On a very muggy day, how many times have you heard people say, “The air is so heavy today”?
Kind of makes sense, given that water weighs nearly nine pounds per gallon, and on a very muggy day (dew point well above 60), the air is chockfull of water vapor. The higher the dew point, the more water vapor there is in the air.
So, if you took a hunk of air on a muggy day and dropped it on a scale, it would of course be heavier than it would be on a non-muggy day.
Except for one problem. It’s the opposite. The air is actually less dense on a very muggy day. That helps baseballs fly out of the park more easily than on a cool, dry night, where the air is more dense.
So, now that we know that, why in the world do we feel so “weighed down” and sluggish on a day where the air itself is lighter? The answer comes from understanding how we as mammals cool off. We sweat. And then the “cooling part” comes from the evaporation of our sweat. But if the air already contains a lot of water vapor, it is harder for it to accept more from the evaporation of our sweat.
So it isn’t that the air is heavy on a very muggy day, it’s just that our cooling mechanism, the evaporation of our sweat, works less efficiently!
I’ve written before of my dew point comfort scale. It allows you to look at a single number and determine how humid it is going to feel outside.
I recently thought of another way to express dew point comfort (with apologies to Clint Eastwood) — the good, the bad, and the ugly.
The good: 40s (Most people find dew points in this range to be very comfortable).
The bad: 60s (Most people become increasingly uncomfortable as dew points rise through the 60s).
The ugly: 70s (Most people find this weather to be truly oppressive. In my formal dew point comfort scale, at 70, I use the word “intolerable.” It’s a strong word, but County folk have strong feelings about that kind of mugginess.
You’ll note the 50s are missing. That is a range where most people notice, as you get toward the mid 50s, that it is starting to get just a touch humid.
It is important to note that my dew point comfort scale is for our region, which is to say it is for people acclimated to our climate. In the southern U.S., 70-degree dew points are much more common, but the people who live there have become acclimated to that weather. I have a colleague who lives in Tulsa, Oklahoma, and he tells me that when the dew point drops to 65, people consider it refreshing, and they open their windows to let it in.
When it gets too muggy for you, and you want relief, what you want is a cold front. Many people think that a cold front brings only cooler weather, but actually the air behind a cold front is typically cooler and/or drier. In fact, just last week we had a cold front passage where the following day was only a few degrees cooler, but the dew point temperature was about 20 degrees cooler, making for a dramatically more comfortable-feeling day.
A final (and unrelated) note. If you are reading this on Wednesday, July 24, the day the Star Herald comes out, you are reading it exactly 50 years after Apollo 11’s crew returned safely to Earth, after their historic mission in which human beings set foot on the moon for the first time.
And lastly, a recommendation: If you can find the series, “When We Left Earth,” watch it. Utterly compelling.
Till next time, be well.
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.
