1. AMS Weather Studies Introduction to Atmospheric Science, 4th Edition
Chapter 5
Air Pressure

2. Case-in-Point Mount Everest
World’s tallest mountain – 8848 m (29,029 ft)
Same latitude as Tampa, FL
Due to declining temperature with altitude, the summit is always cold
January mean temperature is -36 °C (-33 °F)
July mean temperature is -19° C (-2 °F)
Shrouded in clouds from June through September
Due to monsoon winds
November through February – Hurricane-force winds
Due to jet stream moving down from the north
Harsh conditions make survival at the summit difficult
Very thin air
Wind-chill factor
Most ascents take place in May
© AMS

3. Driving Question
What is the significance of horizontal and vertical variations in air pressure?
Air pressure is an element of weather we do not physically sense as readily as temperature and humidity changes
This chapter examines:
The properties of air pressure
How air pressure is measured
The reasons for spatial and temporal variations in air pressure

4. Important changes in weather can accompany relatively small variations in air pressure.
In this chapter we examine the properties of air pressure, how air pressure is measured, and the reasons for spatial and temporal variations in air pressure.
We also consider how air responds to changes in pressure when moving vertically through the atmosphere.
Defining Air Pressure next.

5. Defining Air Pressure
Air exerts a force on the surface of all objects it contacts
The air is a gas, so the molecules are in constant motion
The air molecules collide with a surface area in contact with air
The force of these collisions per unit area is pressure
Dalton’s Law – total pressure exerted by mixture of gases is sum of pressures produced by each constituent gas
Air pressure depends on:
Mass of the molecules and kinetic molecular energy
Air pressure can be thought of as the weight of overlying air acting on a unit area
Weight is the force of gravity exerted on a mass
Weight = (mass) x (acceleration of gravity)
Average sea-level air pressure
1.0 kg/cm2 (14.7 lb/in.2)
Air pressure acts in all directions
That is why structures do not collapse under all the weight
Air Pressure within the house next.

6. • Air Pressure Measurement
The barometer is the instrument used to measure air pressure and monitor its
change.
Mercurial barometer next.

7. There are two basic types of barometers
There are two basic types of barometers. The first is the mercury barometer. The second type is the aneroid barometer. The mercurial (mercury) barometer is the more accurate of the two but also more cumbersome to work with.
Picture of a mercurial barometer next.

8. The height of the mercury column changes as air pressure changes.
The average air pressure at sea level will support the mercury column in the tube to a height of inches.
The height of the mercury column changes as air pressure changes.
Falling pressure will allow the mercury to drop; whereas increasing pressures forces the column of mercury to rise in the tube.
Rising and falling of the mercury means something next.

9. The second type is the aneroid barometer
An aneroid barometer is less precise, but more portable than a mercury barometer
It has a chamber with a partial vacuum
Changes in air pressure collapse or expand the chamber
This moves a pointer on a scale calibrated equivalent to mm or in. of mercury
New ones are piezoelectric – depend on the effect of air pressure on a crystalline substance

10. Air Pressure Measurement
Forecasting uses air pressure and tendency values
changes over time
Barometers may keep a record of air pressure
These are called barographs
provides a continuous trace of air pressure variations with time making it easier to determine pressure tendency.
© AMS

11. Air Pressure Units Units of length Units of pressure
Millimeters or inches
Inches typical for TV
Units of pressure
Pascal – worldwide standard
Metric scale
Sea-level pressure =
101,325 pascals (Pa)
hectopascals (hPa)
kilopascals (kPa)
Bars – U.S.
A bar is inches of mercury
A millibar (mb) is the standard used on weather maps, meaning 1/1000 of a bar
Usual worldwide range is 970 – 1040 mb
Lowest ever recorded mb (Typhoon Tip in 1979)
Highest ever recorded – mb (Agata, Siberia)
© AMS

12. Variations in Air Pressure w/Altitude
Overlying air compresses the atmosphere
the greatest pressure is at the lowest elevations
Gas molecules are closely spaced at the surface
Spacing increases with altitude
At 18 km (11 mi), air density is only 10% of that at sea level
Because air is compressible, the drop in pressure with altitude is greater in the lower troposphere
Then it becomes more gradual aloft
Vertical profiles of average air pressure and temperature are based on the standard atmosphere – state of atmosphere averaged for all latitudes and seasons
Even though density and pressure drop with altitude, it is not possible to pinpoint a specific altitude at which the atmosphere ends
½ the atmosphere’s mass is below 5500 m (18,000 ft)
99% of the mass is below 32 km (20 mi)
Denver, CO average air pressure is 83% of Boston, MA
© AMS

13. Average Air Pressure Variation with Altitude Expressed in mb

14. Air Pressure changes more with elevation than latitude next.

15. Because air pressure drops with increasing altitude at rates that can be readily determined, an aneroid barometer can be calibrated to monitor altitude. Such an instrument is called an altimeter.
Picture of a plane on 870 mb traveling from New Orleans to New York next.

