Some American Airlines regional flights in and out of Phoenix, Arizona, have been cancelled as temperatures push towards 50deg C.

There are certain truths that accompany summer in Phoenix, in the US state of Arizona: Temperatures in the high 30s and 40s Celsius persist well past sundown, it's not considered abnormal to drive with oven mitts or ice packs in the car and after a certain threshold, even the "it's a dry heat" jokes cease being funny.

Usually, the hot season is met with a certain amount of pearl-clutching disbelief by people outside of Arizona.

Meanwhile, locals shrug, knowing simply to stay indoors as much as possible or escape to the cooler climes of Northern Arizona.

But this week has felt different, even for seasoned desert-dwellers.

Today's high of 118F in #Phoenix ties the record set 1 year ago. If we hit our forecast highs Tue & Wed it would set 2 new records. #azheat pic.twitter.com/REMwhT2FQP — NWS Phoenix (@NWSPhoenix) June 20, 2017

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The southwest is experiencing its worst heat wave in decades. Excessive heat warnings have been in effect from Arizona to California and will be for the remainder of the week.

How hot has it been? On Monday (Tuesday NZ Time), temperatures in Phoenix hit 47.8C, according to America's National Weather Service, which announced the record-tying heat against a stock image of a flaming ball of fire.

It's been so hot that even veteran local meteorologists are appending their tweets with #makeitstop.

And it was so hot that at least 50 flights have been cancelled this week at Phoenix Sky Harbor International Airport.

American Airlines alerted its customers over the weekend, offering fee-free changes to upcoming flights that were departing or arriving at Phoenix between 3pm and 6pm, when temperatures peak.

The Fort Worth-based airline cancelled 50 flights on Monday (Tuesday NZT) and 38 flights on Tuesday (Wednesday NZT) in and out of Phoenix.

DANGEROUS heat in the West:

- Las Vegas: 116°

- Palm Springs: 121°

- Phoenix: 119°

- Tucson: 113° pic.twitter.com/NmExXJ47cn — Good Morning America (@GMA) June 20, 2017

Regional flights on American Eagle were the most affected, because they use Bombardier CRJ planes that can only operate at temperatures of 47.8C or below, Feinstein said. Flights on larger Airbus and Boeing planes were not canceled because they are able to operate at higher maximum temperatures: 52.8C for Airbus and 52.2C for Boeing.

Each aircraft manufacturer sets its own parameters for operating temperatures, Feinstein said. As of Tuesday morning (Wednesday NZT), Sky Harbor officials said no other airlines had been affected.

The heat shows no sign of relenting soon.

The National Weather Service broke out the magenta - a colour category little known to the rest of the country - to illustrate parts of Arizona that would be under "rare, dangerous, and very possibly deadly" heat for the rest of the week.

Extreme temperatures today! Reschedule outdoor work/exercise. Take extra water wherever you go. Check on your elderly neighbors. #azwx #cawx pic.twitter.com/0xc8TtO26a — NWS Phoenix (@NWSPhoenix) June 20, 2017

The record for Phoenix was set June 26, 1990, when temperatures reached 50C. Flights out of Sky Harbor that day were grounded, too.

Only three times in recorded history has the temperature hit 49C or above: twice in 1990 and once in 1995, National Weather Service meteorologist Chris Kuhlman told The Washington Post.

Historically, average temperatures for Phoenix this time of year have remained between 40.6 - 43.3C degrees, he said.

Kuhlman, who is based in Phoenix, admitted that he woke up at 4am on Tuesday to do yard work before the sun came up.

Even at that hour, it was 32C outside.

"Normally, it's hot but it's not intolerable. You get acclimated to your surroundings. You kind of get used to the heat," Kuhlman said. "But when it's even this far above what the normal is, even for us that live here ... it's dangerous to be doing stuff outside. Anything, I suppose."