Temperatures are forecast to rise above 50 degrees Celsius in parts of the south-west United States, as an extreme heatwave forces airlines to ground planes and strains power grids.

Major airlines delayed and cancelled flights out of Las Vegas and Phoenix airports as temperatures hit the high 40s, citing difficulty in operating aircraft in extreme heat.

American Airlines cancelled 20 regional flights out of Phoenix, Arizona on Tuesday after temperatures in the desert city soared toward 49 degrees Celsius.

The forecast called for a high of 49 degrees Celsius in Phoenix, 48C in Palm Springs, California, and 52C in Death Valley — a desert valley in Eastern California.

Why can't planes fly in this heat?

Aviation experts said hotter air was also thinner, causing a decline in performance for jet engines, especially during take-offs.

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"As the ambient temperature at a particular airport increases, in this case into the [high 40s] at Phoenix, the amount of lift and power in aircraft engines declines, and the result is that for any given runway length, you can carry less and less payload," Robert Mann, an industry analyst and former airline executive, said.

"As temperatures get that extreme, you have to offload so much fuel or passengers or cargo that it no longer makes sense to fly," he said.

ABC News Breakfast weatherman Nate Byrne explained:

"When the temperatures get that hot, the air is literally just not dense enough to make sure that all of the physics that works to help an aircraft get into the air are enough for it to do that safely."

Mr Mann said aircraft may also be hampered because the charts used in flight manuals to calculate aircraft performance are not written with such heat in mind, making it impossible for pilots to accurately calculate payloads and take-off speeds.

The flights affected by the heat tend to be on smaller regional carriers, which use planes that operate under lower maximum temperatures.

"It doesn't happen very often because these are exceedingly rare temperatures, but there's a point at which either runway length is insufficient or the speed is so high that the tires wouldn't be rated for it," Mr Mann said of those aircraft.

The heat can also create issues for ground crews, where pavement temperatures can reach more 66C, life-threatening conditions if workers are exposed to it for too long.

Booties for pets' paws

Chase the pug gets booties to protect his paws from the searing pavement. ( AP: Angie Wang )

The main burn centre in Phoenix warned people to be careful around car interiors and pavement and with their pets.

People in Phoenix were taking advantage of free elastic booties to put on their pets' paws so they did not burn on concrete and pavement.

Shelby Barnes, 48, picked up three sets of the booties for her dogs. She said she would yell at people who walked their dogs without them in the afternoon heat.

"If you can't put your foot on the sidewalk, neither can they," she said.

One third of people experience 20 days of deadly heat a year

The weather comes as new research found that nearly a third of the world's population experience 20 days a year when the heat reaches deadly levels.

The study of nearly 2,000 deadly heat waves worldwide since 1980 was published Monday in the journal Nature Climate Change, a monthly peer-reviewed journal.

The high temperatures mean the air is thinner. ( AP: Matt York )

The National Weather Service said the current heatwave resulted from a high-pressure system camped over the Four Corners region where Colorado, Utah, New Mexico and Arizona meet.

The sweltering weather was expected to peak on Tuesday or Wednesday (local time) as temperatures in Death Valley and the town of Needles, California, were forecast to spike as high as 53C.

The extreme heat led the operator of California's power grid to call on people to conserve electricity during peak hours.

ABC/wires