# Exner function

$\Pi = \left( \frac{p}{p_0} \right)^{R_d/c_p} = \frac{T}{\theta}$
where $p_0$ is the pressure at the surface, usually taken as 1000 hPa; $R_d$ is the gas constant for dry air; $c_p$ is the heat capacity of dry air at constant pressure; $T$ is the absolute temperature; and $\theta$ is the potential temperature.