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Figuring Out the 'Heat Index'

By Ben L. Kaufman · August 17th, 2011 · On Second Thought

Day and night, local TV weather forecasters tell us how hot and humid it is and will be. They use “heat index” in summer, just as they had “wind chill” in winter, to increase our anxieties about weather conditions.

My question:

Although forecasters state “heat index” as fact, how do they know how hot any of us will feel?

Humidity affects comfort. I knew that from youthful hiking and camping days, decades before anyone invented a heat index. Overseas reporting reminded me that dry heat in Egypt’s Valley of the Kings was less enervating than the Congo rainy season.

Still, I wanted to know if the heat index is more than a way of dramatizing what we already know in Cincinnati: It’s hot and it’s sticky. I wanted to know, in part, because the “heat index” affects decisions on whether Cincinnati opens cooling centers at taxpayer expense.

And I wanted to know what the TV weather terrorists didn’t say: Who created the index meant to tell us something beyond temperature and humidity readings.

I went online and found background research, mathematical formulae, some standards for clothing, and subjective conclusions. On the National Weather Service’s Wilmington, Ohio, website, Seth Binau, science and operations officer, wrote:

“The National Weather Service uses the heat index as a measure of the apparent temperature or what it feels like to a human being outside in the elements.The heat index was developed in 1978 by George Winterling, based on work by Robert Steadman. The index is a complex mathematical equation which takes into account the ambient air temperature, ambient relative humidity and makes assumptions on what the air feels like to a human of average weight/height walking in a light breeze wearing long pants and a short-sleeved shirt.”

Get that? “Apparent temperature,” “assumptions” and “average weight/height.”

Meteorologist Winterling developed the index in the late 1970s as a TV weatherman in Jacksonville, Fla. Steadman was a member of the textiles and clothing department of Colorado State University. His landmark paper on heat/humidity is online.

I found the explanation by Slate.com’s Daniel Engber most useful:

“Moist air feels hotter than dry air because it makes sweating less efficient.

On a hot, dry day, your sweat will evaporate quickly and cool your skin; under humid conditions, sweat evaporates more slowly and doesn't do as much. Just as the wind chill attempts to measure how cold it feels under certain wind conditions, the heat index tries to measure how hot it feels given the humidity.

“ . . . Steadman wrote a paper called ‘The assessment of sultriness,’ in which he used a list of 20 factors to compute how hot you might feel on a given day. These factors included the rate at which you sweat, the type of clothes you're wearing, the surface area of your body, and what you happen to be doing.

“To isolate the effects of temperature and humidity on the perception of heat, Steadman invented a typical situation: A person who's 5 feet 7 inches and weighs 147 pounds walks at about 3.1 miles per hour in a light breeze, wearing long pants and a short-sleeved shirt. Then Steadman filled out his 20 variables with information from this scenario and figured out how hot his fictional person would feel at different outside temperatures and levels of humidity. He put the results in a table: Higher humidity would make his exemplar feel hotter, while drier conditions would make him feel cooler than it really is. For any given temperature, there is a percent humidity at which the weather ‘feels’ exactly as hot as the thermometer indicates.”

The National Weather Service embraces this theory but says it uses a simpler formula than Steadman proposed. At the Wilmington office, Binau said July 11 saw a peak of 114 and six consecutive hours when the heat index reached or exceeded 110 degrees, “making it one of the most uncomfortable days in Cincinnati history since 1930” when the index was 116.

Aha! Comfort. Or how it feels.

But what if I’m not 147 pounds and 5'7”? I haven’t been either in decades.

I asked Binau about that, and he replied, “As far as I know, yes, the Heat Index is based off of Steadman's work and the hypothetical person. Why those size/dimensions? I have no clue. I noticed he cites work done by Newburgh in 1949. I could not track down that paper on how he derives what a ‘standard human’ looks like.”

Well, however it feels different to that mythic person, we’re both inclined to seek air conditioning.

Curmudgeon notes:

• It’s no antidote to heat and humidity, but here’s the story of the index’s evil twin, so beloved of TV forecasters, wind chill. This from Slate.com’s Sam Schechner:

“Despite what you might hear on a weather report, wind chill isn't actually a measure of how cold the air feels. A wind chill temperature is, rather, a calculation of how cold it would have to be to cause the same rate of heat loss from your skin if there were no wind blowing. Wind chill approximates how cold it feels because the rate of heat loss corresponds with skin temperature, and skin temperature is what our nerves sense.

“Wind chill was first measured in 1941, when two explorers, Paul Siple and Charles Passel, spent a long Antarctic winter measuring how long it took water to freeze in plastic containers left outside, hanging in the wind. The stronger the wind, the faster the water would freeze.

“Using this data, they created the first equation to calculate wind chill. Their original formula, published in 1945, was revised slightly in the intervening years, but was always found, especially by those who lived in very cold places, to be a poor measure of how cold the air actually felt to the skin.

“ . . . So, in 2001, a gaggle of agencies from the United States and Canada — collectively, the Joint Action Group on Temperature Indices — agreed on a new formula for calculating wind chill and conducted experiments to measure exactly how much heat is lost from human skin in a windy environment.

“Twelve volunteers donned winter coats and walked on treadmills in a freezing wind tunnel, all while wearing thermal sensors on their faces, inside their mouths, and in their rectums.

The catch with this new model, which is still in use, is that heat loss from skin — known as skin tissue resistance — varies greatly from person to person. Those with thicker skin have greater skin tissue resistance, for example. But high resistance, oddly, puts one at greater risk of frostbite because the skin conducts less heat from inside the body to warm its surface.

“To be on the safe side, the new model is calibrated to unusually high skin resistance — 95th percentile — and therefore overestimates how cold it feels for those with more average skin.”

Aha! “Average.” “How cold it feels.” Is there for someone 5'7” and 147 pounds. Chilling thought.