8.3: Atmospheric Pressure
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Atmospheric (Barometric) Pressure
A barometer is an instrument measuring atmospheric pressure, used especially in forecasting the weather and determining altitude (Figure 8.10).
Air pressure on the planet is directly related to the mass of the air column above at any location under the influence of gravity: Pressure = Force/Area.
Atmospheric air pressure is reported as average air pressure measured at standard sea level. Reported barometric air pressure at elevations above sea level are adjusted to be equivalent to air pressure measured at sea level at locations closest to where measurements are taken.
Figure 8.10. Barometers
How Air Pressure Is Reported
Barometric units most used to describe atmospheric pressure includes atmospheres, millibars, and PSI (pound-per-square-inch).
The weight of ONE ATMOSPHERE (Earth's atmosphere above us) is equal to the weight of the Earth's average air pressure at standard sea level.
One atmosphere (on Earth, on average) is equivalent to:
• 14.7 pounds-per-square-inch (psi) - this might mean something when you add air to you car's tires.
• 29.92 inches of mercury (a historic measure of air pressure that is still widely used).
• 406.8 inches of water (33.9 feet) - how deep you'd need to dive in a freshwater lake to double the weight of the atmosphere.
• seawater (33.4 feet) - seawater is slightly denser than freshwater.
• 1.01325 bars (one bar was supposed to be equivalent to the weight of one average Earth atmosphere; the slight number above 1.0000 bars is from adjustments from atmospheric-pressured data that was later compiled from locations measured around the world. It was determined that the average weight of the Earth's atmosphere was slightly higher than the standard one bar was originally established between the year's 1793 and 1795 by the European science community as an attempt to add an air pressure standard to metric system. When analytical devices are calibrated, they use the revised metric unit: millibars.
The average weight of one Earth atmosphere is now commonly reported as 1013.25 millibars (mb).
Atmospheric Pressure Drops With Increasing Altitude (Elevation)
Elevation and air pressure have an inverse relationship - air pressure decreases with increasing elevation (Figure 8.11). At an elevation of about 18,000 feet you would be above about half of the atmosphere. That, of course, depends on changing weather conditions! An common altimeter is a type of barometer that measures air pressure to report elevation, but altimeters must be adjusted to match local weather conditions.
What is the difference between altitude and elevation?
If you are flying an airplane, you need to know this! Technically, altitude is the vertical distance from the Earth surface (land or water) to an object (such as an airplane). Elevation is a the vertical distance between a location on the ground and global sea level.
Can you feel changes in Atmospheric Pressure?
The answer is a most definite yes! As you go up in elevation, air pressure trapped within your ears is not equalized with the air pressure outside, so your ears tend to occasionally pop as you climb in altitude, as your ear ducts release air when you swallow. Older people commonly complain about bone and joint pain when a storm is approaching and air pressure starts falling.
The opposite is true when you go down in elevation, such as on an airplane descending from high altitude. Anyone who has frequently flown can tell you about crying children complaining ear aches because their ears have not readjusted to air pressures at low elevations.
Figure 8.11. Atmospheric pressure decreases with altitude on a curve.