The formula according to Haude only uses air temperature and relative humidity values measured at 2:00 p.m. This formula is mainly applied in moderate zones and produces good results despite of its simplicity.

ET_p = f_mon × (es₁₄  - ea₁₄) in mm / d

es = 6.11 x 10^{(7.48 × T / (237+T))}
ea = rf × es

ET_p = f_mon x (6.11 × 10^{(7.48 × T / (237+T))} - rf × es)