Blistering temperatures and humidity have gripped swaths of the United States, with the worst yet to come for the densely populated East Coast as the nation co-hosts the World Cup and prepares to celebrate its 250th birthday.
"Heat dome" conditions — in which high-pressure systems trap warm air over a region like the lid on a pot — persisted on Wednesday over the Midwest and South and were moving to the east, with some 46 million people currently under heat alerts.
New York officials say the city is entering what could be the most extreme heat wave this city has seen in over a decade.
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New York has designated hundreds of public buildings as cooling centres, extended public swimming pool hours, dispatched volunteers to check on vulnerable residents, and opened "pop up" cooling stations with misting fans and wet towels.
Multiday warnings of extreme heat landed in New York, Boston and Philadelphia as sultry weather pushed east. Temperatures in the high 90s Fahrenheit (30 degrees Celsius) were forecast for the Northeast; Philadelphia and Boston could top 100 by Thursday. Throw in humidity, and the real-feel heat index will be even higher at times, the National Weather Service said.
More frequent, longer-lasting and more intense heat waves are one of the clearest signs of climate crisis, with Europe also recently hard hit.
Global average surface temperatures have risen roughly 2.3F above pre-industrial averages as a result of climate crisis, mainly driven by the burning of fossil fuels.
World Cup
From Thursday, the US capital Washington DC is forecast to have three consecutive days above 100F (38 C) — the highest on Friday, when the mercury could touch 104F.
If the forecast bears out, the city's daily heat records would be broken each day, while Washington's all-time high temperature of 106F could also be in danger.
The heat comes as the nation prepares to celebrate on Saturday the 250th anniversary of its independence from Britain, with fireworks set to light up the skies over major cities, including New York and Washington.
Fireworks trigger spikes in harmful fine particulate matter that cause respiratory issues, with the combined effects of heat and smoke amplifying health impacts.
Meanwhile, football's World Cup is taking place under punishing conditions.
A high of 102F is forecast for Saturday's clash between Paraguay and France in Philadelphia, where the game will be played in an open-air stadium, unlike other World Cup venues which are air-conditioned.
This edition of the World Cup requires players to take a three-minute "hydration break" every half — with Saturday's searing temperatures, the breaks will be much-needed to stave off heat illness.
Cities at risk
Humidity is not uncommon in the US Northeast.
But Dr Alexander Azan of NYU Langone Health in New York said high air temperatures and humidity are a dangerous combination.
"Their body doesn’t have that level of acclimatization to respond appropriately to the heat, and so heat stress in the form of what we call heat exhaustion, and in more severe cases, heat stroke, can occur at much lower temperatures than we see in people who live in the South," Azan said.
Cities in particular are at greater risk, experts say.
"The concentration of concrete, asphalt, steel, all of those materials help to retain heat," said Vijay Limaye, a climate scientist at the Natural Resources Defense Council. "The number on your phone may actually not reflect the true temperature profile that you’re going out into."
In Washington, DC where the high temperature was 95 F (35 C), thirsty children reached for cold bottles of water from US Park Police as they waited in line for the Ferris wheel on the National Mall.
Jeff Schlegelmilch, associate professor at Columbia University Climate School, said heat is one of the easiest things to attribute to climate crisis.
"We have seen a continued increase in longer summers, hotter temperatures, hotter temperatures earlier on, more evaporation of moisture, higher humidity — effects like that," he said.
