Knowing what a barometer is or how to use it is pointless if you don't understand air pressure. Air pressure, or barometric pressure, is the measurement of how much air is above any one area at a given time. The more air, the more pressure that it exerts down on the space below.he air pressure at sea level is 14.7 pounds per square inch, or 1,013 hectopascals. The phrase a rising barometer means that air pressure is increasing, while a falling barometer indicates a drop in pressure. As the air pressure changes, the amount of oxygen present does too.