Just answering the question ‘what is atmospheric pressure?’ is not enough to give a full understanding of its importance. By definition atmospheric pressure is ‘force per unit area exerted against a surface by the weight of air above that surface’. Atmospheric pressure is closely related to the hydrostatic pressure caused by the weight of air above the measurement point. The term standard atmosphere is used to express the pressure in a system(hydraulics and pneumatics) and is equal to 101.325 kPa. Other equivalent units are 760 mmHg and 1013.25 millibars.
Mean sea level pressure (MSLP) is the pressure at sea level. This is the pressure normally given in weather reports. When home barometers are set to match local weather reports, they will measure pressure reduced to sea level, not your local atmospheric pressure. The reduction to sea level means that the normal range of fluctuations in pressure are the same for everyone.
Atmospheric pressure is important in altimeter settings for flight. A altimeter can be set for QNH or QFE. Both are a method of reducing atmospheric pressure to sea level, but they differ slightly. QNH will get the altimeter to show elevation at the airfield and altitude above the air field. QFE will set the altimeter to read zero for reference when at a particular airfield. QNH is transmitted around the world in millibars, except in the United States and Canada . These two countries use inches (or hundredths of an inch) of mercury.
Atmospheric pressure is often measured with a mercury barometer; however, since mercury is not a substance that humans commonly come in contact with, water often provides a more intuitive way to visualize the pressure of one atmosphere. One atmosphere is the amount of pressure that can lift water approximately 10.3m. A diver who is 10.3m underwater experiences a pressure of about 2 atmospheres (1of air plus 1of water). Low pressures like natural gas lines can be expressed in inches of water(w.c). A typical home gas appliance is rated for a maximum of 14 w.c.(about 0.034 atmosphere).
You can see that understanding ‘what is atmospheric pressure’ is just the tip of the iceberg. Once you have the definition in mind, it really comes together when you see the wide variety of applications.
We’ve also recorded an entire episode of Astronomy Cast all about the Atmospheric Pressure. Listen here, Episode 151: Atmospheres.