For example, the air pressure on the earth's surface is determined by the weight of a 500 km tall air column. Air pressure at any point is also influenced by air density.
Air density represents the number of atoms of nitrogen, oxygen, etc. per volume of air.
Air pressure would decrease uniformly with altitude if gases were evenly distributed throughout the atmosphere. Air pressure declines with increasing altitude as the bulk of air molecules lie close to Earth's surface. One millibar is equivalent to 0.02953 inches of mercury. Close to Earth's surface, air pressure declines at a rate of 1 mb per 8 meter increase in altitude. Air pressure varies with elevation but also with weather systems.