# What are Tetens coefficients?

Tetens coefficients appear in a few meteorological/climatological models that I've been looking at,for example APSIM:

svp_A = 6.106
svp_B = 17.27
svp_C = 237.3


These same values appear in at least two other models, but I can't find any documentation about what they represent, or where they come from. What are these numbers?

Edit: Oroud, I. M., 2001, Dynamics of evaporation from saline water bodies uses these constants and references Tetens, O. 1930 "Uber einige meteorologische Begriffe." Z. Geophys. 6: 297-309, but that text doesn't appear to be available online, or in my university's library.

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.