The ground temperature 4 feet underground in the U.S. is not a single fixed value but remains remarkably consistent, typically falling within a range of 45°F to 70°F, primarily influenced by geographical latitude.
Understanding Ground Temperature at Depth
At approximately 4 feet below the surface, the ground temperature reaches a depth where it is generally below the frost line. The frost line typically ranges from about 3 to 5 feet below the surface in most parts of the U.S. Below this line, the ground temperature remains at a nearly constant level throughout the year, insulated from the significant seasonal and daily air temperature fluctuations.
Temperature Variation by Latitude
While consistent at depth, the specific constant temperature varies depending on whether the location is in northern or southern latitudes of the U.S. This regional difference accounts for the broad range in ground temperatures.
The approximate temperature ranges are as follows:
| Region | Typical Ground Temperature (4 feet underground) |
|---|---|
| Northern U.S. | 45°F - 50°F |
| Southern U.S. | 50°F - 70°F |
Practical Implications
The stable temperature of the earth at these depths is a key principle behind ground source heat exchange systems (often referred to as geothermal systems). This consistent temperature allows for efficient heating and cooling, as the ground can act as a heat sink in summer and a heat source in winter, leveraging the earth's natural thermal stability.
