Time to ‘dew’ it again

“My eyes glaze over when you talk about that. Why not stick to the old percentage thing?” 

That was a recent comment I received, complaining about my frequent summertime use of dew point to describe how muggy or not muggy the air is.

Here’s the answer. “The old percentage thing” doesn’t tell you anything about how comfortable or uncomfortable it will feel outside. By “the old percentage thing,” the person was referring to relative humidity (RH), which is indeed expressed as a percentage. 

Notice that “relative humidity” has the word “relative” in it. OK, relative to what? To the temperature. Keeping actual moisture (water vapor) constant, the relative humidity changes as the temperature does, and it does so in an inverse fashion. As the temperature rises, the relative humidity falls, and as the temperature falls, the relative humidity rises. This is because warmer air has a higher carrying capacity for water vapor. 

Want to dry your clothes on the line? Do it in the afternoon. The low RH air surrounding your wet clothes and sheets will wick (pull) the moisture right out of them, and they will dry more quickly than they would in the morning, when the temperature was lower, and the RH was higher. The lower the RH, the faster the drying; the higher the RH, the slower the drying.

The actual water vapor in the air is expressed as a quantity, such as grams per unit volume or grams per unit mass (weight). Because warmer air has a higher carrying capacity for water vapor, a cubic meter of fully saturated air at, say, 86 degrees F, holds a lot more water vapor than a cubic meter of fully saturated air at 50 F does.

The 86-degree fully saturated air (“fully saturated” means the relative humidity is 100 percent) holds 30.4 grams of water vapor per cubic meter, whereas fully saturated 50 degree air holds only 9.4 grams.

Human comfort is governed by the actual amount of water vapor in the air. When there’s a lot of it, our cooling mechanism, the evaporation of our sweat, can’t work as efficiently, and therefore we feel uncomfortably hot. The stand-in, or proxy, for the actual amount of water vapor is the dew point. The more water vapor there is in the air, the higher the dew point will be. 

You can find the dew point each and every hour by googling NWS GYX RWR. The fourth column in from the left, with the header, “DP,” shows the most recent top-of-the-hour dew point temperatures in Maine. The higher the dew point, the more muggy it will feel. For native northern Mainers, dew points of 50 become noticeable, but it isn’t until it gets to about 60 that most folks find that it feels muggy.

Here’s a great example of why to use the dew point and not the RH as a “comfort gauge.” Let’s take a 90-degree day with a very muggy dew point of 65. This yields a relative humidity of 43 percent. That number means that at that temperature (90), the air is holding 43 percent as much water vapor as it could hold, again, at that temperature. And most folks would think 43 percent is not muggy, after all, it’s less than 50 percent. Then, they walk out into 65 degree dew point air and find out that’s not the case at all. I have found that most “native northern New Englanders” find that it feels muggy when the dew point reaches 60, very muggy when it gets to 65, and extremely muggy when it reaches 70. 

It might seem that the difference of only 10 degrees between a dew point of 60 and a dew point of 70 might not be that big of a deal, but at 70, we’re talking almost 16 grams of water vapor per kilogram of dry air, while at 60, we’re around 11 grams. So a 70-degree dew point has almost half against as much water vapor as a 60-degree dew point, and that’s why 70 degree dew points feel exceptionally muggy.

By the way, water vapor is one of the ingredients needed for thunderstorms, so on high-dew-point days one should be what I like to call “sky aware”. Look for towering, massed-together cumulus clouds. High dew points alone, however, are not a guarantee for thunderstorm formation.

Ted Shapiro holds the Broadcast Seal of Approval from both the American Meteorological Society and the National Weather Association. An Alexandria, Va. native, he has been chief meteorologist at WAGM-TV since 2006. Email him at tshapiro@wagmtv.com.

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