A scorching heat wave that began Monday in the Midwest has since spread to the South and the East Coast of the United States, spilled over into Canada, and is expected to last well into the Fourth of July weekend. Europe is also fighting the heat, well into its second deadly heat wave in weeks, with another one expected to hit the continent next week.

Heat causes the most weather-related deaths in the United States, with 13,000 heat-related deaths since 2018, according to the Centers for Disease Control.

Temperatures are already peaking at dangerously high levels, with the heat index (the actual temperature combined with humidity to produce what the temperature feels like) reaching a whopping 111 degrees Fahrenheit in Philadelphia and Washington, D.C., 106 in Boston, 102 in Chicago, 108 in Detroit, and 110 in Nashville, Tennessee.

Central Park in New York City reached 100 degrees, the first time the city hit the triple digits since 2012, and the heat index in the city soared to 106 degrees.

New York City prepared for the heat by opening cooling centers and asking New Yorkers to conserve electricity to protect the grid. NYC Mayor Zohran Mamdani, in a social media post, recommended setting the air conditioning to 78 degrees and turning off any appliances that are not needed or not in use.

“New York ISO [Independent System Operator, which manages the bulk of NYC’s electric grid] just issued an early warning that we must alleviate the strain on our energy grid statewide,” the mayor wrote. “Please do your part in keeping our power on by setting your AC to 78 degrees, turning off lights and unplugging electronics you’re not using, waiting until early morning or late at night to run appliances like dishwashers or laundry machines.”

“During a historic heatwave, AC will save lives — but only if we keep the grid stable enough for it to stay on,” he explained.

Despite the increase in travel before the holiday weekend, Amtrak canceled dozens of routes due to the heat wave.

What makes this heat wave particularly dangerous is that nighttime temperatures, while dropping somewhat, will still remain high.

The heat is expected to begin to dissipate a little on July 4 for the Northeast but will increase in the Southeast.