Dew Point
The dew point is the temperature at which the moisture in the air will condense onto a surface at that particular time. People talk about "humidity" when they are trying to describe how muggy or sticky it feels out. Dew point is a much better measure of that sensation than "relative humidity" is. The drier the air, the cooler it has to be to form dew. So a low dew point will be drier than a high dew point.
I found this easy to understand chart on the internet. It's probably subjective, but here you go.
Right now in DC, the Dew Point is 76. "Miserable." I think that's a scientific term.
To contrast with that, my parents' town in lovely Maine has a Dew Point of 50 right now.
There's lots of information out there to describe Dew Point better than I just have, but
here are a
couple of links to get you started.