Soil Temperature by Depth

Average Soil Temperature by Month and Depth

Long-term monthly averages at each measured depth, from multi-year climatology at three stations spanning the cold, transition, and mid-south climate bands. The shallow column swings widely across the year while the deepest column stays comparatively flat, and the deep columns reach their yearly high weeks after the surface does.

Current Soil Temperature by Depth

National averages across 309 active USDA SCAN, CSCAN, and NOAA USCRN stations, as of 2026-08-05. Each depth is averaged over the stations that report it, and the station count differs by depth because not every site carries a deep probe.

Across the 261 stations reporting both ends, the shallowest reading currently averages 5.3°F warmer than the deepest.

Why Soil Temperature Changes With Depth

Heat entering the ground at the surface takes time to travel downward, and it weakens as it goes. Two things follow. The daily swing between afternoon and dawn is gone by about 8 inches, so a deep reading does not care what time you take it. And the yearly swing shrinks with depth while arriving later: at the stations above, the 40-inch reading peaks weeks after the 2-inch reading and moves through a much narrower range.

That lag is why deep soil reads cooler than the surface in summer and warmer in winter. It is also why the 2-inch reading is the one that matters for seeding: it responds to this week’s weather, which is what a germinating seed experiences. For the frost side of the same physics, see the frost depth and first frost guide.

Common Questions

What is the ground temperature at 4 feet deep?

Four feet is 48 inches, which is deeper than the federal networks measure. The deepest routine probe is 40 inches, and at that depth the ground stays within a narrow range all year: our monitoring stations typically move about 25°F between winter and summer at 40 inches, against roughly 40°F at 2 inches. Below about 10 feet, soil stops tracking the seasons and settles near the local annual mean air temperature.

How cold is it 20 feet underground?

Deeper than any of our sensors. Soil Temps publishes measured readings from 2 to 40 inches, so we do not report 20 feet. At that depth soil temperature is essentially constant year-round and approximates the local annual average air temperature, which is why it is used as a baseline for ground-source heat pumps and deep foundations.

Why is deeper soil cooler than the surface in summer?

Heat moves slowly through soil. The surface responds to today's sun and air temperature within hours, while a change takes weeks to reach 20 or 40 inches. In midsummer the shallow readings have already climbed while the deep ones are still catching up to spring, so deeper soil reads cooler. The pattern reverses in winter, when deep soil holds summer's heat and reads warmer than the surface.

Which soil depth should I use for lawn and garden timing?

Seeding and pre-emergent decisions use the 2-inch reading, because that is where seeds sit. Established-lawn root-zone questions use 4 inches. The deeper readings on this page are context rather than timing triggers: they show how far the season has moved into the ground.

Related

• 4-inch soil temperature by state
• Live US soil temperature map
• How to measure soil temperature yourself