What are Tetens coefficients?
The Tetens coefficents are the empirically derived coefficients of an equation for calculating the saturation vapour pressure of water. From wikipedia:

a very good approximation can usually be made using the August-Roche-Magnus formula (usually called the Magnus or Magnus-Tetens approximation, though this attribution is historically inaccurate):
$e_s(T)= 6.1094 \exp \left( \frac{17.625T}{T+243.04} \right)$
e_s(T) is the equilibrium or saturation vapor pressure in hPa as a function of temperature T on the Celsius scale. Since there is only a weak dependence on temperature of the denominator of the exponent, this equation shows that saturation water vapor pressure changes approximately exponentially with T.