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Gimme swelter

Why dew point is a more meaningful measure than relative humidity

by Bob Berman
July 28, 2011 12:12 PM | 0 0 comments | 805 805 recommendations | email to a friend | print
Unlike the summer of 1996, when our region never reached 90 degrees even once, this has been a hot one. But the old saw “It’s not the heat, it’s the humidity” is as true as ever. People have flocked to our region from the Big Apple for well over a century, and the reason is not just because we’re usually five degrees cooler; it’s also because we’re drier.

We’ve suffered through some unusually high humidity so far this summer. You newcomers to our region, don’t give up: Unlike in the Deep South where the moisture is almost unrelenting, our area normally spends most of its summer freed from the shackles of muggy air.

A quick way to judge moisture is to glance up. A deep blue sky is the mark of dry air. A light blue sky with a nearly white horizon means average humidity. A sky that’s milky overhead is very humid.

The most important humidity fact is that warm air is capable of holding far more water than cold air. Dozens of times more. The best way to gauge air dampness is to listen for the announcement of dew point. That’s the temperature at which the current air mass, if cooled down, couldn’t hold its moisture any more, when fog forms and dew appears. When you breathe on a mirror, it fogs up because the cool glass has lowered your breath to its dew point.

A dew point of 70 degrees means humid air. Always ignore the humidity percentage business, like when they say, “The humidity is 55 percent.” That’s relative humidity, which makes no sense to most people. If it’s horribly hot and muggy, we expect to hear “90 percent humidity,” and yet we’ll never hear more than “60 percent” during the midday period. Yet around sunrise, that same air somehow earned a “99 percent humidity” rating. What gives? Did the moisture change?

Here’s how to make sense of humidity. Let’s say that early-morning 70-degree air is holding all the water that it can, and fog or dew has formed. We say that this air has a dew point of 70 percent and that the relative humidity is 100 percent. The temperature and dew point are the same; the air is saturated. But six hours later, at midday, the air is 95 degrees and this hot air is now capable of holding twice as much water. So we say that the relative humidity is now 50 percent. Thanks to the increased temperature, the relative humidity has changed. At this new temperature, the air is holding only half the water of which it’s capable. Yet it’s the same air as before, moisturewise. Its dewpoint is still 70 degrees. If you chilled it later that night to 70 degrees, dew and fog would again form.

Dew point is a much better gauge of humidity because relative humidity doesn’t tell you how damp the air is – only how much more moisture the air could hold at the current temperature. To use dew point the way that meteorologists and weather nerds do, just know that a dew point of 70 or higher is extremely humid air. A dew point in the mid or low 60s is somewhat humid, and normal for summer. A dew point in the 50s is always very pleasant. A dew point in the 40s feels euphorically dry, like what they routinely get in Montana.

For comparison, the southeastern and Gulf Coast states often have dew points in the mid-70s all summer long: a steam bath. But Utah and Arizona often have summer dew points in the 30s. The greater the dew point/temperature difference, the drier the air.

Another interesting fact is that air never cools below its dew point. That’s why forecasters can be sure that “the low tonight will be 70 degrees.”

Dew point drops with increasing altitude. And since temperature also drops with altitude (a whopping five degrees per thousand feet), you can always find cooler and drier air in the mountains – which, of course, is why people have traditionally come here for the summer.

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