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Now That's Humidity
Thu Sep 04, 2003
Listen in RealAudio 
Hi, I'm Bryan Yeaton and this is The Weather Notebook. So did you endure a day with 95 degrees
and 95 percent humidity last summer? In fact, that's almost impossible-but almost is the key
word. It's because hot air can carry far more water vapor than it almost ever does. Let's look
at what relative humidity really is.
The RH tells you how much water is in the air compared to the most that could be present at a
given temperature and pressure. The warmer the air, the more water it can hold. Let's assume
the humidity is 50 percent. If the temperature's around 72, then it's a comfortable fall day.
But if it's 90 degrees with the same relative humidity, then that hot air is holding twice as
much moisture as before, and your shirt is probably sticking to your back.
In the U.S., the warmest temperature that ever sees saturation is about 85 degrees. However,
there's one place on earth where you just might get 95 degrees and 95 percent humidity. That's
on the Persian Gulf coast, where the sea water is more like bathwater. In July of 2000, a
station near Dubai in the United Arab Emirates observed a world-record dewpoint of 93 degrees.
That means once you've cooled the air to 93 (if you can call that cool), you'd have a hundred
percent humidity. That's almost twice the moisture you'd have on the muggiest summer days in
Boston or New York. On the plus side, you wouldn't have to go out and get moisturizer for your
skin.
Thanks to writer Bob Henson of Boulder, Colorado. The Weather Notebook is produced by The
Mount Washington Observatory and supported by Subaru of America and The National Science
Foundation.
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