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Posted at 10:30 AM ET, 06/ 9/2009

Reader Mail: Use of the Dewpoint Should Die

By Capital Weather Gang

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.

By Capital Weather Gang  | June 9, 2009; 10:30 AM ET
Categories:  Capital Weather Gang, Education, Lipman  
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"So the air temperature can never go lower than the dewpoint temperature, since the relative humidity cannot exceed 100%."

This is a pretty silly and patently untrue sentence.

Posted by: halaji | June 9, 2009 10:38 AM | Report abuse

Good explanation, guys.

I've liked the dewpoint reporting for the reasons you explain--if you know what it is it gives a pretty good sense of what the conditions are like, especially during the summer. Humidity, because it's relative, requires more information to give a sense of what conditions are like.

Posted by: ah___ | June 9, 2009 10:39 AM | Report abuse

I disagree with Matt from DC. The dewpoint is very useful in determing how much moisture is in the air. In fairness, I am a weather nerd so I have bias, but using dew point in the summer in DC is by far better than using relative humidity.

Posted by: bernydoode00 | June 9, 2009 10:45 AM | Report abuse

I like dewpoint because it refers to the actual amount of moisture in the air.

I had a family member visit from California and he commented that it was "more humid" in California than DC. But he was talking about relative humidity, not dewpoint. The dewpoint here was much higher.

Posted by: psilosome | June 9, 2009 10:52 AM | Report abuse

Normal folks don't care what the humidity, OR the dewpoint is, when it's not warm. Like Matt says, the dew point means nothing to the vast majority of us (like the metric system). In the winter, just say "we're likely to have frost tonight". And from April to October, just tell us what the humidity is and we'll be happy.

Posted by: capsfan77 | June 9, 2009 11:11 AM | Report abuse

I like what CWG does. You say things like "we're likely to have frost tonight" or "it's going to feel oppressively humid" for people who don't care about the numbers, and you give the dew point for those who do.

And for those of us who don't know much about the weather but would like to, the fact that you give both the numbers and the analysis teaches us a lot.

So I say keep giving as much information and analysis as you can fit on a page, the way you always have.

Posted by: LaurainNWDC | June 9, 2009 11:20 AM | Report abuse

Please, please do not drop the dewpoint information! I used to think that relative humidity was a more informative scale, but after much badgering and persuasion from my dear wife, I have had to concede that the dewpoint is far more informative, especially on those sticky summer days when we most care about the value. It takes a little while to learn the new scale, but once you do, it is so very helpful. Please, do not regress in your information to us!

Posted by: colinphillips | June 9, 2009 11:25 AM | Report abuse

I'm a fan of dewpoint numbers. Above 60 is generally getting into the "gross" range, near or above 70 is somewhat unbearable... not too hard to figure out. Relative humidity can mean very different things depending on temperature, etc.

Posted by: Ian-CapitalWeatherGang | June 9, 2009 11:53 AM | Report abuse

Both r. h. and dewpoint should be used. Dewpoint is useful when determining drying conditions.

Drying conditions have been absolutely awful these past few weeks. We seem to have only one or two dry days and then the rain kicks in again. We're even getting Northwoods-style thunderstorms at odd hours of the day or night...and we can't dry out for anything! Mold and mushrooms are popping up all over the place, and I'm even seeing termites and earthworms out in broad daylight. We can now take a complete drought at least through Labor Day.

It's all acting like a very nasty and prolonged hurricane season. Wonder if it will be like this all summer long...

Posted by: Bombo47jea | June 9, 2009 12:07 PM | Report abuse

Why should we give up one over the other??? Just because "Matt from DC" doesn't understand it? Apparently, there are a lot of others that do, judging from the comments.

OK, I'm a weather nerd too.

Dew point is something solid, something that can be used when you know what it is -- the temperature at which dew falls, at which the air is so saturated that it gives up moisture. While relative humidity is just that: relative. Of course, that information has value as well. Together, they provide synergy.

Keep them both! I did not vote in the survey because that option was not given.

Posted by: HistoryAnn | June 9, 2009 1:42 PM | Report abuse

Bombo, we can't take a complete drought until Labor Day. Maybe the water table can take it, but it would be a disaster for the crops.

