Forbes contributors publish independent expert analyses and insights. I make maps, and discuss the climate. Aug 23, 2018, 11:25am EDT Dec 10, 2021, 08:30am EST This article is more than 7 years old.

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?

Friday starts mildly ahead of a hot and humid day. A few showers pop up along the sea breeze moving inland by midday. The best chance for rain and storms will be inland Friday afternoon and evening.

Relative humidity is not a good way to tell if we have muggy air on the way. Here’s why. The key word in relative humidity is ...