We say that "It’s not the heat, it’s the humidity" on a hot day that increases discomfort because the high concentration of water vapor in the air inhibits the evaporation of sweat from the body.
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“It's not the heat, it’s the humidity." What does this popular adage mean? 1. Heat is a measure of the total amount of kinetic energy due to molecular motion in a body of matter. Temperature measures the intensity of heat due to the average kinetic energy of the molecules. Whenever two objects of different temperatures are brought together, heat passes from the warmer to the cooler object until the two are the same temperature, which is called thermodynamic equilibrium. 2. Evaporation is a transformation from a liquid to a gas. Evaporative cooling is a process where a liquid evaporates so the surface remaining liquid cools down. This evaporative cooling occurs because the hottest molecules, those with the greatest kinetic energy, are the most likely to leave as gas. 3. Evaporative cooling of water contributes to the stability of temperature in lakes and ponds and also proves a mechanism that prevents terrestrial organisms from overheating. Evaporation of sweat from human skin dissipates body heat and helps to prevent overheating on a hot day or when excess heat is generated by strenuous activity. The high humidity on a hot day that increases discomfort because the high concentration of water vapor in the air inhibits the evaporation of sweat from the body.