16. Changes in surface air pressure en route are one cause of differences between indicated and true altitude. Aircraft altimeters are equipped with a moveable scale than enables the pilot to adjust altimeter readings based on reports from weather stations along the route.
The pilot is following the 870 mb level from New Orleans to New York. The pilot thinks he is a 1220 m but as he goes north his actual height gets lower so that over New York his actual height is really 1190 m.
Note the pilot is following the 870 mb level from New Orleans to New York. The pilot thinks he is a 1220 m but as he goes north his actual height gets lower so that over New York his actual height is really 1190 m.
STOP

17. Horizontal Variations in Air Pressure
Horizontal variations are much more important to weather forecasters than vertical differences
Local pressures at reporting locations are adjusted to equivalent sea-level values to make up for changes in elevation
This shows variations of pressure in the horizontal plane
This is mapped by connecting points of equal equivalent sea-level pressure, producing isobars

18. Horizontal Variations in Air Pressure
Horizontal changes in pressure can be accompanied by significant changes in weather
In middle latitudes, a continuous procession of different air masses brings changes in pressure and weather
Temperature has a much more pronounced affect on air pressure than humidity
In general, the weather becomes stormy when air pressure falls but clears or remains fair when air pressure rises
Air pressure varies continuously
© AMS

19. Horizontal Variations in Air Pressure
Influence of temperature and humidity
Rising air temperature = rise in the average kinetic energy of the individual molecules
In a closed container, heated air exerts more pressure on the sides
Density in a closed container does not change
No air has been added or removed
The atmosphere is not like a closed container
Heating the atmosphere causes the molecules to space themselves farther apart
This is due to increased kinetic energy
Molecules placed farther apart have a lower mass per unit volume, or density
The heated air is less dense, and lighter.
© AMS

20. Horizontal Variations in Air Pressure
Influence of temperature and humidity, continued
Air pressure drops more rapidly with altitude in a column of cold air
Cold air is denser, has less kinetic energy, so the molecules are closer together
500 mb surfaces represent where half of the atmosphere is above and half below by mass
This surface is at a lower altitude in cold air vs. in warm air
Increasing humidity decreases air density
The greater the concentration of water vapor, the less dense is the air due to Avogadro’s Law
We often refer to muggy air as heavy air, but the opposite is true
Muggy air only weighs heavily on our personal comfort factor

21. Influence of temperature and humidity, continued
Cold, dry air masses are the densest
These generally produce higher surface pressures
Warm, dry air masses generally exert higher pressure than warm, humid air masses
These pressure differences create horizontal pressure gradients
Pressure gradients cause cold or warm air advection
Air mass modifications can also produce changes in surface pressures
Conclusion: local conditions and air mass advection can influence air pressure

22. Horizontal Variations in Air Pressure
Influence of diverging and converging winds
Diverging = winds blowing away from a column of air
Converging = winds blowing towards a column of air
Diverging/converging caused by :
Horizontal winds blowing toward or away from some location (this chapter)
Wind speed changes in a downstream direction (Chapter 8)
© AMS

23. Influence of Diverging and Converging winds
If more air diverges at the surface than converges aloft, the air density and surface air pressure decrease
If more air converges aloft than diverges at the surface, density and surface pressure increase

24. Highs and Lows Isobars are drawn on a map as previously discussed
U.S. convention – these are drawn at 4-mb intervals (e.g., 996 mb, 1000 mb, 1004 mb)
A High is an area where pressure is relatively high compared to the surrounding air
A Low is an area where pressure is relatively low compared to the surrounding air
Highs are usually fair weather systems
Lows are usually stormy weather systems
Rising air is necessary for precipitation formation
Lows are rising columns of air. Highs are sinking columns of air.
© AMS

25. Winds are strong where isobars are closely spaced because of greater horizontal pressure differences, but are weak when isobars are widely spaced as the horizontal differences are less.
Isobars generally are more closely spaced near the center of a Low than near the center of a High.
The Gas Law next.

26. • The Gas Law
We have discussed variability of temperature, pressure, and density → these properties are known as variables of state; their magnitudes change from one place to another across Earth’s surface, with altitude above Earth’s surface, and with time
The three variables of state are related through the ideal gas law, which is a combination of Charles’ law and Boyle’s law
The ideal gas law states that pressure exerted by air is directly proportional to the product of its density and temperature, i.e. pressure = (gas constant) x (density) x (temperature)
Air density drops as temperature increases next.

