Learn how to stay safe during a heat wave, including symptoms of heat stroke, risk factors, and how to cool down in extreme ...

Summer is just getting started, and millions of people are under heat advisories as a major heat wave spreads across the eastern U.S.

Instead of just looking at temperature when planning outdoor activities, check the heat index, which accounts for heat illness risk associated with temperature and relative humidity ... 15 F higher ...

As temperatures rise across the U.S., here’s what you need to know about high humidity and how it can affect you.

For example, when the air temperature is 100 degrees Fahrenheit, and the relative humidity is 55 percent, the heat index can shoot up to a sweltering 124 degrees Fahrenheit. And that’s in the shade.

When humidity is high, sweat doesn’t evaporate as effectively – and that’s a big problem for cooling down. "In the UK, the relative humidity is higher than in countries with dry heat.

Some heat safety tips are obvious — but some are surprising. Know how much water to drink and why humidity actually matters.