A depth below Earth's surface which stays at the same temperature regardless of surface seasonal and daily temperature variations (see Fig. 1).

Strictly speaking the surface temperature variations propagate down the Earth infinitely and it goes down to measurement accuracy to assess where exactly the surface temperature variations decay below the measurement threshold  $//<![CDATA[ \begin{array}{l}\delta T_{\rm cut}\end{array} //]]>$, which is usually varies between the most common value of  $//<![CDATA[ \begin{array}{l}\delta T_{\rm cut} = 0.1 \, {\rm °C}\end{array} //]]>$ down to the most accurate  $//<![CDATA[ \begin{array}{l}\delta T_{\rm cut} = 0.001 \, {\rm °C}\end{array} //]]>$.

Fig. 1 shows a difference between Neutral Temperature Layer location for  $//<![CDATA[ \begin{array}{l}\delta T_{\rm cut} = 0.1 \, {\rm °C}\end{array} //]]>$ (34 m) and for  $//<![CDATA[ \begin{array}{l}\delta T_{\rm cut} = 0.01 \, {\rm °C}\end{array} //]]>$ (49 m).

In this sense, the Neutral Temperature Layer is more of an engineering concept rather than true physical property of the subsurface medium.

The temperature of the Neutral Temperature Layer is called Subsurface Neutral Temperature and usually denoted as  $//<![CDATA[ \begin{array}{l}T_n\end{array} //]]>$.

Within the accuracy of temperature measurements, the value of Subsurface Neutral Temperature equals to average annual surface temperature  $//<![CDATA[ \begin{array}{l}T_0\end{array} //]]>$ based on weather reports: $//<![CDATA[ \begin{array}{l}T_n \approx T_0\end{array} //]]>$.

See Also

Geology / Geothermal Temperature Field 

[ Geothermal Temperature Field  ] [ Geothermal Temperature Profile ] [ Neutral Temperature Layer (NTL) ]

[ Neutral Temperature Layer @model ]

Reference

GeothermalTemperatureProfile.xlsx