Is Dry Heat a Myth?

Posted: July 29, 2015, 10:21 am by jmartucci
Images above courtesy of Wikipedia.

For many in the Northeast and across the southern states, people often complain that the oppressive summer heat and humidity is ruining their outdoor plans and many head straight for the air conditioning. Some also say, “I should move to the desert Southwest. It’s a dry heat down there”. Some are taking the expression seriously as Nevada, Arizona, Utah, Texas and Colorado are in the top ten fastest growing U.S. states according to the 2010 Census1. But what exactly is dry heat? And is it really better than the heat of the Northeast and South?

Heat index chart, with air temperature on the left and dew point temperature on top. The different colors correspond to the caution level one should take. (Courtesy of the Oklahoma Climatological Survey).


To understand dry vs humid heat, it’s best to look at the heat index. The heat index combines the air temperature and the dew point to find what the apparent temperature is on the body. This phenomenon occurs because moist air is closer to saturation than dry air. Since the body cools down by evaporation of sweat, less evaporation occurs in a more saturated environment. When it’s “dry” outside, there is less moisture in the air and which allows the evaporation of sweat to occur more readily. So basically, the more moisture present in the air, the hotter it will feel, the less the cooler it will feel.

Back to the dry heat debate, let’s take a look at an example below from July 3, 2014:

As you can see above, even though Las Vegas’s air temperature was well into the 100’s, it’s heat index was 99°F because of the very low humidity. By contrast, DC was only 91°F but reached a heat index of 102°F, actually feeling hotter than Las Vegas on this day! Judging by this, the dry heat of the Desert Southwest does seem more tolerable than the East.

Wondering how to calculate the heat index? Well, grab a calculator because here is the formula:


Heat Index = -42.379 + 2.04901523*T + 10.14333127*RH - .22475541*T*RH - .00683783*T*T - .05481717*RH*RH + .00122874*T*T*RH + .00085282*T*RH*RH - .00000199*T*T*RH*RH (where RH = Relative Humidity and T = Temperature)2


Or more simply, use this handy heat index calculator online:

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