t's not often air traffic would be delayed on a crystal-clear sunny day, but what's happening in Phoenix is no ordinary sunny day.
An intense heat wave is building in the Desert Southwest, threatening to tie or topple all-time record high temperatures. And you know that's saying something for Phoenix.
The high temperature reached 118 degrees there Monday and was expected to reach near 120 degrees Tuesday -- and possibly even into Wednesday. The 118 degree reading tied a record high on Monday -- set just last year -- and the National Weather Service says it was tied for their fifth hottest day on record.
If Phoenix manages to reach 120 degrees Tuesday, it will only be the third time in the city's history. Phoenix's all-time record high is 122 degrees.
Those lofty temperatures were an issue for some air travelers going through Sky Harbor Airport because extreme heat creates changes in the air density that make it harder for airplanes to take off. American Airlines cancelled 38 flights in and out of Phoenix on Tuesday afternoon because of the expected extreme heat.
The cancellations are for operations by smaller regional jets that have lower maximum operating temperatures than full-size airliners. Those jets can't operate when it's 118 degrees or above.
American Airlines pilot Shane Coffey said extreme heat means pilots have to use more thrust or impose weight restrictions such as flying with less cargo.
Air density on a 90-degree day in Denver at more than 5,000 feet elevation is similar to a 120-degree day in Phoenix at 1,100 feet above sea level, he said.
In 1990, amid a similar heat wave when Phoenix hit that record 122 degrees, flights were cancelled at the Phoenix airport because there was too much uncertainty about how the heat would affect aviation performance. Now, airlines have a better understanding, but the heat is still a concern — primarily for smaller, regional jets.
To that effect, American Airlines has been warning passengers that it may have to ground flights in Phoenix and is letting passengers flying during peak heat times through Wednesday to change flights without a fee.
While a touch cooler in other parts of the Southwest that should not impact air traffic, some other all-time record highs could be threatened. For example, the temperature reached 113 in Las Vegas Monday and was forecast to hit 117 on Tuesday, which would tie their all-time hottest day.
In fact, much of the West Coast will be broiling in summer-time heat as the solstice arrives on Tuesday. People in Arizona typically flee for cooler mountain climates when it gets hot, but going north won't provide much of an escape this time.
Flagstaff is expected to spend most of the week with highs above 90 degrees, which is so rare many residents don't have air conditioning.
"Extremely high temperatures are a little unusual for northern Arizona," said Coconino County's Deputy Chief Health Officer Mike Oxtoby.
Residents without air conditioners are advised to "pull shades over the windows and use cross-ventilation and fans to cool rooms."
One place you can beat the heat? Western Washington and Western Oregon -- where we'll be basking in the comfort of highs in the 70s:
The desert Southwest will "cool" off about 5 degrees or so to the 110-115 range by the end of the week, but on the flip side, the Pacific Northwest will begin to warm up a bit with highs climbing into the low-mid 80s by next weekend.
The Associated Press contributed to this report.