Air density is affected by the temperature and humidity of the air.
On a hot day, or at high altitude, or on a moist day, the air is less dense. A reduction in air density reduces the engine horsepower and also reduces the aerodynamic lift. This calculator shows you how much the air conditions on any given day will alter the density of the air. Density altitude is defined as the pressure altitude corrected for the effects of temperature and humidity. Density altitude also affects your engine horsepower as you can see using my Relative Horsepower Calculator .