Fig 2-19 a:  Atmospheric density

• Atmospheric pressure is a more explicit and scientifically correct term than is "air" pressure.
• Atmospheric pressure is the weight of the atmosphere per unit of area on which it rests.
• The higher the elevation, the fewer the atmospheric molecules pressing down from above; conversely, the lower the elevation, the more molecules pressing down.

Fig 2-19b: Vertical profile of atmospheric pressure

The higher the elevation, the fewer the atmospheric molecules pressing down from above; the lower the elevation, the more molecules pressing down.

• Hence the higher the elevation above the surface, the lower the pressure.
• The lower the elevation, the higher the pressure.

Another expression of Standard Pressure is 1013.2 millibars (at sea level).

Common behavior of atmospheric pressure with respect to elevation:

• In the mountains, water boils at lower temperatures; foods don't cook properly.
• High performance athletes don't get as much oxygen at high elevation venues.
• Climbers of Mt Everest (nearly 6 miles high) generally need bottled oxygen.
• Those flying in unpressurized aircraft above 10,000 feet would be prudent in using auxiliary oxygen.
• Crew and passengers in high altitude commercial aircraft need pressurized cabins, period!.

• The key word root for atmospheric pressure is bar, a scientific unit of measure from the field of physics.  (Can you handle that?  A new use for a common word?)
  • Many atmospheric pressure terms contain the root, bar.
• A barometer is used to measure atmospheric pressure.

Fig 4-2b:  Idealized mercury barometer

• Mercury, a very heavy, liquid metal occupies an otherwise empty tube.
• Gravity pulls the mercury down into a pool in an open container.
• The container does not overflow.
• The mercury does not totally leave the tube.
• The weight/pressure of the atmosphere presses the mercury up into the tube.
• A measuring stick along side the tube measures the height of the mercury column above the top of the pool.
• When atmospheric pressure is higher, the mercury column is pushed taller.
• When atmospheric pressure is lower, the height of the column of mercury drops.
• A mercury barometer is large and impractical for ordinary purpose, and costly!.  
• Its contents are dangerous; mercury is poisonous!
• Water is not a suitable substitute for mercury; it would have to be 33+ feet tall!!!

An aneroid barometer

• Aneroid:  Actuated without the use of liquid
• This is a practical, compact, inexpensive instrument for common use.
• The mechanism is a small, sealed canister with some air removed.
• The sides of the canister expand outward when atmospheric pressure decreases.
• The sides of the canister contract inward when atmospheric pressure increases.
• The movement of the sides of the canister are transmitted, mechanically, to a pointer that indicates atmospheric pressure on the faceplate of the instrument. 

Sam Scripter photo

Barograph

• Note in your text the barograph of Fig 4-2c.
• This is an aneroid barometer that registers its reading with a pen on a paper chart.
  • Bar-o-graph = graphical barometer
  • Typically it can be operated by a wind-up spring, much like in some clocks
  • The chart encompass two weeks of readings.
• Hence the instrument may be placed in remote locations nor requiring daily attention by a person.
• The instrument in the text figure is in fact a recording aneroid barometer.
• In some unattended weather stations, today, the chart recording barometer is not used and instead barometric pressure is reported automatically by radio or a hard wire connec- tion to a computer

Atmospheric pressure units; Calibrating barometers

Units of measurement and standard atmospheric pressure

• Remember, standard atmospheric pressure means at sea level,  14.7 pounds per square inch (psi)

• 760 millimeters (mm) of mercury (Hg) (height of a mercury column in a barometer)

• 76 centimeters (cm) of mercury (Hg) (height of a mercury column in a barometer)

• 29.92 inches (in) of mercury (Hg) [You may round it off to 30]

• 1013.2 millibars (mb)   [You may round it off to 1000]

Adjusting barometers to sea level:

• Barometer readings vary from place to place because of differences in elevation above sea level.

• To be able to compare pressure readings from one geographic place to another, barometers must be adjusted to read as if they were at sea level.  Otherwise, one would not be able to compare the readings from city to city, from place to place.

• Barometers must be adjusted to read as if they were at sea level!

A variation on the Aneroid Barometer:  The Altimeter