Humidity steams city but keeps it cool

It was steamy at midday along the Augusta Canal Towpath but Horace Duncan, 59, slogged ahead on a 14-mile run/walk.


"We've had a few humid days," he said. "A little sweat helps."

Humidity and its effect on sweating to cool the body is why forecasters use a heat index to calculate how hot the temperature really feels. But before you curse the humidity, think of what the alternative might be like, Georgia's state climatologist said.

Humidity makes it feel hotter because it interferes with the sweat evaporation process. Sweating is the body's primary way of cooling itself, said Wendy Bollag, a Medical College of Georgia researcher who specializes in the skin. Actually creating sweat dissipates some heat energy, and then the process of evaporation dissipates more heat energy, Dr. Bollag said. Think of it as a pot of water on the stove, which needs an input of heat energy to get the pot boiling.

Sweat is "water there that then gets converted to vapor, and as it does so it extracts heat from you," Dr. Bollag said.

Humidity is a measure of water vapor in the air, and the higher it is, the less capacity there is to absorb more from evaporating sweat.

"If there's a large amount of moisture in the air, it just makes it much more difficult to evaporate that, and thus you don't cool," said state climatologist David Stooksbury, an associate professor of engineering and atmospheric sciences at the University of Georgia in Athens.

There also might be a "double whammy" from that sweat clinging to your skin, because water stores heat better than air, Dr. Bollag said.

"Once you get that water build-up on your skin, and there's no place for it to go, it's actually going to start storing the heat, which I think to a certain extent that makes you feel hotter," she said.

Though humidity makes the heat feel worse and potentially more dangerous, the moisture also could be keeping things cooler.

Augusta and Phoenix, for instance, are on about the same latitude geographically but have much different climates, Dr. Stooksbury said, and moisture plays a role in that. The energy from the sun goes into the soil in both places, but in Augusta part of that energy is dissipated by evaporating moisture in the soil that becomes water vapor in the air, he said.

"It increases the humidity, but it doesn't heat the air itself," Dr. Stooksbury said. But in the desert in Phoenix, the soil is dry.

"So that energy that would go into evaporating moisture here in Georgia, in Phoenix that energy goes into heating the soils. And then of course the air above it," Dr. Stooksbury said.

The heat and the humidity were all relative to Brandice Williams and Candice Barr from Fort Lauderdale as they strolled along the canal path.

"If this was Florida we wouldn't even be outside," said Ms. Williams, 19.

"Or at a pool," said Ms. Barr, 17. "Or at the beach."

Reach Tom Corwin at (706) 823-3213 or