No matter how much rain we got this spring, it would still be nice to see at least a weekly rain through the summer!

Posted by: LaurainNWDC | June 9, 2009 1:48 PM | Report abuse

"So the air temperature can never go lower than the dewpoint temperature,..."

Wrong! There is nothing keeping the air temperature from dropping below 32 degrees F. When it does, the dew that fell freezes, becoming frost.

Of course, once some moisture falls out of the air, the air becomes less humid. As the tempurature gets colder and the air gets less humid, the relative humidity changes. As the humidity lessens, the dew point drops. When the dew point drops, the dew stops falling unless the temperature drops so much that it reaches the dew point again.

In the real world, the dew that falls overnight (when it usually happens) stays on the ground until the temperature reaches the evaporation point or the sun comes out and causes sublimation.

Posted by: HistoryAnn | June 9, 2009 1:56 PM | Report abuse

Um, I think the dew point is useful for knowing if I'm going to be driving into pea soup or not. The frost point is just as useful.

I could care less about relative humidity.
If you know the dew point then you can figure out humidity.

Posted by: Havoc737 | June 9, 2009 2:17 PM | Report abuse

Ann, maybe they put the statement a little too simply, but isn't it still the case that the dew point is never higher than the temperature, no matter the mechanism that caused it?

Posted by: LaurainNWDC | June 9, 2009 2:57 PM | Report abuse

First, the dewpoint is preferable precisely because it is not "relative."

Second, the air temperature CAN NOT fall below the dewpoint temperature. In the example given by HistoryAnn, when you have dew formation or "dewfall", that will lower the dewpoint since less moisture is in the air, and the air temperature can continue to fall. But the air temperature will not go below whatever the dewpoint is, but it can go lower than what the dewpoint was.

Posted by: firstweatherman | June 9, 2009 3:03 PM | Report abuse

It does seem public reports, as opposed to in-house meteorological information, ought to be geared toward common knowledge - except where catering to tradition denies us the bennies of more relevant measurements, as sounds to be the case with ye olde relative humidity versus dewpoint.

As for today's forecast, all I can say is, OH, NOOOOOO, it's Mr. Heavy Rainstorm!!!

Posted by: jhbyer | June 9, 2009 3:40 PM | Report abuse

I'm with Matt (but I'm also not a "weather nerd"). As soon as I hear "dew point," I think, "how does that translate to relative humidity?" or more to the point, how can I find a forecast that will just tell me what the darn humidity is?

Posted by: westsloper | June 9, 2009 3:41 PM | Report abuse

I also disagree with Matt. If he has a problem with dew point, let him think of it as absolute humidity, i.e., the total amount of moisture in the air.

As Ian notes, that's far more an indicator of how yucky the air is going to feel than the relative humidity: on a stifling 102-degree day, the relative humidity might be a pleasant-sounding 40%. But that corresponds to a totally disgusting dew point of 73 degrees.

Also, the dew point is more consistent during the course of the day, while the relative humidity will vary inversely as the temperature. Thus, knowing the dew point actually tells you something in and of itself, while the relative humidity is a completely meaningless figure unless you also know the temperature.

Posted by: kevinwparker | June 9, 2009 3:52 PM | Report abuse

kevinwparker's example shows what several commenters have said about the problem with relative humidity: It's too relative. In that example, the heat index would be 113°. If dewpoint is too hard a concept to deal with, what about heat index?

Posted by: CapitalClimate | June 9, 2009 4:27 PM | Report abuse

Keep the dew points. They are a far better way of describing the amount of water vapor than relative humidity, which varies wildly depending on the temperature. In addition, a dewpoint of around 50 is generally the required threshold for most warm-season thunderstorms; 55 or greater for severe weather.

Posted by: MMCarhelp | June 9, 2009 4:28 PM | Report abuse

Humidity. Why? Because hearing people endlessly whine "it's not the heat, it's the dewpoint' just sounds silly.

Posted by: idiparker | June 9, 2009 4:59 PM | Report abuse

Folks, lets get our story straight.

