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

Dictating the Superiority of Dew Point

By Jason Samenow

* Moderate Dew Points: Full Forecast Through the Fourth *

I can't say I was surprised that, in a recent reader poll, relative humidity beat out dew point as the preferred metric for describing humidity. Dew point is a technical -- some would say geeky -- term. And the words "dew" and "point" offer no contextual clue that the term has anything to do with humidity, whereas relative humidity speaks for itself.

But the margin of relative humidity's victory shocked me. After nearly 1,000 votes were cast, relative humidity obliterated dewpoint in a 76% to 23% landslide. So does this mean the Capital Weather Gang will, from now on, favor the use of relative humidity?

Actually, as much as I aim to please, no. Not unlike Iranian President Mahmoud Ahmadinejad, I intend to ignore these results. Before anyone riots in the streets, hear me out as to why...

Keep reading to find out why CWG will continue to refer to dew point and largely ignore relative humidity...

If you care, here's the technical definition of dew point (but feel free to skip): The dew point is the temperature to which a given parcel of air must be cooled, at constant barometric pressure, for water vapor to condense into water.

The average person, understandably, might have a difficult time relating this jargon-filled definition to how humid it is. But they don't need to. To understand the link the between dew point and humidity during the summer, one simply needs to be able to comprehend this table:

Summer Dew Points for Dummies

Dew point
How Humid it Feels (and subjective description)
Below 55 Dry (Pleasant)
55-60 Hint of humidity (Still comfortable)
60-65 Moist (Tolerable)
65-70 Sticky (Becoming unpleasant)
70-75 Muggy (Gross)
Above 75 Sultry (Oppressive and unbearable)

The bottom line is the higher the dew point, the more humid it is. A dew point of 65 or higher indicates it's pretty humid, and if it goes above 70, it's becoming oppressive for many people. Dew points in the 50s or lower are dry and comfortable. "Typical" dew points in the D.C. area during the summer are in the upper 60s. This week, dew points are hanging out around 60 which is pleasantly dry for this time of year.

Some of the consistently highest dew points in the U.S. are found along the Gulf Coast and in Florida. For example, the dew point is a sultry 75 degrees in Miami, Fl. as I write this. Dew points, unsurprisingly, tend to be lowest in the desert climates. The dew point in Tehran, Iran is currently a bone-dry 34 degrees even though the air temperature is 88.

So perhaps I've convinced anti-dewpites that understanding how dew point relates to humidity is not hard. But why is it preferable to relative humidity?

Though the overwhelming majority of voters preferred relative humidity, nearly all the comments in response to the poll spoke out in favor of dew point and for good reason. Consider the following arguments, all of which I agree with:

*CWG reader "kwparker" explains low relative humidity readings on hot days may be misleading:

...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.

*CWG reader "xandersun" presented the following analogy, cogently illustrating why dew point is the preferable metric:

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 dew point 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 dew point or the annual salary any day.

*And finally, BikerJohn speaks to the practical advantage of dew point over relative humidity:

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.

If you're still not convinced you want to deal with dew points, don't worry. Even though I'm not keen on using relative humidity, as a benevolent dictator I'll complement my use of dew points with adjectives. (And there's another metric I'll sometimes use which we haven't really even talked about: the heat index.) So don't yet add me to your axis of evil. Then again, when I forecast brutal and disgusting humidity levels, maybe you should...

By Jason Samenow  | June 30, 2009; 10:30 AM ET
Categories:  Capital Weather Gang, Education  
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All hail the dictator! An excellent decision, for all of the reasons you cite. Dew point is by far the more useful metric.

Posted by: lp2004 | June 30, 2009 11:08 AM | Report abuse

Nerd fight!

What the poll shows is that apparently you and other weather folks need to do a better job of educating the public about the usefulness, if not superiority, of the dew point in describing weather conditions.

The thing that showed me how (relatively) useless relative humidity is is that typically the highest humidity is in the middle of the night, and the lowest during the heat of the day. But in DC what I'm concerned about is the humidity in the middle of the day when the sun is beating down.

Posted by: ah___ | June 30, 2009 11:22 AM | Report abuse


Posted by: omarthetentmaker | June 30, 2009 12:53 PM | Report abuse

We could start using mixing ratio and just mess everybody up!

Posted by: Brian-CapitalWeatherGang | June 30, 2009 12:58 PM | Report abuse

great decision and LOVE the useful table.

i do love a heat index, too...

Posted by: dinermail | June 30, 2009 1:33 PM | Report abuse


You are wrong.

Maximum relative humidity does NOT occur in the middle of the night.

On a calm night, maximum relative humidity occurs at the point of minimum temperature, usually at or just after sunrise.

