Dear Tom,
Why does air pressure fluctuate?
—John DePue
Dear John,
Air pressure fluctuations occur primarily because of changes in temperature, elevation and humidity. Regarding temperature, warm air is less dense than cool air. As air is warmed, its molecules move faster and are spaced farther apart, causing its density and consequently its air pressure to decrease. Changes in elevation cause air pressure to change because air pressure at any given level is the weight of the air above you. As you move upward, air pressure always decreases because there is less atmosphere above you. Changes in humidity also affect air pressure. As humidity increases, air pressure decreases because water molecules have less mass than the oxygen and nitrogen molecules which dry air is primarily composed of.