Too damned hot for planes, that's how hot.
Officials at the Phoenix, Arizona airport say 40 flights have been cancelled because some airplanes aren't designed to fly when temperatures hit 49 degrees Celsius today. The superheated air is just too thin to generate enough lift for airliners such as Bombardier's CRJ class.
It's a well-known problem - a 2016 report from the International Civil Aviation Organization (ICAO) even warned that higher temperatures caused by climate change could "have severe consequences for aircraft take-off performance, where high altitudes or short runways limit the payload or even the fuel-carrying capacity".
Those problems are why many countries in the Middle East, and some high-altitude airports in South America, tend to schedule long flights for the evening or night, when it is cooler.
Today's cancellations at Phoenix airport come just a day after the publication of a new study from Dr. Mora at the University of Hawaii warning of a significant increase in heatwave intensity and frequency around the world.