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Why is the temperature below ground more constant than the temperature at the surface?

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In the context of green architecture, the stability of underground temperatures is a key factor for energy efficiency. The annual fluctuation of air temperature from winter to summer is reflected in the upper few metres of the ground, but this effect diminishes rapidly with depth.1

While surface temperatures are subject to significant seasonal shifts, these fluctuations become subdued as you move deeper into the earth. At a depth of approximately 7.5 metres (25 feet), temperature variations are minimal, and by 9 to 15 metres (30 to 49 feet)—a point known as the level of zero amplitude—seasonal fluctuations are barely detectable. Below this depth, the temperature is no longer governed by atmospheric conditions but instead increases steadily due to the geothermal gradient, which is influenced by heat flow from Earth’s interior. This natural thermal buffering allows for more consistent indoor climates in earth-sheltered or subterranean building designs.12