AUSTIN (KXAN) — “It’s not the heat, it’s the humidity” is part of every Central Texan’s vernacular. But did you know there actually is some truth to this statement?
The heat index, also called “apparent temperature,” is what the temperature feels like when interacting with the human body. This is a result of when relative humidity and air temperature are combined.
This is very important to remember because when the body gets too hot, it begins to perspire and sweat to cool itself off. When sweat is evaporated off your skin, it effectively cools your internal temperature.
If the perspiration is not able to evaporate, the body can’t regulate its temperature — which can result in heat exhaustion and heat stroke.
An example you’ll see a lot in the coming days is air temperatures in the low 90s with relative humidity in the 55 to 60% range, making it feel like 100 to 105 degrees.
The opposite is true when the relative humidity decreases because the rate of perspiration increases. The body actually feels cooler in arid conditions like you find in West Texas and the Desert Southwest.
Interestingly, areas with high air temperature and low relative humidity can actually have an apparent temperature cooler than the air temperature.
In fact, there is a direct relationship to air temperature and relative humidity: meaning as air temperature and relative humidity increase, so does the heat index.
Courtesy of the National Weather Service, here’s a weather calculator if you’d rather enter numbers manually.
If you’re really mathematically inclined, there’s an equation that gives a very close approximation to the heat index. However, this equation was obtained using a multiple regression analysis, and therefore, it has an error of ±1.3°F.
Heat Index = -42.379 + 2.04901523T + 10.14333127R – 0.22475541TR – 6.83783 x 10-3T2 – 5.481717 x 10-2R2 + 1.22874 x 10-3T2R + 8.5282 x 10-4TR2 – 1.99 x 10-6T2R2
T – air temperature (F)
R – relative humidity (percentage)