As temperatures climb in the West Coast especially in Phoenix, Arizona, where more than 40 flights have been cancelled – because it is too hot for the planes to fly.
The weather forecast for the US city suggests temperatures could reach 120F (49C) on Tuesday.
That is higher than the operating temperature of some planes.
American Airlines announced it was cancelling dozens of flights scheduled to take off from Sky Harbor airport during the hottest part of the day.
The local Fox News affiliate in Phoenix said the cancellations mostly affected regional flights on the smaller Bombardier CRJ airliners, which have a maximum operating temperature of about 118F (48C).
The all-time record for temperatures in Phoenix is just slightly higher, at 122F, which hit on 26 June 1990.
The cancelled flights were scheduled to take off between 15:00 and 18:00 local time.
Why can’t planes fly?
At higher temperatures, air has a lower density – it is thinner. That lower air density reduces how much lift is generated on an aircraft’s wings – a core principle in aeronautics.
That, in turn, means the aircraft’s engines need to generate more thrust to get airborne.