(1)	$\begin{array}{l}\displaystyle H_N = \sqrt{\frac{a_e \, A_T }{\pi}} \, \ln \frac{\delta T_A }{\delta T_{\rm cut} }\end{array}$

where

$\begin{array}{l}H_N\end{array}$	true vertical depth of the Neutral Temperature Layer with reference to the Earth's surface
$\begin{array}{l}a_e\end{array}$	Local average Thermal diffusivity of the soil between Earth's surface and NTL
$\begin{array}{l}A_T\end{array}$	Period of annual temperature variation cycle: $\begin{array}{l}A_T = 1 \, {\rm year}\end{array}$
$\begin{array}{l}\delta T_A\end{array}$	Annual average Earth's surface temperature variation in a given location based on weather reports
$\begin{array}{l}\delta T_{\rm cut}\end{array}$	Temperature measurement threshold (usually $\begin{array}{l}\delta T_{\rm cut} = 0.01 \, {\rm °C}\end{array}$ )

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