American Airlines, the world's largest carrier by fleet size, revenues and destinations served, cancelled almost 50 regional flights out of Sky Harbor International Airport in Phoenix, Arizona, as the day temperature was forecast to touch 120 degrees Fahrenheit (nearly 49 degrees Celsius) on Tuesday.
If a plane does try to take off beyond its temperature threshold, it may keep racing down the runway and not get up in time and have to abort.
As temperatures increase, more people turn on air conditioners, which use a lot of electricity and can strain the power system. Their temperature limitations are 127 and 128 degrees respectively.
In recent days, American Airlines has been forced to cancel more than 40 flights in Phoenix, Arizona. In Phoenix, American Airlines cancelled flights as they couldn't take off in such high temperatures.
"Today is the peak and then tomorrow will be a little bit cooler, but we're still expecting temperatures above 115 for Phoenix", she said. Las Vegas was forecast to hit 117 (47 Celsius) on Tuesday on the first day of summer, and excessive heat warnings cover nearly all of California.
Forecasters say the current heat wave should break by early next week, returning Phoenix to more typical summer temperatures - around 110 degrees. On Monday night, Las Vegas dropped to only 88 degrees - which also happened to be the high temperature in Washington, D.C., on Tuesday. San Diego County Animal Services says it has received around 30 rattlesnake calls in the past week as the weather has warmed.
In the Southwest U.S., the heat has caused several problems this week.
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High-altitude airports like those in Denver, Colorado, have thinner air by nature, so lift is even more affected by higher temperatures.
Allen says the Las Vegas Strip's tall, close buildings and long stretches of concrete cause the area to heat quickly and cool slowly.
American Airlines alerted its customers over the weekend, offering fee-free changes to upcoming flights that were departing or arriving at Phoenix between 3 and 6 p.m., when temperatures peak.
"Typically in the hotter days of the summer, you may have to bump payload, which includes cargo and/or passengers", said Mr David Wilhelm, a senior dispatch manager at Southwest Airlines.
The Death Valley National Parks Service has issued a warning to visitors to avoid hiking after 10am, and to "travel prepared to survive". Overnight lows aren't going to drop below the upper 80s in parts of Arizona. Drinking extra water and sports drinks will help prevent dehydration that can trigger a health crisis like a heat stroke. Here are five facts to keep in mind when dealing with intense heat. Flights out of Sky Harbor that day were grounded, too. The mercury hit 117 degrees there.
American media organisations quoted a release from American Airlines saying the Canadian-built Bombardier CRJ aircraft used by its American Eagle regional services were not certified to fly in temperatures higher than 118°F (48°C).
In 2015, Dr Radley Horton, a research scientist at Columbia University's Earth Institute, published a joint study on the effect of extreme heat on aviation.
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