- Energy suppliers in the form of radiation sources ( sun ) and geothermal energy ,
- the wind speed ,
- the humidity , represented by the saturation deficit .
a version of the Penman equation:
- E = evaporation rate (kg m −2 s −1 )
- Δ = slope of the saturation vapor pressure curve as a function of the air (vapor) temperature (PaK −1 )
- R n = net radiation (W m −2 )
- G = soil heat flow (W m −2 )
- ρ L = air density (kg m −3 )
- c p = heat capacity of the air (1004 J kg −1 K −1 )
- C at = atmospheric conductivity ( ms −1 )
- δe = water vapor saturation deficit (Pa)
- γ = psychrometric constant (approx. 0.6 hPa K −1 )
- λ w = heat of evaporation of water (2.5 MJ kg −1 )
The Penman formula only applies to open water areas or areas well supplied with water. The maximum possible evaporation caused by the energy supply is referred to as potential evaporation . The current evaporation , or also real evaporation, is the measurable evaporation on the observed surface and therefore always smaller or equal to the potential evaporation. The relationship between potential and current evaporation is essential for the formation of climatic zones , which creates arid , semi-humid or humid climates .
- HL Penman: Natural evaporation from open water, bare soil and grass. In: Proc. Roy. Soc. London A. 194, 1948, pp. 120-145.