Thursday, June 13, 2013

Heat & Humidity Combine for Added Discomfort!

-- Jay Grymes / WAFB Storm Team
Afternoon temps in the 90°s coupled with an abundance of Gulf humidity will continue to push the Heat Index readings up to near or even above 100° for most WAFB neighborhoods just about every day this week.
As you know, the Heat Index represents the perceived ‘feels like’ temperature for humans and is often referred to as the “apparent temperature.” The Heat Index (HI) is a subjective measure that makes a number of assumptions about the state of an individual, including type of clothing, the amount physical activity, even the person’s size and weight. The HI, therefore, is a reference number, a guideline, designed to approximate what one would “feel” given a combination of temperature and humidity under light winds and shaded conditions. And remember, the “apparent temperature” can increase by as much as 10° to 15°F under direct mid-day sunshine.
The human body has a built-in “cooling system” -- perspiration! Yet we fight perspiration constantly throughout the South Louisiana summer. It may not seem to make sense, but sweating is actually designed to cool the body. How? Heat is removed from the body by the evaporation of sweat -- it’s the same as the effect when you dab your skin with rubbing alcohol. The alcohol quickly evaporates, pulling heat out of your skin through the evaporation process and leaving you with a cooling sensation.
So why doesn’t the sweating process work better? Because of the normally high water-vapor content in our south Louisiana summer air -- our high humidity.
The more humid the air around us, the less efficient the perspiration/evaporation process. And when the air is very humid, the opportunity for our sweat to evaporate can go to near zero -- and then say hello to that sweat-soaked shirt!
The dew point temperature is one of our better indicators of the humidity level of the air. (You may recall that when the air temperature and the dew point temperature are equal, the relative humidity is at 100%.) The higher the dew point, the more water-vapor being held by the air. For Baton Rouge, summer season outdoor dew points are routinely in the low to mid 70°s and can climb into the upper 70°s to near 80° when conditions are right. By comparison, air-conditioned locations often will try and maintain an indoor dew point temperature below 60°. (To get a sense of what this means in terms of humidity, to go from a dew point temperature of 60° to a dew point of 75° requires that the amount of water vapor in the air must nearly double!)
For Baton Rouge, a somewhat typical summer afternoon will see outdoor air temperatures climb into the low to mid 90°s with dew point temperatures in the low to mid 70°s – no doubt about it, that combination is unpleasant to say the least! For example, a combination of an air temperature of 95° with a dew point of 75° yields a Heat Index of 107° -- considered “Dangerous” when one is exposed to that kind of heat for prolonged periods without relief. Of course, on individual days it can get even hotter and more humid for the Red Stick. What’s more, there are a number of days during just about every summer when the Heat Index stays in the triple digits for several hours. In fact, we sometimes see these extreme heat days come back-to-back-to-back.

The bottom line: heat can be a killer. In fact, some estimates suggest that, on average, 300 or more Americans die each year as a direct or indirect result of extreme heat.

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