Chapter 5 Air Pressure

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

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?

Air Pressure Measurement Barometers: Mercury Aneroid

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

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

Air Pressure Measurement Pressure tendency!

Air Pressure Units TV meteorologists normally use inches of mercury Scientists use the pascal (Pa) U.S. meteorologists use millibars (mb) 1mb=0.02953 in. Hg. = 1 hPa = 100 Pa Average pressure at sea level: 101,325 Pa = 1013.25 hPa = 1013.25 mb = 29.92 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.

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. (1013.25 mb vs. 840.99 mb)

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.

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

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.

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…

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)

Influence of convergence and divergence on horizontal Air pressure differences

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)

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)