We get a lot of complaints about humidity. It is, after all, peak summer in D.C. — the nation’s swamp — which it turns out is hard to drain.
Specifically, some of our readers (and listeners) can’t stand that we don’t use “percent humidity.” You may be familiar with this metric from things like third-grade science lessons and the generic weather app on your phone. It’s really easy to understand — on a scale of 1 to 100, how humid is it?
Except that’s not at all what percent humidity is describing. Key to this is the modifier to the term that’s often dropped in casual conversation: relative humidity. It’s relative to the temperature.
The amount of moisture that air can “hold” changes with temperature. Cold air can hold less moisture. Hot air can hold much more moisture. (This is a very simplistic explanation of a complex thermodynamic principle, but it accurately describes the outcome, which is why I use it when I’m talking to people who really don’t care about my thermodynamic principles.)
For example, it might surprise you to know that right now — at 11:45 a.m. Wednesday — the relative humidity is just 52 percent in D.C.
Wrong. So wrong. Because it’s relative.
Here’s another example. Air that’s 85 degrees with 25 percent humidity contains twice as much moisture as 35-degree air with 75 percent humidity. My colleague Don Lipman uses a little story to illustrate this:
I like to use the analogy of a windowless spaceship returning to Earth after a long voyage. It lands in Washington, but the crew has no idea what season it is. A garbled radio message informs the crew only that the relative humidity is 75 percent. What kind of weather should they dress for? Many might say warm, humid weather. They would not necessarily be right, however, as the outside air temperature could be 20 degrees Fahrenheit and it could be the middle of winter.
Have I dogged relative humidity enough? Are you ready for me to get off my high humidity horse? Okay.
This is why we use dew point instead. The dew point is the temperature at which dew forms on things like the grass and your car windshield. If the air temperature drops to the dew point, the air is saturated, which is exactly what it sounds like — wet. The inverse is true, as well. If the dew point rises to the temperature, the air is saturated. The closer the dew point is to the temperature, the more humid it feels, especially in the summer when the air can hold a lot of moisture.
Let’s go back to today’s conditions, which gave us a 52 percent humidity. At 11:45 a.m., the temperature was 90 degrees and the dew point was 70. That’s pretty high. According to our very scientific dew point chart above, it feels “sticky” going on “muggy.”
In Washington, when it’s both hot and humid, relative humidity levels are usually between 40 and 50 percent, and very rarely higher than 55 percent. If we said that the humidity is 40 percent when it’s 100 degrees, our readers may not appreciate that it’s actually quite humid outside.
We’re sticking with dew point.