Density altitude

Pressure altitude = elevation + (1013.25 − QNH) × 27 ft. Density altitude = pressure altitude + 120 ft × (OAT − ISA temperature). Hot, high and low-pressure days raise density altitude, so the engine, propeller and wings all perform as if the airfield were higher. The take-off figure is a rule of thumb of about +10% per 1,000 ft of density altitude.