1. Bowen ratio The Bowen ratio is defined as follows (Ponce, 1989):
in which B = Bowen ratio; Q_{h} = sensible heat transfer from water body to the atmosphere by convection and conduction; Q_{e} = energy expended in the evaporation process; T_{s} = water surface temperature, in °C; T_{a} = overlying air temperature, in °C; e_{s} = saturation vapor pressure at the water surface temperature, in millibars (mb); e_{a} = vapor pressure of the overlying air, in mb; p = atmospheric pressure, in mb; γ = psychrometric constant, in mb °C^{1} (Note that 1000 has the units of mb). 2. The psychrometric constant γ The psychrometric constant γ is expressed as follows:
in which c_{p} = specific heat of air at constant pressure, in cal gr^{1} °C^{1}; p = atmospheric pressure, in mb; r_{MW} = ratio of the molecular weight of water vapor to dry air: r_{MW} = 0.622, and λ = latent heat of water vaporization, in cal gr^{1}. The specific heat of air between 0°C and 40°C is: c_{p} = 1.005 J gr^{1} °C^{1} = 0.24017 cal gr^{1} °C^{1}. The mean sealevel atmospheric pressure is: p = 1013.25 mb. Therefore, at sea level, the psychrometric constant is:
which reduces to:
with λ in cal gr^{1} and γ in mb °C^{1}. For any atmospheric pressure p (mb), the psychrometric constant is:
which reduces to:
with p in mb, λ in cal gr^{1} and γ in mb °C^{1}. 3. Variation of γ with temperature Since λ varies with temperature, γ also varies with temperature. Table 1 shows the variation of the psychrometric constant γ with temperature, at standard sealevel atmospheric pressure (p = 1013.25 mb) (Fig. 1).
Fig. 1 Variation of the psychrometric constant with temperature, at standard atmospheric pressure. 4. Summary The slight variation of the psychrometric constant γ with temperature has been explained and clarified. The aim is to increase the accuracy of online evaporation calculations (http://onlinecalc.sdsu.edu)
Fig. 2 Ira Sprague Bowen. References
Ponce, V. M. 1989. Engineering Hydrology: Principles and Practices. Prentice Hall, Englewood Cliffs, New Jersey.

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