WBRZ Weather

The heat index, dew points, and evaporative cooling

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Posted: Jun 16, 2014 11:41 AM by Meteorologist Robert Gauthreaux
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You may have heard of the "heat index." The heat index is a way to express exactly how the temperature feels outside. One day listening to one of my favorite comedians Bill Engvall, I heard him go into a rant about the heat index. A "made-up" term he claimed, which he compared to his fabricated "speed index" where he tells an officer "...well it only felt like I was going 55mph." It may be the butt of many jokes but the heat index has a real application here in the South.

Evaporation is a cooling process which is one of the reasons humans sweat to lower body temperature. As the sweat evaporates, it cools the skin in an attempt to bring the body back to homeostasis, or a state of relative constant. Where there is a high level of moisture in the atmosphere, it is more difficult for sweat to evaporate into the air, thus harder for human beings to cool down. That's why 100 degrees in the California desert feels a lot more pleasant relative to 100 down here in Dixie. It may be 100 degrees, but it feels more like 115!

An air conditioner uses evaporative cooling as well. With more moisture present in the air, it is harder for air conditioners to use evaporative cooling to cool a building thus requires more energy to do so.

The heat index is dependent on the dew point. You've likely seen our dew point graphics on our newscasts showing that dew points in the 70s are considered oppressive. The dew point is a much better measurement in evaluating moisture in the atmosphere as opposed to a humidity percentage. If you think about it, 100% humidity can happen with an outside temperature of 40 degrees and a dew point temperature of 40 degrees. You don't typically hear people complaining about the humidity when it's only 40 degrees.

The way the temperature "feels" outside can be used in the winter too. You may have heard of the wind chill factor. A breeze of cool dry air against your skin enhances evaporation of moisture from the surface of your skin and you feel cooler than it actually is. While the wind chill factor according to Bill Engvall is what the temperature would feel like standing outside naked in a snow storm, we guarantee you that the job certainly does not exist. At WBRZ, we stick with math.

Tomorrow's article will discuss the July 1995 Chicago heat wave and the actual impact of the heat index.

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