Air pressure decreases with an increase in altitude.
In a standard atmosphere, air pressure decreases at a rate of one (1) inch of mercury per 1,000 feet increase in altitude. As height increases, the volume or column of air is less. As a result, the weight of the air or pressure is less. When the pressure is less, the density of the air is less. The performance of the helicopter is less when in a low-pressure environment, as density is a factor in lift production. In addition, normally aspirated piston engines perform less efficiently in lower pressure as the lower density of air molecules means there is less air being used for combustion.
FAA AC-00-6B Aviation Weather pg. 5-4