Humidity throws a wrench in that system of evaporative cooling, though. As relative humidity increases, the evaporation of sweat from our skin slows down. Instead, the sweat just drips off of us, which leaves us with all of the stinkiness and none of the cooling effect. Thus, when the humidity spikes, our bodies effectively lose a key tool that could normally be used to cool us down.  What's relative about relative humidity?  We all know humidity is the amount of water the air contains. However, as the air's temperature changes, so does the amount of water the air can hold. (Air can hold more water vapor as the temperature heats up.) Relative humidity compares the actual humidity to the maximum amount of water vapor the air can hold at any given temperature.  Whose idea was the heat index?  While the notion of humidity making days feel warmer is painfully apparent to anyone who's ever been outside on a soupy day, our current system owes a big debt to Robert G. Steadman, an academic textile researcher.  In a 1979 research paper called, "An Assessment of Sultriness, Parts I and II," Steadman laid out the basic factors that would affect how hot a person felt under a given set of conditions, and meteorologists soon used his work to derive a simplified formula for calculating heat index.  The formula is long and cumbersome, but luckily it can be transformed into easy-to-read charts. Today your weatherman just needs to know the air temperature and the relative humidity, and the chart will tell him the rest.  Is the calculation the same for everyone?  Not quite, but it's close. Steadman's original research was founded on the idea of a "typical" person who was outdoors under a very precise set of conditions. Specifically, Steadman's everyman was 5'7" tall, weighed 147 pounds, wore long pants and a short-sleeved shirt, and was walking at just over three miles per hour into a slight breeze in the shade.
Is it really not the heat, but the humidity? With temperatures spiking around the country and weathermen warning of days where the heat index surges into the triple digits, we thought it might be a