At a depth of below four feet, the ground temperature stays a constant 50 to 55 degrees year-round.
During the winter, a geothermal system absorbs this extra heat from the earth and transfers it into your home. During the summer, the system takes heat from indoors and moves it back underground.
Annual air temperature, moisture content, soil type and vegetative cover (i.e., trees and plants) all have an effect on underground soil temperature. As you might expect, the earth's temperature changes in response to weather changes, but there is less change at greater depths.