As altitude increases, atmospheric pressure decreases. On average, with every 1,000 feet of increase in altitude, the atmospheric pressure decreases 1 “Hg. As pressure decreases, the air becomes less dense or “thinner.” This is the equivalent of being at a higher altitude and is referred to as density altitude (DA). As pressure decreases, DA increases and has a pronounced effect on aircraft performance.

Differences in air density caused by changes in temperature result in a change in pressure. This, in turn, creates motion in the atmosphere, both vertically and horizontally, in the form of currents and wind. The atmosphere is almost constantly in motion as it strives to reach equilibrium. These never-ending air movements set up chain reactions which cause a continuing variety in the weather.