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?

ST. LOUIS – In this heat, days start warm and get hot fast. The key is not just the heat, it’s also the dew point (the temperature at which water vapor condenses). The higher the dew point, the higher ...