Tuesday, August 17, 2010

It was hot today: the dirt on the heat index.

Fer reals.  The National Weather Service predicted that it would feel like 110°F (Fahrenheit) today.  Same for tomorrow, I hear.  This does not bode well for the following polar bear:
Maryland: Where men are almost as tall as mountains.
What are they saying when they predict it will feel like 110°F, even if it is only [only: ha!] 102°F?  They are talking about the Heat Index.  At first I thought it was the same as the term "effective temperature".  Oh, but no; when I looked up that definition, I got this from Columbia University: 
The effective temperature of a planet is the temperature it would have if it acted like a black body, absorbing all the incoming radiation received at its surface and reradiating it all back to space.
That's not what I want! This is what I was looking for, from NOAA's National Weather Service:
The Heat Index (HI) or the "Apparent Temperature" is an accurate measure of how hot it really feels when the Relative Humidity (RH) is added to the actual air temperature.
Please click on their link for all the details, it is pretty good text, but in the mean time, here is my favorite part:
The body's blood is circulated closer to the skin's surface, and excess heat drains off into the cooler atmosphere by one or a combination of three ways...
  • radiation,
  • convection, and
  • evaporation.
At lower temperatures, radiation and convection are efficient methods of removing heat. However, once the air temperature reaches 95°F (35°C), heat loss by radiation and convection ceases. It is at this point that heat loss by sweating becomes all-important. But sweating, by itself, does nothing to cool the body, unless the water is removed by evaporation (sweat changing to water vapor). The downside of this method of cooling is that high relative humidity retards evaporation.  (reference link)

Considering their explanation, I'm surprised that wind speed isn't factored into it.   Doesn't wind promote evaporation?  Cuzzzzzz it blows the humidity away?  

But here is a funny thing part: the heat index was first introduced by R.G. Steadman (1979) in his document called  "The Assessment of Sultriness, Parts 1 and 2." 

Sultriness. Hee!


  1. Scientifically speaking, you are the Sultan of Sultriness.

  2. Hi Mortl! Yes, wind facilitates evaporation by helping maintain a vertical moisture gradient so long as the air in the upward direction isn't near saturation point (100% relative humidity). This, of course, varies with temperature due to air capacity (for moisture) varying with temperature. --Horticle

    this is a pretty good link

  3. Great thoughts, Horticle! I like the link too. Do you know of a math equation I can use to describe it? I can just make one up ;-) Thanks for the visit!