starts cooling, it is now colder than the air, so convective heat transfer begins to occur from the air to the drop. Furthermore, the evaporation rate depends on the difference of concentration of water vapor between the drop-stream interface and the distant stream (i.e. the "original" stream, unaffected by the drop), and on a convective mass transfer coefficient, which is a function of the components of the mixture (i.e. water and air). After a certain period, an equilibrium is reached: the drop has cooled to a point where the rate of heat carried away in evaporation is equal to the heat gain through convection. At this point, the following balance of energy per interface area is true: water content of interface at equilibrium (kgH2O/kgdry air) (note that the air in this region is and has always been saturated) water content of the distant air (same unit as above) mass transfer coefficient (kg/m2⋅s) air temperature at distance (K) water drop temperature at equilibrium (K) convective heat transfer coefficient (W/m2·K) Note that: is the driving force for mass transfer (constantly equal to throughout the entire experiment) is the driving force for heat transfer (when reaches , the equilibrium is reached) Let us rearrange that equation into: Now let's go back to our original "thermodynamic wet-bulb" experiment, Experiment 1. If the air stream is the same in both experiments (i.e. and are the same), then we can equate the right-hand sides of both equations: Rearranging: If then the temperature of the drop in Experiment 2 is the same as the wet-bulb temperature in Experiment 1. Due to a coincidence, for the mixture of air and water vapor this is the case, the ratio (called psychrometric ratio) being close to 1. Experiment 2 is what happens in a common wet-bulb thermometer, meaning that its reading is fairly close to the thermodynamic ("real") wet-bulb temperature. Experimentally, the wet-bulb thermometer reads closest to the thermodynamic wet-bulb temperature if: The sock is shielded from radiant heat exchange with its surroundings Air flows past the sock quickly enough to prevent evaporated moisture from affecting evaporation from the sock The water supplied to the sock is at the same temperature as the thermodynamic wet-bulb temperature of the air In practice the value reported by a wet-bulb thermometer differs slightly from the thermodynamic wet-bulb temperature because: The sock is not perfectly shielded from radiant heat exchange Air flow rate past