The program ONLINE_PENMAN calculates potential evapotranspiration [ET]_o by the Penman method, given specified climatic conditions.

The input is: (a) the chosen month, (b) mean monthly air temperature T_a (^oC), (c) mean monthly net radiation Q_n (cal/cm²/day), (d) relative humidity RH (%), and (e) wind velocity at 2-m depth v₂ (km/day).

The output is daily potential evapotranspiration, in cm/day, and monthly potential evapotranspiration, in cm.