A punishing heat-wave has settled over central and eastern parts of the US, pushing temperatures as high as 37C (99F) and causing up to 22 deaths.
The National Weather Service warned of "dangerous" levels of heat and humidity creeping east, with no relief expected 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.
On Thursday, many regions in the central US and parts of the eastern seaboard also saw heat indexes - a combination of temperature and humidity - topping 43C.Turkeys 'hit hard'
"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.
Stay-cool tips from an Arkansas farmer
Douglas Holmstrom, a 67-year-old businessman and cattle farmer in Lonoke County, tells the BBC:
- When doing outdoor construction work, keep the tools in the shade so they don't burn your hands
- Start work early in the day
- Wear a wet rag around your neck
- "I wear a straw hat, I take plenty of breaks, and I make sure to work with somebody most of the time. You have to watch out for one another"
- Don't get too used to the air conditioning: "The kids these days are so tuned to TV and to doing their games, they can't deal with the heat. If you stay outside and you're used to it, you can deal with it"
- Eat fresh vegetables and fruits - stay away from greasy food
"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.
Meanwhile, asphalt and concrete pavements and buildings in cities were "re-radiating" the heat, he added.
"There's no good place to be," Mr Jacks said.
Heat is the number one weather-related killer in the US, according to the National Weather Service.
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.
Mr Jacks said the combination of high heat and high humidity made it hard for the human body to cool itself - because sweat does not evaporate efficiently.
In the town of Hutchinson in Kansas three elderly people were found dead in separate homes on Wednesday, while the body of a woman in her 80s was found in her bedroom in the nearby state of Missouri.
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 were hit especially hard, the paper reported.But Seattle shivers
In South Dakota, as many as 1,500 head of cattle have died in the heat, state veterinarian Dustin Oedekoven told Reuters news agency.
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 utility 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.
Chicago was 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 heat-wave in Chicago in 1995, when more than 700 people died over three days.
By Friday, the low 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 of the micro-blogging website 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 are responsible for killing 162 people on average in the US each year.
The most severe heat-wave 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.