A desiccant lowers the humidity, thus allowing cooling via evaporation of water in a climate that is too humid for an ordinary evaporative "swamp cooler" commonly used in desert climates. The adsorption of water vapor by the desiccant must be exothermic delta H < 0: the configurational entropy of the water is reduced as it is concentrated at the desiccant, so there must be a more-than-compensating increase in thermal entropy

|delta H|/T. This heat of adsorption must be dissipated to the outside air. To recharge the desiccant it must be heated to a high enough temperature T so that |delta H|/T becomes smaller than the gain of configurational entropy upon water escaping from the desiccant. 

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