Instead of looking at the relative humidity, pay attention to dewpoint

(Photo by Dion Ogust)

It has been like Brazil. We’ve endured a very humid few weeks in late June and now early July, with welcome relief lasting only briefly.

Meteorologists express humidity with the word dewpoint. Air with a 65-to-75-degree dewpoint is very humid. If the dewpoint is around 60 degrees, it’s slightly humid. A dewpoint in the 50s feels dry and comfortable.

During late June, the dewpoint was stuck above 70 degrees, which is what they routinely experience in the Carolinas and the Gulf Coast. One quick indicator of extreme humidity is when your windows fog up on the outside. This only happens when the dewpoint is higher than your air-conditioned room’s temperature, so the cool glass chills the air hitting it to where it can no longer hold its moisture as vapor. Likewise, humid air chilled to its dewpoint is what makes toilet bowls and pipes sweat – which they’re been lately doing like crazy. It also guarantees that early-morning lawns are wet with dew.


But most people mistakenly judge air dampness by the “relative humidity” percentage. Why is this misleading? Glad you asked. A humid morning may be misty because the air’s cool dawn temperature hovers near the point where it can’t hold its moisture. Thus, the relative humidity at 7 a.m. may be 99 percent. As the day warms and we reach an afternoon reading of 88 degrees, the radio announcer says that the humidity is now 35 percent. Does this mean that the air has dried out? Not at all. But since warm air can hold far more water, it simply means that at 88 degrees the air is now holding only 35 percent as much moisture as it can absorb. But back at 7 a.m., the chilly dawn air was holding virtually all the water it could, which is why it was starting to condense into liquid droplets as mist, fog and dew.

The change from 99 percent to 35 percent relative humidity during the course of just a few hours make many think the air has become drier. It has not. It’s still the same air mass. It still feels just as sticky. Thus, instead of trusting the relative humidity, pay attention to dewpoint. You won’t go wrong if you remember that dewpoints in the 40s and 50s mean dry air, low 60s mean medium-dry air, high 60s and 70s indicate steamy conditions. During weather programs, listen for the dewpoint.

Humid air contains countless tiny droplets that whiten the atmosphere. Thus, on humid days, the sky near the horizon is white and not blue at all. Even the overhead sky is light blue, not cobalt.

At night, such conditions manifest as a sky limited in star-count. This isn’t a good time to look for faint galaxies or the Milky Way. But one very excellent benefit unfolds for backyard astronomers: On a humid night, the air loses its heat very slowly. Air can never cool below its dewpoint, which limits how much the nightly low can plummet. The resulting air temperature homogeneity allows light from the universe to arrive without being refracted (bent) one way and then another by varying air densities (colder air is denser). Incoming light thus takes a straighter path.

Result: Stars don’t twinkle on humid nights. And telescopically, fine detail emerges when observing planets. Images are rock-steady. Point a telescope at the Moon, or Saturn (the medium-bright star one-third of the way up the sky in the southeast the first couple of hours after nightfall), and you’ll be treated to exquisite views of rare quality. This is the happy-face side of summer moisture.