|
|
CMSE Online Front
Page |
In North Carolina's muggy summer weather, we often say, "It's not the heat, it's the humidity" causing our discomfort. And in a general way we understand what that means. Humidity refers to the moisture content of the air. If the air is dry, then perspiration evaporates rapidly. This cools our bodies because evaporating water requires heat, and that heat is removed from the surface of our skin. If, on the other hand, the air is very moist, then perspiration tends to cling to us without evaporating and we lose the benefit of this cooling.
To understand humidity, we need two important facts about moisture in the atmosphere:
U.S. weather reports traditionally include the relative humidity, telling us how much water there is in the air as a percentage the maximum possible amount. At 3:00 pm July 28, 1997, RDU reported a temperature of 96°F and a relative humidity of 46%, meaning that the air contained 46% of the moisture it could possibly hold at that temperature.
The problem with relative humidity? It doesn't tell us how the air feels. If you were outside on the afternoon of July 28, you know it felt terrible. But when the temperature is only 80°F, a relative humidity of 46% feels very comfortable.
To improve reporting of humidity, meteorologists have adopted two strategies:
So what's the dewpoint? If we cool air without changing its moisture content, eventually we'll reach a temperature at which the air can no longer hold the moisture it contains. Then water will have to condense out of the air, forming dew or fog. The dewpoint is this critical temperature at which condensation occurs.
The dewpoint is a measure of how moist the air mass itself is; that's why the Weather Channel's new dewpoint maps show us the movement of moist air masses across the country. Ordinarily the dewpoint doesn't vary much during a 24-hour period. Unlike temperature and unlike relative humidity, the dewpoint is usually the same at night as it is the daytime.
In North Carolina's summers, dewpoints typically range from about 60°F to 80°F. When the dewpoint rises above 70°F the air begins to feel uncomfortably muggy. At a dewpoint of 75°F the air is definitely unpleasant, and at a dewpoint of 80°F the humidity is hazardous: sweating no longer cools human bodies effectively and heat stroke is the price of overexertion. On the afternoon of July 28, a man in Durham died of a heat-induced heart attack while working on the roof of a school building.
So it's not wrong to say, "It's not the heat, it's the humidity," but it's a lot more precise to say "It's not the heat, it's the dewpoint". If you want to know how humid it is on a hot August day, listen for the dewpoint!
Internet Sources
FEEDBACK: We'd be happy to have your comments and suggestions.
Originally posted August 1, 1997; updated June 30, 1998. Features remain online as long as they remain current; they may be updated if new information becomes available.
Copyright © 1998, Center for Mathematics and Science Education. Teachers have permission to duplicate this page for use in teaching their own classes. All other rights reserved. You are welcome to link to this page, but do not copy its contents.
|
June 30, 1998 http://www.unc.edu/depts/cmse/nature/dewpoint.html |
CMSE Online Front
Page |
