Hot and high

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. The reduced density reduces the performance of the aircraft's engine and also provides less lift, requiring a higher speed to lift the plane off the ground. Aviators gauge air density by calculating the density altitude.
An airport may be especially hot "or" high, without the other condition being present. Temperature and pressure altitude can change from one hour to the next. The fact that temperature decreases as altitude increases mitigates the "hot and high" effect to a small extent.
Improving hot and high performance.
Some ways to increase aircraft performance in hot and high conditions include:
Jet or rocket assisted take off.
Auxiliary rockets and/or jet engines can help a fully loaded aircraft to take off within the length of the runway. The rockets are usually one-time units that are jettisoned after takeoff. This practice was common in the 1950s and 60s, when the lower levels of thrust from military turbojets was inadequate for takeoff from shorter runways or with very heavy payloads. It is now seldom used.
Auxiliary jets and rockets have rarely been used on civil aircraft due to the risk of aircraft damage and loss of control if something were to go wrong during their use. Boeing did, however, produce a version of its popular Boeing 727 with JATO primarily for "hot and high" operations out of Mexico City Airport (MMMX) and La Paz, Bolivia. The boosters were located adjacent to the main landing gear at the wing root on each side of the aircraft.
Specialized aircraft.
Several manufacturers of early jet airliners offered variants optimized for hot and high operations. Such aircraft generally offered the largest wings and/or the most powerful engines in the model lineup coupled with a small fuselage to reduce weight. Some such aircraft include:
The marketing failure of most of these airplanes demonstrated that airlines were generally unwilling to accept reduced efficiency at cruise and smaller ultimate load-carrying capacity in return for a slight performance gain at particular airports. Rather than accepting these drawbacks, it was easier for airlines to demand the construction of longer runways, operate with smaller loads as conditions dictated, or simply drop the unprofitable destinations.
Furthermore, as the second generation of jet airliners began to appear in the 1970s, some aircraft were designed to eliminate the need for a special "hot and high" variant – for instance, the Airbus A300 can perform a 15/0 takeoff, where the leading edge slats are adjusted to 15 degrees and the flaps kept retracted. This takeoff technique is only used at hot and high airports, for it enables a higher climb limit weight and improves second segment climb performance.
Most jetliner manufacturers have dropped the "hot and high" variants from their model lineups.
Hot and high airports.
Notable examples of hot and high airports include: