Question

What is the "heat index" that the weatherperson talks about during the summer? Also, why is the air comfortable at only 80 degrees F even though body temperature is 98 degrees F?

Answer

During an average day your body burns about 2,000 calories (when you are exercising heavily it burns a lot more). During waking hours that means you are burning 2 calories or so a minute. These 2 calories have the ability to raise the temperature of 1 kilogram of water 2 degrees C. If you weigh 50 kilograms (110 pounds), your body temperature rises 1/25th of a degree C (1/12th of a degree F) every minute.

Your body needs a way to dump that excess heat. If it doesn't, then body temperature rises into the danger zone in a matter of half an hour. Up to about 80 degrees F (24 degrees C) it's easy to dump excess heat simply through radiation (this is why air temperature "feels" comfortable up to about 80 degrees F). Above 80 degrees F your body does not have enough surface area to radiate the heat away fast enough, so your body turns on your sweat glands to make evaporative cooling possible.

Evaporative cooling works great if the air is dry. In high humidity, however, sweat doesn't work very well - the sweat cannot evaporate because the air is already saturated with humidity. In high temperature/high humidity environments your body can get into a dangerous situation where it cannot radiate or evaporate the heat away. The heat index that you see on the evening news is designed to make you aware of these dangerous situations.

The heat index takes the day's temperature and humidity into account and calculates what the temperature would be if the air were at 25% humidity or so (very dry). On this scale, high humidity can be excruciatingly hot because your body has no way to eliminate excess heat. For example, 100 degrees F with 100% humidity is the equivalent to 195 degrees F - nearly the boiling point of water!

Here are several interesting links:

• Heat and Humidity can be a deadly combination
• Heat index calculator
• Contains the actual formula for the heat index

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