From the ABC 7 Weather team

Heat Index: NO AC and it's brutal

July 1, 2012 - 12:01 PM
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How hot would you think it was if you didn’t know the temperature?

Power still out for hundreds of thousands in our area and the Heat Index is going to be 100º or higher again today.  But what is the "Heat Index" and where did it come from?

Heat Index for for July 2 from NWS


I’m going to date myself with this post, but here goes. I grew up before there was such a thing called the “heat index.” (I almost grew up before there was a “wind chill,” too. Almost.) And yet I still sort of knew how hot it was during summertime in the beautiful Hudson Valley. At least my body did, and that’s what the heat index is all about, a “feels-like” temperature.

But how hot would you think it was if you didn’t know the temperature? The great baseball player Satchel Paige – baseball has never been the same for me since the Dodgers left Brooklyn – once tackled this sort of question when he was asked how old he was. Paige replied, “How old would you be if you didn’t know how old you are?”

heat index 1 satchel paige

Similarly, the answer for temperature is you will think it’s as hot as your body feels. When the air is hot (forget about humidity for one moment), you perspire. Your body does not want you to be broiled in the sun. That perspiration evaporates from your skin and removes heat. How's that? Gas, in this case water vapor, has more heat energy than liquid water (sweat) and liquid water has more heat energy than ice. It takes heat to change water from liquid to gas (think about heating water on the stove) and when your sweat evaporates, it moves heat from your body into the air.

When the air is very dry, you may actually feel chilled by moisture evaporating from your skin. But not this July. That's because there is lots of water to the air – yes, that sticky humidity. Oh no, he’s going to use that dreaded term “dew point”! You bet, and now look at this dew-point chart.

heat index 1 dew

The higher the dew point for one temperature, the higher the relative humidity and the lower the rate of evaporation of sweat from your body. The hotter your body feels the hotter you feel, thus a higher heat-index reading. That’s why the heat index is important: It’s a measure of how hot your body thinks it is. The hotter your body "feels," the more stress on your body and your internal organs. That's why heat is harder on the very old and very young: Their bodies are more sensitive to heat stress. I bet Satchel Paige would see the value of the heat index.

heat index 3

And then what happens when summer is over? It’s time for your body to feel cold. How cold does your body/skin think it is? Why, our body gets chilled by cold air and the wind removes heat from our body, so both the cold air and wind make our exposed skin (not fully exposed in winter, for sure) feel cold. Down below you can see the wind-chill chart. Don’t worry, I won’t mention wind chill again before October, although we sure could use some of it this week with so many without power and air conditioning.  Heat can be a killer, especially after several days without air conditioning and heat indexes 100º or more. Plenty of cool water and some ice will help.  October will help more.

heat index 4
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