Everything, especially humidity, is relative

10 years ago

In the second installment of our new weather column, I want to talk about a single number you can look at to determine how humid it feels outside. Most everyone looks at “humidity,” which is actually short for “relative humidity.” The word “relative” is in there because it is dependent upon the temperature.

It is commonplace to see summertime relative humidity at 95 percent at sun-up, then have it fall to 40 percent by mid-afternoon. That 40 percent simply means that the air is holding 40 percent as much water vapor as it could hold at that temperature, and warm air can hold more water vapor than cool air.
Dewpoint, on the other hand, is a proxy (stand-in) for the ‘actual amount’ of water vapor in the air … so the higher the dewpoint, the more water vapor there is. It’s that simple.
So in terms of human comfort, I have found that most “Native Northern New Englanders” find that it feels muggy when the dewpoint reaches 60, very muggy when it’s 65, and extremely muggy when it’s 70. This information can easily be found by simply Googling “Current Maine Dewpoints.” If you have trouble finding them, please feel free to send me an email.
By the way, available water vapor is one of the ingredients for thunderstorms. So high dewpoint days should be considered “thunderstorm candidate days.” On those days, one should be what I like to call “sky aware.” High dewpoints alone, however, are not a guarantee of thunderstorms.
Finally, relative humidity, while not a good gauge for human comfort, is useful for other things though — for example, higher relative humidities slow wildfires, while lower relative humidities are great for drying your clothes outside on the clothesline!
   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 tedintheclouds @gmail.com.