How does seasonal air temperature variation change with depth below the ground surface?

Seasonal air temperature variations significantly influence the ground temperature, though this effect is restricted to the upper layers of the Earth's surface. In the uppermost few metres of the ground, annual fluctuations in air temperature are reflected in a subdued manner.1

As depth increases, these fluctuations diminish rapidly. At a depth of 7.5 metres (about 25 feet), the temperature variation is reduced to only a few degrees, and by 15 metres (49 feet), it is barely detectable. The level of zero amplitude—the depth at which seasonal fluctuations are essentially imperceptible—typically occurs between 9 and 15 metres (30 to 49 feet) below the surface.1

Below this level of zero amplitude, the influence of seasonal air temperature changes ceases. Instead, the ground temperature is governed by the geothermal gradient, where temperatures increase steadily due to heat flowing from Earth’s interior.1