October 16, 1998 transcript # 255-5
Subject(s): dew point, humidity
Title: LISTENER QUESTION-WHAT’S THE DEW-POINT?Well, it’s listener question day again here on The Weather Notebook, and here’s one from George Walker, a WFIU listener in Bloomington, Indiana.
"My wife used to agree with the old statement ‘It ain’t the heat, but the humidity.’ Now she maintains, as she watches the various TV weather services ‘It ain’t the humidity, it’s the dew point.’ I remember from high school physics how you determined dew point and I know how to compute humidity. I’m not quite sure how the two relate, and I’d love an explanation."
Well, here’s an explanation; hope it’s a good one. Humidity is another word for the amount of moisture or water vapor in the air at any given place or time. But like the word "muggy," "humid" is a qualitative word. So to describe humidity more precisely than saying, "wicked muggy," You need a measurement. That’s what dewpoint and relative—I repeat—relative humidity are: measurements of air moisture. I think dew point is a more meaningful indication, because, for one thing, it doesn't change as fast as relative humidity.
Dewpoint only changes with changing air masses, which can hang around for days. The dewpoint temperature simply tells how much moisture there is in the air mass. It depends on one thing: moisture. A twenty-degree dewpoint is dry. Eighty-degree dewpoint? It’s wet.
Now relative humidity, on the other hand, depends on temperature and moisture. It’s a ratio between the dewpoint and the temperature. So different air masses with the exact same amount of moisture could have different relative humidities. Besides, relative humidity actually goes up when the sun goes down, and goes down when the sun goes up. Confused? Well, see, there you go. That’s why I like dewpoint: the higher the dewpoint, the muggier the whole day will be.
Thanks for the question, and thanks for listening to the Weather Notebook, a production of the Mount Washington Observatory.