Pressure is a word with many connotations, some of them more foreboding than others: Your tire pressure looks low. The pressure is on for the final exam. The barometric pressure drop signals a strong chance of ...
Wait. Was that part good news, or was it bad news? You are probably familiar with the term "barometric pressure" (often "atmospheric pressure") if you're a regular watcher or listener of weather forecasts. But have you ever stopped to wonder what its contribution to weather actually is?
Also, like a lot of physical quantities, pressure comes in many units. The one most commonly used for automobile tire inflation in the United States, for example, is different from the one used to chart blood-pressure measurements, and both differ from the pascal, the SI (international system) or metric unit of pressure.
What Is Pressure In Physics?
Pressure is often described as a force, and it probably feels like one to you when you ponder the physical concept. That's not quite accurate, but it's close. Force is mass times acceleration (including the acceleration owing to gravity); pressure is force per unit area. If you apply force to your skin with the corner of your cell phone and then the same amount of force using a pin, you'll appreciate the difference.
When measuring pressure that results from the weight of air (weight having units of force), the molecules in air are distributed evenly above the surface of the ground, choosing any area allows for the measurement of pressure as long as you know its exact magnitude. This works because the net force is assumed to act vertically downward with respect to this chosen system.
What Is Atmospheric Pressure?
As hinted at already, atmospheric pressure is a measure of the weight of air molecules at ground level over a particular surface area. You are not aware of this pressure because, like all organisms on land, the species to which you belong is adapted to perceive this pressure as "neutral." But it is actually considerable, nearly half that of a well-inflated car tire.
The atmosphere consists mainly of the diatomic gas molecules nitrogen (N2) and oxygen (O2). Other players include CO2 (carbon dioxide) methane (CH4) and locally varying mounts of water vapor (H2O); while constituting only a small percentage of the air overall, methane and carbon dioxide exert significant effects as greenhouse gases. which contribute to warming of the planet.
How Is Atmospheric Pressure Measured?
A device called a barometer is used to measure air pressure. (A panoply of pressure gauges exist; the old-style one used to measure blood pressure goes by the easy-to-recall name of sphygmomanometer.) The principle is simple; because air has mass, it will tend to push other fluids (liquids or gases) out of its way with its own weight.
A barometer contains a base reservoir of mercury into which a tube sealed at the top has been inserted. The top of the mercury column naturally rests at the level of normal atmospheric pressure, which is about 101,325 pascal (Pa) or 101.325 kilopascals (kPa). If the pressure drops, this is reflected by a lower mercury level, as the liquid is being pushed upward less forcefully; when pressure rises, so does the mercury level.
What Are Pascals?
Pa units are not actually convenient for measuring pressure on the scale of most everyday things. As you just saw, you need over a hundred grand of them to equal atmospheric pressure. They are the SI unit of pressure because 1 Pa = 1 M/m2, and newtons and meters are the SI units for force and length respectively.
The metric system does make it easy to convert within its own units; for example, to convert hpa to pa (hectopascals to pascals), just multiply by 10.
Units typically used to express atmospheric pressure and the value of normal atmospheric pressure in these units include millimeters of mercury, also called torr (760 mm Hg or torr); inches of mercury (29.9 in Hg); pounds per square inch (14.4 psi); and bars, where 1 bar = 100 kPa (1.013 bars).
Pressure Conversion Tool
See the Resources for a page that allows you to convert between the aforementioned pressure units, plus a great many more.
About the Author
Kevin Beck holds a bachelor's degree in physics with minors in math and chemistry from the University of Vermont. Formerly with ScienceBlogs.com and the editor of "Run Strong," he has written for Runner's World, Men's Fitness, Competitor, and a variety of other publications. More about Kevin and links to his professional work can be found at www.kemibe.com.
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