Barometric pressure is the measurement of air pressure in the atmosphere, specifically the measurement of the weight exerted by air molecules at a given point on Earth. Barometric pressure changes constantly and is always different depending on where the reading takes place.
Average barometric pressure at sea-level is commonly cited as 14.7 pounds per square inch (PSI). However, this figure is just an average. In reality, barometric pressure varies across the world, especially at higher elevations where atmospheric pressure is much lower than at sea level. In fact, there are 50% fewer air molecules at 18,000 ft. than there are at sea level. One of the ways that aircraft can determine what altitude they are flying at is by measuring outside air pressure. Altimeters can read air pressure relative to a calibrated ground reading and convert that information to a readout in feet or meters.
Barometric pressure also changes with the weather—or rather, the weather changes with changes in barometric pressure. Being able to measure and analyze small changes in atmospheric pressure helps meteorologists track the weather and predict storms. is the absolute atmospheric pressure existing at any given point in the atmosphere. It is the weight of a unit column of gas directly above the point of measurement. It varies with altitude, moisture and weather conditions.
(See also, “Atmospheric pressure”)