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 lower air density reduces the power output from the aircraft's engine and also requires a higher true airspeed before the aircraft can become airborne. 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. Negative effects of reduced engine power due to hot and high conditions Airplanes require a longer takeoff run, potentially exceeding the amount of available runway. Reduced take-off power hampers an aircraft's ability to climb. In some cases, an aircraft may be unable to climb rapidly enough to clear terrain surrounding a mountain airport. Helicopters may be forced to operate in the shaded portion of the height-velocity diagram in order to become airborne at all. This creates the potential for an uncontrollable descent in the event of an engine failure. In some cases, aircraft have landed at high-altitude airports by taking advantage of cold temperatures only to become stranded as temperatures warmed and air density decreased. While unsafe at any altitude, an overloaded aircraft is much more dangerous under hot and high conditions. Improving hot and high performance Some ways to increase aircraft performance in hot and high conditions include: Reduce aircraft weight. Weight can be reduced by carrying only enough fuel to reach the (lower-altitude) destination rather than filling the tanks completely. In some cases, unnecessary equipment can be removed from the aircraft. In many cases, however, the only practical way to adequately reduce aircraft weight is to depart with a smaller passenger, cargo, or weapons load. Consequently, hot and high conditions at the originating airport may prevent a commercial aircraft from operating with a load large enough to be profitable, or may constrain the firepower that a combat aircraft can bring to bear when conducting a long-range airstrike. Increase engine power. More powerful engines can improve an airplane's acceleration and reduce its takeoff run. More powerful engines are generally larger and heavier and use more fuel during cruise, however, increasing the fuel load needed to reach the same destination. The added weight