Reader Mail: Use of the Dewpoint Should Die
By Don Lipman, Volunteer Contributor
"Matt from D.C." wants to abolish popular use of the dewpoint. In an email to the Capital Weather Gang, he rants:
It just seems pointless to introduce a term that seems of no use to the average person who isn't a weather nerd. Personally, I'd like to see relative humidity make a comeback. We all know that relative humidity has a maximum of 100 percent, and it seems a lot easier to understand than the mysterious and unhelpful "dew point." To me, a relative humidity of 71 percent tells me much more than I can ever learn by knowing that the dewpoint is 71.
More specifically, Matt wants to know (a) what the "dewpoint" really means; (b) why it's being mentioned so much by over-the-air weathercasters and; (c) why should we care about it?
Keep reading for the answers to these questions...
The answer to the first part of the question is easy: the dewpoint, or frostpoint, is the temperature at which dew or frost will form should the air temperature fall sufficiently. Other things being equal, as the temperature falls, the relative humidity rises, reaching 100% at the dewpoint, at least at ground level. So the air temperature can never go lower than the dewpoint temperature, since the relative humidity cannot exceed 100%.
The relative humidity, however, is the amount of moisture in the air compared to the amount of moisture the air is able to hold at that temperature. Warm air can hold far more moisture than cold air--thus, 85 degree air with 25% relative humidity contains about twice as much moisture as 35 degree air with 75% relative humidity. (We don't use the term absolute humidity--e.g., grams per cubic meter--because to the average person the numbers would be meaningless.) By the way, a misconception is that the relative humidity must be 100% for it to rain. Usually, while precipitation is falling, the RH is in the 70s or higher (at ground level). At cloud level, that's a different story.
Yes, relative humidity and dewpoint are obviously related: so why not use just the term "relative humidity" in the popular media? I like to use the analogy of a windowless spaceship returning to Earth after a long voyage. It lands in Washington but the crew has no idea what season it is. A garbled radio message informs the crew only that the relative humidity is 75%. What kind of weather should they dress for? Many might say warm, humid weather. They would not necessarily be right, however, as the outside air temperature could be 20 degrees Fahrenheit and it could be the middle of winter.
If, instead, in the above illustration, the crew was informed only that the dewpoint was 68 degrees, they would know at least, that mild to warm (and humid) conditions existed. In other words (and in answer to the 2nd part of Matt's question), the dewpoint is much more informative than the relative humidity alone and once people come to realize that a dewpoint in the middle to upper 60s is relatively uncomfortable, that's a chunk of information. (Almost everyone is uncomfortable with a dewpoint over 70 degrees.)
Equally important is the spread between the air temperature and dewpoint temperature: the larger the spread, the more comfortable it feels because the air is drier and can evaporate perspiration from our skin more easily (nature's cooling mechanism). This is the reason that Washington's 92 degrees with a 67 degree dewpoint feels so much more oppressive than Phoenix's 105 degrees with a 40 degree value. (A 25 degree spread here but a 65 degree spread there.)
Part (c) of Matt's question was "Why should we care about the dewpoint?" Aside from the above reason, the frostpoint is useful to know about in the winter for two additional reasons: (1) to determine whether a car parked outside might have to be scraped in the morning * and; (2) to get a rough estimate of the minimum temperature that night. **
Got a question for the Capital Weather Gang that you'd like to see answered on the blog? Contact Us.
About the author: After a 30 year "Intelligence Community" career, Don's second life focuses on tennis and weather. When not on the courts, he writes weather articles, which have appeared in community newspapers/newsletters and gives weather talks at senior centers and on cruise ships. He's doing a weather "gig" on the Emerald Princess on August 24th in the Baltic. Interested in going along?
*Again, other things being equal, if the dew/frostpoint is at or below freezing in the evening, the air temperature is below 50 degrees, and there are clear skies and light winds (i.e. ideal radiational cooling conditions) there is a reasonable chance of frost forming, at least in suburban areas.
**The air temperature cannot drop below the dew or frost point because the relative humidity cannot exceed 100% and because when frost (or dew) forms, a slight amount of heat is released into the air, stabilizing the temperature.
Capital Weather Gang
| June 9, 2009; 10:30 AM ET
Categories: Capital Weather Gang, Education, Lipman
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