This does not hold on a windy night, since wind may advect moisture into or out of the observation area. If it's breezy or windy, maximum relative humidity may occur at any time, whenever maximum moisture passes over the observation point.

As for determining how much water vapor is in the air, the best standard is the rarely-reported quantity absolute humidity or precipitable water as a function of absolute humidity. This would probably be reported in terms of grams of water per cubic meter of air. Precipitable water is expressed in terms of inches or millimeters of rain possible from moisture currently in the air. This quantity is difficult to locate but does appear on the radiosonde Skew-T or Stuve diagram for a weather station which releases radiosonde balloons. BTW the latest precipitable water reading for our nearest balloon station, Dulles Airport (IAD), is 0.90 inch. Generally I tend to use Crown Weather Services' U.S. Weather Page to find the radiosonde reports.

Posted by: Bombo47jea | June 30, 2009 1:33 PM | Report abuse

To paraphrase: The higher the relative humidity, the more humid it is.

Relative humidity rocks again!!

Posted by: shoveit | June 30, 2009 2:07 PM | Report abuse

Why dew point (TD)? Let's consider some basic biology and physics; if you'd rather skip the details, the bottom line is that body heat loss - the more the cooler you feel - is more closely related to dew point than relative humidity (RH),

The human body must continually loose heat that is generated by its metabolic processes. For air temperatures below 98.6, body heat is lost directly by contact with the environment and by evaporation of perspiration.

Consider the temperature rising from 75 to 95 from early morning to mid afternoon. The conductive heat loss is directly proportional to the difference in body and air temp, which here amounts to a 26% decrease as the the temperature rises.

As the air temperature increases from 75 to 95 the TD will ordinarily remain constant, while the RH decreases (take my word for it). For a TD of 60, the RH decreases from 85% to 69%. For a TD of 75, RH decreases from 100% to 81%. If cooling of the body was directly proportional to the decrease in RH, the maximum change would be 19% (100 - 81) - insufficient to make up for the reduced conductive heat loss as the day wears on.

On the other hand, the difference between the lower and higher TD (60/75) is 25%, which pretty much offsets the reduction in the body's ability to loose heat by conduction.

In reality, it's a bit more complicated, but fundamentally the body's ability to loose heat necessary to feel more comfortable is much more related to TD than RH

Posted by: SteveT-CapitalWeatherGang | June 30, 2009 3:51 PM | Report abuse

The body's reaction to the combination of heat and humidity is behind the theory of the heat index.

Posted by: CapitalClimate | June 30, 2009 4:12 PM | Report abuse

Yes, CapitalClimate. The Heat Index is a better measure on how you feel than either TD or RH alone. I was addressing only the question of RH or TD. The Heat Index parallels the concept of Wind Chill, which also addresses how the body feels as a function of environmental conditions and body heat regulation. See also:

Posted by: SteveT-CapitalWeatherGang | June 30, 2009 5:07 PM | Report abuse

I like this blog a lot, but please leave your baseless political commentary outside of the posts. I've yet to see any indication that the election results were either valid or not (and of course no analysis from the washington COMpost, lacking any sense of journalistic credibility...) SO please don't contribute to the American media/neo-con drumbeat.

Posted by: nykr29 | June 30, 2009 5:07 PM | Report abuse

"To paraphrase: The higher the relative humidity, the more humid it is.

Relative humidity rocks again!!"


No, not really. There's a lot more moisture in the air ("absolute humidity") at 40% RH and 102 degrees than there is at 100% RH and 50 degrees.

Posted by: kevinwparker | June 30, 2009 6:34 PM | Report abuse

Congragulations. You made the correct decision. Dew point, for most purposes, is a FAR better measure of water vapor...and discomfort......than relative humidity. Dew point also gives a definite, and constant, standard to measure from. Relative humididy is just that............."relative", depending on air temperature.

Posted by: MMCarhelp | June 30, 2009 10:27 PM | Report abuse

Can you post a permanent link to the dew point explanation chart next to the current number in current conditions? It is very useful and easy to understand. But I know I will forget it when I want to use it again. Thanks.

Posted by: jbc3 | July 2, 2009 9:08 AM | Report abuse

This is a relatively elementary enquiry: How does one calculate relative humidity? Does one need only temperature/dewpoint readings to get those percentages, or, one must have some other data as well. In any case, in my understanding, the dew point tells one a lot more about conditions (e.g., whether the air-mass is tropical or arctic), as opposed to RH which at the freezing point might be close to 90% but still uncomfortable for a lot of people! Thanks!

Posted by: chatpk | July 2, 2009 5:06 PM | Report abuse

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