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.
Meteorologists must correct for changes in altitude to understand the pressure characteristics of air masses in order to forecast future weather patterns. Pressure data on weather maps are therefore recalculated to reflect pressure measurements for hypothetical sea-level elevation. The condensation of moisture in the atmosphere forms clouds . Air cools as it rises, eventually becoming saturated with moisture when the temperature passes the below the dew point . This section describes the factors that cause air to rise and that affect condensation.