperatures do not vary much with depth, at depths greater than 15 feet (4.57 m). However, there is seasonal variation indicating that the average ground temperature at a depth of 15 feet (4.57 m) is 66°F (18.9°C) during winter, and it is 58°F (14.44°C) during summer months. In addition, there is a temperature differential of 20 to 25°F (11.1 to 13.9°C) between the ground temperature at 15 feet (4.57 m) and ambient temperature. This temperature differential suggests that there is a potential for pre-heating and pre-cooling the ambient air used in conditioning in commercial and institutional buildings. The empirical equations for predicting average monthly subsurface ground temperatures are presented. This information is useful in the design and analysis of ground coupled heat pumps.