Heat Index

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The HEAT INDEX accounts for the fact that at a given hot temperature, it feels hotter if the relative humidity is higher. A higher relative humidity means that there is more moisture in the air, and more moisture in the air slows your body's ability to cool. This is because the primary cooling mechanism is the evaporation of sweat, and sweat will not be evaporated as easily if the air is already quite moist.

The value of the HEAT INDEX tells you how hot it feels outside given the temperature and relative humidity. For example (refer to chart below), if the temperature is 95, and the relative humidity is only 30%, the heat index is 96. If, however, the temperature is 95, but the relative humidity is 65%, then the heat index is 119! (Indicated by the blue box).

These examples demonstrate it can feel MUCH hotter if there is significantly more moisture in the air. The heat indices during the middle of July 1995 in Chicago reached dangerous values approaching 115.