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 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.