It’s called density altitude: “Too hot to fly at Portland International Airport?”

An aircraft’s performance is determined by density altitude.

At a high altitude airport, the air is less dense. At a hot airport, the air is less dense. Same with humidity.

The combination is called “density altitude” and refers to the pressure altitude after corrected for temperature and humidity.

On a hot day, the density altitude of a near sea level airport on a hot day might be more like 3,000 feet than sea level (that’s a guess – I did not do the calculation).

Portland Airport is basically at sea level so aircraft can accommodate a higher temperature and humidity than if this were an airport at 9,000 feet.

At 115 degrees, it wasn’t quite hot enough to affect air density to the point where it hindered flights.

Source: Too hot to fly at Portland International Airport? Not quite. – oregonlive.com