You probably think a good soaking rain shower cools things off a bit on a hot summer day, don’t you? Well, I’m here to testify that’s not always the case. When cold water hits a hot surface the properties of both the water and the surface change. Or as we like to say here in Mississippi in mid-July when the day time temperatures have been around 95°F for several days in a row, “Sure is steamy out there.”

It has been known for quite some time that one of the nicer features of being a human being is the ability to think, ask questions, and learn. And so today I was thinking about the Heat Index or “HI” as the experts like to call it. What, exactly, is the heat index? Sure, sure. It’s what it “feels” like when the temperature at 9pm is 82°F and the relative humidity is 89%.

Even Tiger knows it “feels” steamy. I am told it “feels” like it’s 92°.

But what does the HI mean? How is the HI calculated? If I wanted to determine what the HI was under the shade of the cedar tree over by the tomato garden, how would I do that?

To answer that last question, I searched for “how to calculate heat index” on the World Wide Web. The World Wide Web told me that I’d need to get a thermo-meter, and a humido-meter, and read the two numbers. I should be careful to read the F-scale not the key of C on my thermo-meter– AND AND!!! (GASP!!!) I’d have to convert a % to a decimal!!!!!!!!!!!!! on my humido-meter. And I’d need to plug those two numbers into the boxes on the web site and then I’d know what the heat index was under the cedar tree.

That was not the sort of answer I was looking for.

(I have tried and tried to train up that stupid WWW and it jist keeps on a spitting back e-How and About answers at me when I ask it a question. When will it learn that I am a thinking human being who wants real answers and who is not afraid of numbers?)

You may be wondering why I didn’t just look the formula up in a crappy old book and that’s a good question. The answer is a) HI is a relatively new statistic; I don’t know as I have a crappy old book new enough to have that formula, and b) dude. It’s hot out there. When I come inside from being outside where it feels like it’s 107° in the shade to pass the time in idle inquiry, the last thing I want to do is be rooting around looking for something I don’t even know I have. (I’ve been thinking about this all day, by the way, so pardon the disparate temperatures.) So I default to the World Wide Web.

I hate to say it but Wikipedia to the rescue.

I still have a lot to learn– there are a lot of assumptions and I *think* we’re talking about fitting lines but I’m not clear on the biological/physical justifications for the constants (see the graphs above)– but according to Wikipedia, which tells me HI wasn’t even a thing until the late ’70s (which would explain my reaction 40 years ago when I asked, “What the hell is the heat index?”) the general form of the equation is this, with a different constant for each of the nine elements.

Where

T = Temperature in degrees F equal to or greater than 80

and

R = Relative humidity equal to or greater than 40

and not included here, but including C1…9 (each of the nine terms has a numerical constant that can be tweeked if T or R are tweeked)

HI = T + R + TR +T² + R² + T²R + TR² + T²R²

Not entirely satisfying, is it?

Nevertheless. It is hot and steamy out there this evening.