27. Conclusions from the ideal gas law
Density of air within a rigid, closed container remains constant. Increasing the temperature leads to increased pressure
Within an air parcel, with a fixed number of molecules:
Volume can change, mass remains constant
Compressing the air increases density because its volume decreases
Within the same air parcel:
With a constant pressure, a rise in temperature is accompanied by a decrease in density.
Expansion due to increased kinetic energy increases volume
Hence, at a fixed pressure, temperature is inversely proportional to density

28. Conservation of energy
Expansional cooling – when an air parcel expands, the temperature of the gas drops
Compressional warming – when the pressure on an air parcel increases, the parcel is compressed and its temperature rises
Conservation of energy
Law of energy conservation/1st law of thermodynamics → heat energy gained by an air parcel either increases the parcel’s internal energy or is used to do work on the parcel
A change in internal energy is directly proportional to a change in temperature
More on expansional cooling and compressional cooling next.

29. Conservation of Energy
The law of energy conservation states that energy is neither created nor destroyed but can change from one form to another.
When the air is compressed, work is done on it but when air expands, the air does work on its surroundings. Energy is either supplied (during compression) or released (during expansion).
Adiabatic process next.

30. Conservation of Energy
If the air is compressed, energy is used to do work on the air
If we allow the air to expand, the air does work on the surroundings

31. Adiabatic Processes
During an adiabatic process, no heat is exchanged between an air parcel and its surroundings
The temperature of an ascending or descending unsaturated parcel changes in response to expansion or compression only
Dry adiabatic lapse rate = 9.8 C°/1000 m (5.5 °F/1000 ft)
Once a rising parcel becomes saturated, latent heat released to the environment during condensation or deposition partially counters expansional cooling
Moist adiabatic lapse rate = 6 C°/1000 m (3.3 °F/1000 ft) → this is an average rate
© AMS

32. Dry adiabatic lapse rate describes the expansional cooling of ascending unsaturated air parcels

33. Should a rising air parcel cool to the point that the relative humidity nears 100% (saturation) and condensation or deposition (vapor to solid) takes place, the ascending air parcel no longer cools at the dry adiabatic rate. It now cools more slowly at the moist adiabatic lapse rate of 60C per 1000 m.
Picture of dry and adiabatic lapse rate next.

34. More on moist adiabatic lapse rate next.

35. Warm saturated air has more water vapor than cool saturated air
Warm saturated air has more water vapor than cool saturated air. For the same temperature change, the greater condensation or deposition in warm saturated air releases more latent heat than condensation or deposition in cool saturated air.
The moist adiabatic lapse rate ranges from about 4 Celsius degrees per 1000 m for very warm saturated air to very near 9.8 Celsius degrees per 1000 m for very cool saturated air.
For convenience, we use an average value of 6 Celsius degrees per 1000 m for the moist adiabatic lapse rate.
Conclusions next.

36. Atmospheric Stability
To view this animation, click “View” and then “Slide Show” on the top navigation bar.

37. • Conclusions
Air pressure drops rapidly with altitude in the lower atmosphere and then more gradually aloft.
Adjusting a station’s barometer readings to sea level eliminates the influence of weather station elevation on air pressure.
More conclusions next.

38. Surface air pressure depends on air density, which in turn, is governed by air temperature and, to a lesser extent, by the concentration of water vapor in air.
Diverging or converging winds may also affect air density and surface air pressure.
Changes in air temperature trigger the phase changes of water that cause clouds to form or to dissipate.
Basic Understandings next.

39. • Basic Understandings
Air pressure is the weight of a column of air acting on a unit area of Earth’s surface. The pressure at any specified altitude is equal to the weight per unit area of the air column above that altitude.
A barometer is a weather instrument that monitors changes in air pressure.
Air pressure and air density decrease rapidly with increasing altitude in the lower atmosphere and then more gradually aloft.
More Basic Understandings next.

40. About 50% of the mass of the atmosphere occurs below and altitude of 5
About 50% of the mass of the atmosphere occurs below and altitude of 5.5 km, and 99% is below an altitude of 32 km.
Cold air masses are denser and exert higher pressure at Earth’s surface than do warm air masses.
As a rule, temperature has a much more significant influence on air density and air pressure than does humidity.
Air pressure may fluctuate in response to divergence and convergence of air, which is produced by changes in wind speed and direction.
More Basic Understandings next.

41. Important changes in the weather often accompany relatively small changes in air pressure at Earth’s surface.
High or rising pressure signals clearing or continued fair weather, whereas low or falling pressure means stormy weather.
During an adiabatic process, no heat is exchanged between an air parcel and its environment.
Last slide next.

42. AIR ASCENDS, EXPANDS AND COOLS. AIR DECENDS, COMPRESSES AND WARMS
LAST