Nearly half of the United States population — around 161 million people — are facing an "extremely dangerous" heat wave, with temperatures soaring above 38°C across the East Coast, including New York, Philadelphia and Washington, DC.

The US National Weather Service (NWS) warned that the combination of heat and humidity would drive the heat index — the "feels like" temperature — to as high as 43°C in parts of the Northeast.

In New York City, temperatures climbed to 37°C, marking the hottest day in the city since 2012.

Nearby Newark, New Jersey recorded 39.4°C, while Philadelphia reached 38°C.

"This kind of heat is extremely dangerous and not to be taken lightly," the NWS said, urging residents to avoid strenuous activity and stay hydrated.

The heat triggered a blackout affecting more than 34,000 homes in the Bronx borough of New York, prompting energy supplier Con Edison to urge customers to reduce electricity use.

Public spaces in several cities were nearly deserted by mid-day.

In Washington, the National Park Service closed the Washington Monument, citing the "extreme heat in the DC area."

Train company Amtrak announced reduced speeds on its East Coast routes due to the extreme temperatures, warning of delays.

'Heat dome'

The high temperatures are part of a weather pattern known as a "heat dome" — a system of high pressure that traps hot air near the surface, intensifying heat levels over several days.

Meteorologists and climate experts say such events are increasing in frequency, length, and intensity due to global warming.

The year 2024 was the hottest year on record globally, with 2025 projected to rank among the top three.

Extreme heat is the leading cause of weather-related deaths in the US, surpassing hurricanes, floods, and tornadoes.