A punishing heatwave has settled over central and eastern parts of the US and Canada, pushing temperatures as high as 37C (99F) and causing up to 22 deaths.
The US weather service warned of "dangerous" levels of heat and humidity creeping east on Friday, with no relief in eastern states until Sunday.
As much as 50% of the US population was under a heat advisory, officials said.
Meteorologists have put the temperatures down to a "dome" of high pressure in the atmosphere.
Many regions in the central US and parts of the eastern seaboard have seen heat indexes - a combination of temperature and humidity - topping 43C.
The blistering heat has also hit parts of Canada, with temperatures in Toronto expected to top 35C on Friday, according to Environment Canada.
At 38C (101F), Syracuse in New York State has already witnessed its hottest day since 1936, forecasters say.
"This is an exceptionally strong ridge of high pressure that really has an exceptional scope and duration," Eli Jacks, a meteorologist with the National Weather Service, told the BBC.
"The air is sinking, as it sinks it compresses and gets warmer. It also dries out, so few clouds form to block the high early-summer sun," he said.
The combination of high heat and high humidity make it hard for the human body to cool itself - because sweat does not evaporate efficiently, he added.
Across the central and eastern US, people and animals alike have been struggling to keep cool amid the oppressive heat and humidity.
As the heat peaks in major population centres on the east coast, the number of deaths is expected to rise, officials warn.
Officials in the central state of Missouri say 13 people have died, and there have been fatalities in neighbouring Oklahoma, including a three-year-old boy.
In the town of Hutchinson in Kansas three elderly people were found dead in separate homes.
In Minnesota - a northern state known for its frigid winters - farm livestock have been dying from heat stress at a rate not seen in three decades, the Minneapolis Star Tribune newspaper reported. Turkeys have been hit especially hard, the paper reported.
In South Dakota, as many as 1,500 head of cattle have died in the heat, state veterinarian Dustin Oedekoven told Reuters news agency.
More than three-quarters of Texas is suffering from drought amid the worst dry spell in the state for decades.
Urban areas have opened cooling centres for the poor and elderly, and the National Weather Service has warned people in normally cool areas to be especially cautious.
Philadelphia has deployed police officers to manage hot, irritated crowds at the city's public swimming pools.
Electricity company Con Edison said scattered power cuts were likely in New York in the next several days amid a surge in usage of air conditioning units.
The city's fire chiefs have hired additional crews and officials have placed mobile fountains around the city to allow people to refill water bottles.
In Nashville, Tennessee, hospital staff have reported several cases of heat exhaustion and heat stroke.
Chicago is experiencing unhealthy smog levels caused by the heat. The Illinois Environmental Protection Agency asked the city's residents to reduce polluting activities, such as mowing lawns and keeping the engines running on stationary vehicles.
Forecasters said the damage caused by the heat could be worse that that brought about by a heatwave in Chicago in 1995, when more than 700 people died over three days.
Asphalt and concrete pavements and buildings in cities were "re-radiating" the heat, forecasters say.
By Friday, the high pressure system moving east is expected to bring thunderstorms with hail to the north-eastern US.
Meanwhile, as half of the country's 310 million residents sizzle, states in the north-western region of the country were experiencing abnormally cool temperatures.
"I didn't know it could be this cold in July. It is absolutely freezing here in Seattle," said one user on Twitter.
During the past month, Seattle has only experienced three days with temperatures hitting the 27C (80F) mark, with most days falling in the (mid-60s, according to the city's Sea-Tac Airport measuring station.
High temperatures - the number one weather-related killer in the US - claim 162 lives on average in the country each year.
The most severe heatwave in modern North American history took place during the Great Depression in 1936. The heat that summer was blamed for more than 5,000 deaths in the US and Canada.