1. Weather Studies Introduction to Atmospheric Science American Meteorological Society
Chapter 5
Air Pressure
Credit: This presentation was prepared for AMS by Michael Leach, Professor of Geography at New Mexico State University - Grants

2. Case-in-Point Mount Everest
World’s tallest mountain – 8850 m (29,035 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

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. 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

5. Air Pressure Measurement
A mercury thermometer employs air pressure to support a column of mercury in a tube
Air pressure at sea level will support the mercury to a height of 760 mm (29.92 in.)
Height of the mercury column changes with air pressure
Adjustments required for:
The expansion and contraction of mercury with temperature
Gravity variations with latitude and altitude

6. Air Pressure Measurement
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
Home-use aneroid barometers often have a fair, changeable, and stormy scale
These should not be taken literally

7. 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

8. 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)

9. 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

10. Average Air Pressure Variation with Altitude Expressed in mb

11. Horizontal Variations in Air Pressure
Horizontal variations are much more important to weather forecasters than vertical differences
In fact, local pressures at elevations are adjusted to equivalent sea-level values
This shows variations of pressure in the horizontal plane
This is mapped by connecting points of equal equivalent sea-level pressure, producing isobars

12. 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

13. 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.

14. 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

15. Air pressure varies continuously

16. Horizontal Variations in Air Pressure
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

17. 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)

18. