# Evaporation

Measurement in an atmometer

The evaporation is a meteorological term that the evaporation of water designated for an adult or free land or water surfaces.

## Penman equation

HL Penman established a widely used approximation formula in 1948. This takes into account that the evaporation rate (measured in mm / d ) has three influencing factors:

a version of the Penman equation:

${\ displaystyle E = {\ frac {\ Delta (R _ {\ mathrm {n}} -G) + \ rho _ {\ mathrm {L}} c _ {\ mathrm {p}} C_ {at} \ delta e} {\ lambda _ {\ mathrm {w}} \ left (\ Delta + \ gamma \ right)}}}$

With

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 .

In this context , transpiration refers to evaporation in living beings , whereby the transpiration of plants is particularly important.

The sum of evaporation and transpiration is called evapotranspiration .