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The Penman equation describes evaporation (E) from an open water surface. It is widely regarded as one of the most accurate models, in terms of estimates.^{[citation needed]} It was developed by Howard Penman from Kelso in his laboratory.

There are numerous versions of the Penman equation, one of which is:

<math>E=\frac{m R_n + \rho_a c_p C_{at} \delta e}{\lambda_v \left(m + \gamma \right) }

</math>

where:

m = Slope of the saturation vapor pressure curve (Pa K^-1)

R_n = Net irradiance (W m^-2)

ρ_a = density of air (kg m^-3)

c_p = heat capacity of air (J kg^-1)

C_at = atmospheric conductance (m s^-1)

δe = vapor pressure deficit (Pa)

λ_v = latent heat of vaporization (J kg^-1)

γ = psychrometric constant (Pa K^-1)

which (if the SI units in parentheses are used) will give the evaporation E in units of kg/(m²·s), kilograms of water evaporated every second for each square meter of area.