On March 25, we saw a post by Phil M. indicating that “At a point of 6′ below the surface of the earth, temperatures stays constant at around 60°F.” This may only be true for very specific locales. For most locations, constant ground temperature reflects the average yearly air temperature for the locale, and ground temps are only constant at a depth of about 30 ft. and below. At depths above 30 ft., ground temperatures begin to increasingly modulate up and down following seasonal air temperature. In areas of the world with seasonal temperature changes similar to the United States, temperatures at a 6′ depth can swing +/- ~10°F from the constant for a given locale, and at the surface, soil temps may swing as much as +/-20°F or more from the locale’s constant. Ground temps generally reach their maximum in August and reach their minimum in February.
For example, here in Northwest Florida, the constant ground temp at 30′ is ~69°F (water from a 200′ well is only slightly cooler at ~67°F). In August, the soil temperature 6′ down will be close to 79°F, and in February, temperature at that same 6′ depth will be close to 59°F.
Readers can get a general idea of what their own U.S. locales may look like at a Virginia Tech web page.
JWR Adds: In northern latitudes, the ambient ground temperature can also be depended on for year-round food refrigeration.