How does perspiration, coolers, earthen pots, cotton clothes and spirit on hand display cooling effect of evaporation?
Evaporative cooling works by removing the high-velocity tail of the kinetic energy distribution. That is, only the fastest molecules escape the liquid, leaving the rest to thermalize at a lower temperature. If there is capillary action taking water to the outside of the pot and that is evaporating, then the pot cools down as it is losing heat to the leaving molecules' kinetic energy. This then cools down the water inside by conduction.
One can then ask why, if the air is hotter than the water, can heat flow from the water into the hot air? The answer to this is that there is also a reverse process which is also possible: on a wet day, water molecules in the air can rejoin the water on the pot walls, and if the air is hot then in the mean this process will heat the pot and the water.
In an equilibrium situation, both of these processes happen at the same rate and there is no heat flow. For evaporative cooling to work, the air needs to be dry so that more molecules leave the water than condense into it. In a closed environment, though, evaporation will raise the air's humidity until both processes are equally likely and everything thermalizes. On a windy day, though, the pot is trying to raise the humidity of the whole atmosphere, which is not going to happen soon. This is an open system, in contact with an infinitely dry reservoir of dryness
Evaporative cooling works by removing the high-velocity tail of the kinetic energy distribution. That is, only the fastest molecules escape the liquid, leaving the rest to thermalize at a lower temperature. If there is capillary action taking water to the outside of the pot and that is evaporating, then the pot cools down as it is losing heat to the leaving molecules' kinetic energy. This then cools down the water inside by conduction.
One can then ask why, if the air is hotter than the water, can heat flow from the water into the hot air? The answer to this is that there is also a reverse process which is also possible: on a wet day, water molecules in the air can rejoin the water on the pot walls, and if the air is hot then in the mean this process will heat the pot and the water.
In an equilibrium situation, both of these processes happen at the same rate and there is no heat flow. For evaporative cooling to work, the air needs to be dry so that more molecules leave the water than condense into it. In a closed environment, though, evaporation will raise the air's humidity until both processes are equally likely and everything thermalizes. On a windy day, though, the pot is trying to raise the humidity of the whole atmosphere, which is not going to happen soon. This is an open system, in contact with an infinitely dry reservoir of dryness
