The interpolation the saturated humidity depends on the temperature $T$ (in Kelvin). 
It is set by the following equation :
\begin{equation}
q_{sat}(T)= \dfrac{q_s(\lfloor T \rfloor +1) - q_s(\lfloor T \rfloor).\lbrace T \rbrace + q_s(\lfloor T \rfloor)}{P}
\end{equation}
where : \\
\begin{itemize}
 \item  $ \lfloor T \rfloor $ represents the largest integer not greater than $T$. 
 \item  $ \lbrace T \rbrace = T -\lfloor T \rfloor $ is the fractional part of $T$.
 \item  $q_s$ is the array caclulated in the subroutine qsfrict\_init. 
\end{itemize}