When I took meteorology, in days of yore, relative humidity referred to the amount of moisture, percentage wise in the air, while the dewpoint was considered the saturation point where fog is created.

Although both are valuable metrics, I prefer dewpoint, because it requires us people to THINK.

Posted by: Computer_Forensics_Expert_Computer_Expert_Witness | June 9, 2009 5:27 PM | Report abuse

Thanks for your wisdom!

Yes, we need frequent showers throughtout the summer because of the very high evaporation rates. Otherwise we will have many parched lawns and agricultural interests will suffer. The end result will be higher produce prices.

Keep both dewpoint and RH. No big deal!!

Posted by: AugustaJim | June 9, 2009 5:34 PM | Report abuse

I've read all the posts and am still wondering why weathercasters waste time giving us the dewpoint temperature. I can only guess that they do it because they can.

Posted by: shoveit | June 9, 2009 5:43 PM | Report abuse

This is the problem -- people hear "humidity" and understand that term. However, when you put the adjective "relative" in front of it, the truth is they no longer understand what the term "relative humidity" means, BUT, they THINK they do, because they know what humidity means by itself. The "dewpoint" is a much more useful piece of information in determining if it truly is humid and sticky outside because it is "absolute".

An analogy would be "wealthy". People know what "wealthy" means. People THINK they know what "relatively wealthy" means, but they don't really, unless they're given additional information. You can call a person on welfare "relatively wealthy" compared to a starving North Korean. The dewpoint is like the salary point. You don't really want to know if the person is "relatively wealthy", you want to know how much s/he makes, goddamit! US$100,000 per year? or US$15,000 per year? Now THAT's useful information.

So 90% relative humidity? What the heck does that tell me? It's like telling me that person is 90% relatively wealthy. Give me the dewpoint or the annual salary any day.

Posted by: xandersun | June 9, 2009 5:54 PM | Report abuse

"So the air temperature can never go lower than the dewpoint temperature, since the relative humidity cannot exceed 100%."

Uh, if the temperature falls below the dewpoint, then won't the dewpoint go down? Oh, and it does this by a process called condensation. This often forms...DEW!

Posted by: praxis22 | June 9, 2009 6:17 PM | Report abuse

You want people to THINK about the dewpoint and what it means? Most of us have too many other important things to think about than try to calculate the humidity from the dewpoint or the what the heat index is or how close the freakin isobars are to the jet stream. Just tell me the temp and the humidity and I'll be happy.

Posted by: capsfan77 | June 9, 2009 7:34 PM | Report abuse

CapitalClimate -- I always like heat index. Dewpoint is good and RH I don't usually pay attention to, but heat index really lets me know what to wear!

idiparker -- i am TOTALLY going to start saying "it's not the heat, it's the dewpoint." silly or not, i love it!

Posted by: dinergirl | June 9, 2009 7:58 PM | Report abuse

Keep Dewpoint:
My guidelines for open vs closed windows at home= close them when the temperature is above 78F or the dew point is above 57F.

Relative humidity tells me nothing, as far as interior comfort goes.

Posted by: bikerjohn | June 10, 2009 7:43 AM | Report abuse

I say, keep the DP and bring back the RH, while educating the public, so readers become comfortable with only the DP, then drop the RH - or not. The more information, the merrier, right?

My pet peeve is pseudoscientific weather measures, what I call sexing-up the weather, to enable even us women to feel macho, e.g. "At IAD, it's 40 F, but the wind chill makes it feel like 19.83651 F." No, it doesn't. Factor in sunlight reflecting off snow, and it feels a bit springlike.

I expect that peeve has been aired before here.

Posted by: jhbyer | June 10, 2009 9:20 AM | Report abuse

I don't understand what is so hard about understanding the dewpoint (maybe they should have a laymen's version of the name like "mugginess factor point"). In the summertime, anything below a dewpoint of 60 degrees F is really comfortable. Anything above a dewpoint of 65 degrees F is starting to get uncomfortable. Or even simpler, just try to remember "65 and below, open a window, 65 and above, it's starting to get sticky, luv".

Posted by: xandersun | June 10, 2009 12:22 PM | Report abuse

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