Equilibrium temperatures of water surfaces as related to air temperature and solar radiation

Equilibrium temperature T e is the water surface temperature at which net energy exchange to the atmosphere is zero. Since heat loss rate is a function of ( T w − T e ), where T w is the actual water surface temperature, the concept of equilibrium temperature is useful in predicting water temperatures. By selecting a set of equations to describe the heat exchange processes at the surface, one can produce curves of heat exchange rate (excluding shortwave radiation) versus ( T w − T a ), where T a is air temperature. These curves can be approximated by linear functions. The slopes q and intercepts Q o of these curves are in turn linear functions of wind speed for a specified set of weather conditions (clear and low humidity or cloudy and high humidity). Specifying these weather conditions, wind speed, air temperature, and net incoming solar radiation Q R , one can calculate q and Q o and compute the equilibrium temperature as T e = [( Q R − Q o )/ q [ + T a . This relation provides a simple yet general means of calculating T e and can be used to investigate time variations of T w in response to meteorologic conditions.