Scenario A: An icicle on the eaves of Uncle Ivan’s dacha in far off Verkhoyansk vaporizes a number of water vapor molecules at an ambient temperature of -80°C. Over the succeeding months, one of these molecules makes its random way to the surface of a cloud droplet over Ouagadougou. Along the way, it undergoes an average of some 109 molecular collisions and consequent changes in kinetic energy of translation per second. Once in Ouagadougou, it condenses onto that cloud droplet, thereby transferring its associated enthalpy of condensation to that surface. The ambient surface temperature of the droplet is some 25°C. Scenario B: Aunt Tillie’s teakettle in Tunbridge Wells comes to a rolling boil, producing many water vapor molecules at an ambient temperature of 100°C. Over the succeeding months, one of these molecules makes its merry way to Ouagadougou in its turn. During its journey, it undergoes a similar number of molecular collisions and changes of kinetic energy of translation as Uncle Ivan’s molecule. And, like the latter molecule, it also condenses onto the same cloud droplet, thereby transferring its associated enthalpy of condensation to that same surface. Aunt Tillie’s molecule and Uncle Ivan’s molecule are of the identical isotopic construction and ionic condition. There is no known way of telling the two apart. Comment: It is my understanding that classical thermodynamics considers that the transfer of the enthalpy of condensation from Uncle Ivan’s molecule to the cloud droplet does not constitute heat transfer, because the transfer of energy goes from the colder (at its origin) to the warmer. Conversely, the transfer of the enthalpy of condensation from Aunt Tillie’s molecule to the cloud droplet does constitute heat transfer, because the transfer of energy goes from the warmer (at its origin) to the colder. Question 1: How then do we consider the transfer of enthalpy of condensation to be a form of heat transfer in its entirety (as in the atmospheric heat budget) when we don’t know the sources of the water vapor? Question 2: Let us postulate that the ambient humid air in Ouagadougou is at the same temperature as the surface of the cloud droplet. Net condensation will still take place as long as the ambient vapor pressure exceeds the equilibrium vapor pressure. Since there is no difference in temperature, is this still heat transfer?