It’s a T-shirt kind of day in Washington, DC: 85 degrees with 63 percent humidity. We all know those numbers mean hot, but what exactly does that humidity percentage tell us? And why should we care?

Imagine you’re sitting among friends on the porch with a big pitcher of ice cold lemonade. The heat is searing and the air is still and stifling. It’s almost hard to breathe. You look to your friends ...