The dangerous heat wave sweeping the US isn’t just high temperatures. Heat index is all about how you sweat.

Kids and adults cool off in a fountain on the Rose Kennedy Greenway on the first day of a forecasted summer heatwave in Boston, Massachusetts, July 19, 2019.

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Kids and adults cool off in a fountain on the Rose Kennedy Greenway on the first day of a forecasted summer heatwave in Boston, Massachusetts, July 19, 2019.
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Brian Snyder/Reuters

  • A dangerous heat wave is projected to sweep the Central and Eastern United States this weekend, with heat warnings issued for over 147 million people. Forecasters project record-breaking temperatures, and heat indices will rise past 110 degrees in some places.
  • Heat index is how hot the air feels to the human body. It’s a combination of temperature and humidity. Sweat evaporates to cool you off, and humidity reduces evaporation.
  • Heat is the deadliest form of extreme weather in the US.
  • Because of global warming, the world is likely to see hotter, longer, and more frequent heat waves like this one.
  • Visit Business Insider’s homepage for more stories.

A dangerous heat wave is expected to sweep the Central and Eastern United States this weekend.

The National Weather Service has issued excessive heat warnings and watches for over 147 million people across large swaths of the country through Sunday evening, with heat indices exceeding 110 degrees in some places. Forecasters also expect overnight temperatures to stay high – breaking or tying 123 records – limiting relief and recovery between scorching days.

That kind of forecast can be deadly. Heat is the leading cause of weather-related deaths in the US, killing about 136 people each year.

Forecasters use heat index to determine when the wave gets dangerous and where to issue warnings. Here’s what that means and why it’s important.

Heat index accounts for humidity

Sweat might be annoying, but it’s part of your body’s temperature regulation system, and it could save your life. As that moisture evaporates, it cools you off.

High humidity saturates the air with moisture, reducing its capacity to absorb more water vapor. That means sweat can’t evaporate so easily. Your body’s best cooling mechanism is disarmed.

Heat index combines temperature with humidity to quantify what temperature it feels like to the human body. More humidity means less sweat evaporation, which means it feels hotter. The heat index, not the temperature, is the number that your body reacts to. It might be 90 degrees outside, but if it’s humid enough your body will react like it’s 117.

Heat index calculations assume shady, light wind conditions, so full sun exposure can increase heat index values by up to 15 degrees.

If your body reaches dangerously high temperatures, it can cause heat stroke and even death.

A girl cools off from the heat in water from an open fire hydrant in the Washington Heights section of upper Manhattan in New York City, New York, U.S., July 19, 2019.

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A girl cools off from the heat in water from an open fire hydrant in the Washington Heights section of upper Manhattan in New York City, New York, U.S., July 19, 2019.
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Mike Segar/Reuters

Scientists expect global warming to cause more extreme heat events like this one. A 2016 report from the National Academy of Sciences linked extreme heat in general to climate change. Since the 1960s the average number of heat waves in 50 major US cities has tripled, according to the US Global Change Research Program. The average heat wave season in those cities is 45 days longer than it was in the 1960s.

“Monthly heat records all over the globe occur five times as often today as they would in a stable climate,” Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research told The Associated Press during last month’s heat wave in Europe. “This increase in heat extremes is just as predicted by climate science as a consequence of global warming caused by the increasing greenhouse gases from burning coal, oil, and gas.”

As heat waves become more frequent, keeping track of the heat index can help people know what to expect and prepare for it.

Even if the humidity is stifling your sweat, a number of strategies can help you cool down. Above all the National Weather Service advises everyone to stay out of the heat, avoid strenuous activity, drink water, and check on family, neighbors, and pets. If you must be outside in the heat, stay hydrated and take regular breaks in the shade.