The atmospheric pressure, temperature and humidity all affect the density 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 amount of oxygen available for combustion and therefore reduces the engine horsepower and torque. For tweaking the fuelair mixture, the air density is the most important consideration.

Website: http://wahiduddin.net/calc/calc_hp_dp.htm