The south-western US is braced for some of the worst heat it has seen in years.
Some flights have been cancelled in Phoenix, where the forecast is for temperatures of 119F (48.3C), because certain planes cannot take off in extreme heat.
Arizona is seeing the most stifling temperatures, but the wrath of the heatwave is being felt across Nevada and California as well.
Las Vegas was forecast to hit 117F (47.2C) on Tuesday and excessive heat warnings cover almost all of California.
Phoenix hit 118F (47.7C) on Monday, which the National Weather Service says is quite rare.
In fact, temperatures of 118F or above have only been recorded 15 times since records were first kept in 1896.
Forecasters also warned Phoenix could hit 120F (48.8C) degrees this week.
Phoenix has hit the 120F mark only three times in recorded history - the last time 22 years ago. The record high was 122F (50C), recorded on June 26 1990.
In addition to grounding flights of smaller planes, airlines have been taking other measures on larger jets to reduce their weight because of the heat.
American Airlines spokesman Ross Feinstein said the carrier began limiting sales on some flights to prevent the planes from exceeding maximum weight for safe take-off in the hot conditions.
Airlines can use other strategies for dealing with limitations on the planes during hot weather, including reducing the amount of luggage and cargo and not fuelling completely to make the plane lighter for take-off.