1. Chapter 5
Air Pressure

2. Defining Air Pressure
Air exerts force on all of the surfaces of all objects in contact with it
Molecules are always in rapid, random motion, and each molecule exerts force when it collides with a surface – and the total air pressure at the surface is the cumulative force of all of these molecules colliding with a surface area

3. Weight = (mass) x (acceleration of gravity)
Defining Air Pressure
In general, we can think of air pressure as the weight per unit area of the atmosphere above that point
Weight = (mass) x (acceleration of gravity)
Average air pressure at sea level is 1.0 kg per square centimeter
So the weight on the roof of a typical house is about 2.1 million kg (4.6 million lbs) – so why don’t all of our houses collapse? Or why aren’t we all squished?
Air pressure is in ALL DIRECTIONS…so the net pressure
Is generally zero. – what about popping ears?

4. Air Pressure Measurement
Barometers:
Mercury
Aneroid

5. Mercury Barometer Invented in 1643 by Evangelista Torricelli
Glass tube about 1 meter long – closed at one end and open at the other
Filled with mercury and inverted into an open container of more mercury
Mercury settles into the tube until the pressure of the weight of the mercury column exactly balances the pressure of the atmosphere acting on the surface of the mercury in the container

6. Aneroid Barometer Invented in 1843 by Lucien Vidie
Less accurate than mercury barometer – more portable
Consists of a flexible chamber from which air has been evacuated – along with a spring that keeps the chamber from collapsing
As air pressure changes, the chamber expands ( ) or contracts ( )
Lower air pressure
Higher air pressure

7. Air Pressure Measurement
Pressure tendency!

8. Air Pressure Units TV meteorologists normally use inches of mercury
Scientists use the pascal (Pa)
U.S. meteorologists use millibars (mb)
1mb= in. Hg. = 1 hPa = 100 Pa
Average pressure at sea level:
101,325 Pa = hPa = mb = in. Hg
Worldwide range at sea level is 970 to 1040 mb.
Lowest reading ever is 870mb in the eye of Typhoon Tip near Guam – tornadoes probably much lower…but not too easily measured.

9. Air Pressure Variation with Altitude
Air is compressible
The pull of gravity compresses the air in such a way that results in the highest density of air is near the surface – gas molecules more closely packed near the surface
Remember the definition of pressure? Well…since the density falls with altitude, so does the pressure.
Average air pressure in Denver (1766 meters) is 83% that of Boston. ( mb vs mb)

10. Horizontal Air Pressure Variation
Variations in pressure at the surface are not always due to elevation…and these differences are of most interest to meteorologists. – so pressure’s are adjusted to approximate a station’s pressure if it were at sea level…
Sea level pressure
One way of adjusting to sea level pressure is by assuming an imaginary column of air extending from the station, down to sea level, having the properties of an “average” atmosphere.
So…for a station above sea level, sea level pressure is (greater than / less than) surface pressure.

12. Horizontal Air Pressure Variation
Air masses are continually shifting across the nation…but why do different air masses exert more or less pressure on the surface?
Air density
Convergence/divergence of winds

13. Influence of temp and humidity on horizontal Air pressure differences
TEMPERATURE CHANGES:
Cold air is more dense than warm air
Increasing temperature = decreasing density, and thus decreasing weight, and decreasing pressure.

14. Influence of temp and humidity on horizontal Air pressure differences
HUMIDITY CHANGES:
Which is MORE dense: moist air or dry air ?
The molecular weight of water is less than that of dry air (18 vs. 29 atomic mass units)
According to Avogadro’s Law, the number of molecules in a gas mixture of constant temperature and pressure must remain the same
So when air becomes more moist, water molecules replace dry air molecules…

15. Influence of convergence and divergence on horizontal Air pressure differences
Convergence/divergence produced in 2 ways:
Horizontal winds blowing toward or away from a source
Wind speed changes in a downstream direction (more on THIS one in Chapter 8)

16. Influence of convergence and divergence on horizontal Air pressure differences

17. Air pressure = (constant) x (density) x (temperature)
The Ideal Gas Law
In the book: “The Gas Law”
I have described the atmosphere in terms of variations in temperature, pressure, and density – variables of state
Variables of state are interrelated by the Ideal Gas Law
Air pressure = (constant) x (density) x (temperature)

18. Using the Eulerian framework in x-y-p coordinates, the primitive equations can be written as:
(1)
(2)
(3)
(4)
(5)
(6)
P=ρRT (7)