Dictating the Superiority of Dew Point
* 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
How Humid it Feels (and subjective description)
|Below 55||Dry (Pleasant)|
|55-60||Hint of humidity (Still comfortable)|
|65-70||Sticky (Becoming unpleasant)|
|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...
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