Whether a summer day is great or awful often comes down to the humidity.

Relative humidity measures how much moisture is in the air compared to how much it could hold at a given temperature. It is simply the dew point temperature divided by the air temperature, times 100.

Extreme levels of heat and humidity are forecast to spread across the country over the next two weeks, blanketing the Southwest, Midwest, and East Coast with high temperatures beginning this June 19 ...

The number on the thermometer only tells half the story. Here’s what to know about heat index—and why some scientists are rethinking how it is calculated.