Both give a measure of the amount of moisture contained in the air. Absolute humidity measures the actual amount of water vapor in a given volume of air and is expressed in units of grams per cubic meter. In contrast, relative humidity is a ratio of the amount of moisture in air in comparison to the maximum volume of moisture the air would contain when saturated. Relative humidity increases with decreasing temperature but absolute humidity doesn't change. The more air over your head, the greater the air pressure.
Air pressure is greater at low elevations and less at high altitudes.
The air pressure at sea level averages 1013 millibars and declines gradually with increasing elevation, dropping to just 12 mb at 30 km. Adiabatic temperature changes occur as a result of the compression or expansion of air. These temperature changes represent the conversion of mechanical energy into heat energy. Compressed air becomes warmer, expanding air becomes cooler. Rising air undergoes adiabatic cooling, resulting in increasing relative humidity and eventual saturation.