U.S. summers are heating up fast, and a new study shows nearly every major city is hotter than in 1970, with extreme heat risks on the rise.

At its most basic: the heat index is what the temperature feels like when you're outside. The chart above shows the combination of air temperature with humidity. It's very important to note the chart ...

USA TODAY analysis found that 90-plus heat index days are becoming more frequent, starting earlier, lasting longer and occurring in longer streaks ...