So there you are, on a sunny summer day, pondering which great County hike you are going to enjoy.! You check the weather and see that the humidity is 48 percent, so you know it won’t feel muggy while hiking, right? Wrong!
“Humidity,” in the way it is used in weather reports, is short for, “Relative Humidity.” Relative Humidity expresses how much water vapor there is, compared to how much there could be if the air was completely saturated.
But here’s the key. The warmer it is, the more water vapor that can be held. So keeping actual moisture constant, as the temperature goes up, the relative humidity goes down. Relative humidity is always dependent on the temperature. It’s meaningless as a number to use to determine whether it’s muggy or not; for that, you use the dew point.
Dew point, as I’ve written in the past, is a stand-in, or proxy, for the actual amount of water vapor there is in the air. The actual amount is expressed in various ways, including grams per cubic meter.
Now at the start of this column, I wrote that you checked the weather and found the humidity, which you now know is short for “relative humidity,” to be 48 percent. Sounds comfortable to most people because it’s less than 50, so it isn’t even half! Well, on the day in question, that 48 percent RH day, the temperature was a very warm 83 and the dew point was 63. That’s squarely in the muggy range on my Dew Point Comfort Scale. (“muggy” starts at 60, “uncomfortable” starts at 65, and “Intolerable” starts at 70).
Another way to say it is that on that day, the 83 degree air was holding 48 percent as much water vapor as it could. It was about one-half saturated. But for northern Mainers, even that would be too much water vapor for them. Many people prefer dew points that never rise above 60. The more water vapor there is in the air surrounding us, our cooling mechanism, the evaporation of our sweat, works less efficiently, so many people tend to feel sluggish and weighed down.
A quick review: the day was sunny and the winds were light. You got ready to go out, thinking it would not be muggy because the humidity was 48 percent. But had you known that the dew point is the best summer comfort indicator, and had you known my dew point comfort scale, at a dew point of 63, you would have prepared for muggy conditions.
But back to Relative Humidity. It is a useful number to know, for a number of things. The higher the RH, the slower things will dry. The lower the RH, the faster things will dry. And when you get very low RH, breezy to windy conditions, and an antecedent dry spell, the stage is set for wildfires.
Maine gets wildfires. There was a big one down in Bar Harbor back in 1947 that burned a number of the famous old wooden hotels built in the 1800s.
Many years ago, I lived in a place where the RH was less than 10 percent from time to time. I’d get out of the shower and I’d barely need to dry my hair, the evaporation was that fast. The lower the RH, the greater the rate of evaporation. This rate is called “flux.” So a very low RH would lead to a high moisture flux (and now I’ve given you a good new way to use an “x” in Scrabble).
In terms of water conservation, the best time to lose the least water to evaporation is when the RH is high, because the moisture flux will be low. And the highest daily RH is typically during the early morning hours.
A final summer safety note about low RH days, the evaporation of your sweat is almost instantaneous.That means you can be losing a lot of water without realizing it, because you don’t see yourself sweating a lot. It’s critical to stay sufficiently hydrated on low RH days.
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.
