What is "hot and high"

abbr. (context aviation English) (of aircraft operations) hot climates and high altitude.

In aviation, hot and high is a condition of low air density due to high ambient temperature and high airport elevation. Air density decreases with increasing temperature and altitude. At any given true airspeed, lower air density reduces the amount of lift generated by the wings or the rotors of an aircraft, which may hamper an aircraft's performance and hence its ability to operate safely. The reduced density also reduces the performance of the aircraft's engine, compounding the effect. Aviators gauge air density by calculating the density altitude.

"Hot" and "high" do not have to be mutually inclusive of one another, though this tends to be the exception. If an airport is especially hot or high, the other condition need not be present. Temperatures can change from one hour to the next, while the elevation of an airport always remains constant. The fact that temperatures decrease at higher elevations mitigates the "hot and high" effect to a certain extent.