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Sunday, July 15, 2007

index schmindex

Has there ever been a more meaningless datum than the heat index? (Yes, the wind chill factor, but that blog entry can wait until the winter.) Our experience of climate depends on a number of factors, chiefly temperature and humidity. Obviously, high humidity exacerbates the effects of high temperature on the human body. But the heat index is an empty number that provides meaningless data. The meteorologists may as well say, 'Yo, it's hella hot out today.' That would mean a lot more than a heat index of 105.

The heat index is an American measurement maintained by the National Oceanic and Atmospheric Administration. Strangely, the NOAA is part of the U.S. Department of Commerce, but we'll have to let that tangent go for now. Canadian meteorlogists use a different scale called the humidex, wherein the dew point takes the place of relative humidity in the American formula. I'd like to think the rest of the world does not waste their time with these bogus indices.

According to this chart at the NOAA's website, a temperature of 90°F and relative humidity of 70% yield a heat index of 105, i.e. the climate is said to feel like 105°F. But why does no one ask the obvious question: it feels like 105°F at what humidity? Are we meant to infer that that is what 105°F would feel like at 0% humidity, not that anyone would necessarily know what that feels like anyway? If so, how is that any more meaningful than telling us that the temperature is 90°F and the humidity 70%?

What the heat index really measures is scientific illiteracy. The teleprompter-readers on the news programs disseminate the heat index, people hear it and repeat it to each other, and no one bothers to ask what it means. And the more one looks into it, the more ridiculous it reveals itself to be.

According to this technical document, at the website of the Southern Regional Headquarters of the National Weather Service, the heat index equation is based on several assumed magnitudes including the following:

'# Dimensions of a human. Determines the skin's surface area. (5' 7" tall, 147 pounds)
# Effective radiation area of skin. A ratio that depends upon skin surface area. (0.80)
# Significant diameter of a human. Based on the body's volume and density. (15.3 cm)
# Clothing cover. Long trousers and short-sleeved shirt is assumed. (84% coverage)
# Activity. Determines metabolic output. (180 W m-2 of skin area for the model person walking outdoors at a speed of 3.1 mph)
# Effective wind speed. Vector sum of the body's movement and an average wind speed. Angle between vectors influences convection from skin surface (below). (5 kts)
# Clothing resistance to heat transfer. The magnitude of this value is based on the assumption that the clothing is 20% fiber and 80% air.'

Basing models on assumed conditions is not unusual, but are we better off for assuming a height of 5'7" and trousers? This is science? I have a better idea. From now on, meteorologists should tell us the temperature and the humidity and, when the combination of the two is dangerously high, they can tell us, 'Yo, it's dangerously hot.' That is a warning we could all readily understand.



Blogger Doug said...

As someone who grew up in the Sonoran Desert, where the high was 105 (with very little humidity) for three months straight every year, I can tell you emphatically that 90 degrees with 90 percent humidity does not feel like 105 with no humidity.

The Heat Index has irked me for a long time, too. It's bullshit. Down with it.

Kudos to you.

10:33 PM, August 01, 2007


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