A massive heat wave is sending temperatures into the triple digits in much of the eastern United States, just as the World Cup enters the crucial knockout rounds.
Some areas are expected to set new heat records. Climate change means record-breaking heat waves are becoming more frequent.
Conditions are particularly dangerous in the Northeast, where temperatures in the 90s combined with very high humidity have prompted the National Weather Service to issue warnings about extreme heat, including in cities hosting World Cup matches.
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"We are definitely getting into the hottest part," says Daniel Vecellio, a climate scientist who studies extreme heat at the University of Nebraska at Omaha. "This is definitely the week, and coming up this weekend, where some of these systems will be tested."
An extreme heat warning is in effect in Philadelphia through the evening of July Fourth, when the city is hosting a match between Paraguay and France. The forecast calls for high temperatures around 100 degrees Fahrenheit, along with high humidity which will make it feel even hotter.
The match is scheduled to start at 5 p.m. ET. Although typically the weather cools in the evening, the multi-day heat wave will keep temperatures high well after sunset.
"Very warm low temperatures in the mid 70s to low 80s at night will not offer any relief from the heat," the weather service advisory for Philadelphia warns.
High temperatures may also affect games in Kansas City and Miami, and could linger into Sunday, when New Jersey will host a match at 4 p.m. ET, says Vecellio.
Dangerously hot, humid weather is no surprise at this year's World Cup. An NPR analysis of past weather conditions found that more than one-third of the tournament's matches are at high risk for dangerous heat and humidity.
That analysis identified Philadelphia as one of the highest-risk host cities. It also found that the July 4 match in Philadelphia was particularly likely to take place in dangerous conditions.
In an email to NPR, a spokesperson for FIFA, international soccer's governing body, said that the organization has added one extra water break per half for players and referees and installed air conditioning on the sidelines for those on the benches. Fans in the U.S. and Canada are also allowed to bring one factory-sealed water bottle into the stadium with them.
FIFA did not respond to questions from NPR about whether it had put additional heat-related protections in place this week, such as additional shade for people waiting in line, misting systems, cooling buses or free cold water.
Heat exhaustion is dangerous and can progress to heat stroke, which is potentially deadly. Drinking a lot of water and staying out of the sun can help prevent heat illness, according to the U.S. Centers for Disease Control and Prevention (CDC).
Vecellio warns that older adults and people who have medical conditions that put them at higher risk from heat should carefully consider whether to attend outdoor World Cup matches in the hottest locations this week.
"There are lots of different ways to be vulnerable to the heat. One of the biggest ones is age," he explains, because older people generally sweat less and have more difficulty regulating their body temperature. People with heart disease also have increased risk, because hot humid weather stresses the cardiovascular system.
Rahul Mukherjee contributed graphics and reporting.
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