(Bloomberg) -- How hot is hot? Temperature alone doesn’t tell the tale.

The combination of heat and humidity can make temperatures feel hotter, and kill you quicker. That’s bad news for the elderly, the very young and the sick, and the reason the U.S. National Weather Service has been listing heat indices factoring in humidity since the mid-1980s.

The forecast for this weekend shows the heat index -- the "feels like" number beloved of TV weathermen -- reaching as high as 115 from Philadelphia to Raleigh-Durham, according to Rich Otto, a forecaster with the U.S. Weather Prediction Center in College Park, Maryland. New York may hit 111.

By now, people may be getting used to the heat. This past June was the world’s all-time hottest June in records dating back to 1880, according to the National Centers for Environmental Information in Asheville, North Carolina. That means nine of the 10 warmest Junes on record have occurred since the decade began.

Moving forward, “it is virtually certain 2019 will end among the five warmest years on record,” said Karin Gleason, a scientist with the National Centers. Already, the first six months of the year has tied 2017 as the second warmest start to a year, she said.

Some groups are already adjusting for the weekend’s weather.

The New York Road Runners group has called off Saturday’s 10-mile marathon training session, according to its Twitter account. Pools run by the New York City Parks Department will be kept open an hour later to 8 p.m. for the general public, starting on Friday. And horse racing is canceled on Saturday at Saratoga Race Course about 180 miles (290 kilometers) north of the city. It’s been 13 years since extreme heat caused that to happen, according to the New York Racing Association.

Why does humidity matter so much? In a word, sweat. High humidity curbs the evaporation of sweat, limiting the body’s ability to cool itself.

About 618 people die from heat-related illness every year in the U.S., according to the Centers for Disease Control and Prevention in Atlanta. A year ago, a similarly high heat index in Quebec, where many homes don’t have air conditioning, killed as many as 70. So far in 2019, nine toddlers have died in cars during high index days, according to the U.S. weather service.

High Heat

Symptoms of heat stroke can include a throbbing headache, dizziness, lack of sweating despite the heat, muscle weakness or cramps, nausea and rapid shallow breathing, according to the WebMd website. The solution: Dress in light colors, drink lots of water, avoid the sun and hard work during the hottest times of the day, and retreat to an air-conditioned room if possible.

And stay positive, the heat is set to break by Monday. “By the middle of next week it will be a welcome relief from this heat,” Otto said. “It’s July, but it is going to feel much, much better.”

(Adds the cancellation of horse racing at upstate Saratoga in the seventh paragraph.)

To contact the reporter on this story: Brian K. Sullivan in Boston at bsullivan10@bloomberg.net

To contact the editors responsible for this story: Tina Davis at tinadavis@bloomberg.net, Reg Gale, Will Wade

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