Your FOX Weather app says the relative humidity is only 60%, but sweat is pouring down your face because the air feels so muggy despite a measly 60% not sounding all that humid. Now you can’t even fathom what a relative humidity of 100% must feel like.
But the next morning, the relative humidity does reach 100%, yet you need a jacket for your walk to work or school. What is going on here?
Relative humidity is exactly that: relative to the air temperature. It could be 100% with a temperature of 80 degrees or a temperature of 20 degrees. Most likely, it’s not going to feel muggy at 20 degrees no matter what the relative humidity is at that temperature.
But it can feel humid if the temperature is 80 degrees and the humidity is only 60%. That’s because the dew point is 65 degrees.
The dew point is the temperature to which the air must be cooled in order for it to become saturated. It provides a measure of the actual amount of water vapor in the air – so the higher the dew point, the more moisture in the air.
Relative humidity increases as the air temperature drops to the dew point or the dew point rises to the air temperature (since the humidity is relative to the air temperature). Once the air temperature and dew point meet, the air becomes saturated and the relative humidity reaches 100%. The dew point can never be higher than the air temperature.
Similarly, as the air temperature rises and/or the dew point drops, the relative humidity decreases because the air is getting farther away from saturation.
Using the 100% relative humidity example at an air temperature of 20 degrees, that would mean the dew point is also 20 degrees. That’s indicative of a bone-dry air mass even though the relative humidity says otherwise.
In general, once the dew point exceeds 60 degrees, there’s a lot of water vapor in the air, which is what gives the air that muggy feel.
At 60% relative humidity with a temperature of 80 degrees and a dew point of 65 degrees, you’re probably going to break a sweat while doing anything strenuous – even at night or on a cloudy day – because dew points in the mid-60s are indicative of a muggy air mass.
This is why relative humidity is practically useless when trying to determine how humid the air is outside. You’re probably not going to break a sweat at 20 degrees, even with 100% relative humidity.
Weather conditions affect everyone differently, but a good generalization is that dew points in the 50s or lower are comfortable for most people, the 60s are humid and somewhat uncomfortable and the 70s are oppressive and very uncomfortable. On rarer occasions, dew points can even get into the 80s, mainly along the Gulf Coast or in the corn-covered upper Midwest; that’s essentially unbearable to just about anyone.
However, these comfort and discomfort levels can vary by region because people who live in the very humid climates of the southern United States might be able to tolerate dew points in the 60s far better than those who live in the much drier climates of the northern U.S.
"Dew points in the Seattle area typically hover in the 40s or 50s in the summer due to our proximity to the cool waters of the Pacific Ocean," said FOX Weather meteorologist Scott Sistek, a resident of the Pacific Northwest. "It's a rare occasion when dew points climb into the low 60s, with mid-60s nearly unheard of. Thus, on the few occasions they reach that level, Seattle residents surely note the very muggy conditions."
In contrast, Sistek said that in the tropical climate of Florida, dew points in the low 60s during the summer are likely a cause for celebration since much of the season has dew points in the 70s there. So it all depends on what your body is accustomed to experiencing each year.
Next time you open your FOX Weather app in search of the humidity, perhaps you’ll consider looking for the dew point instead?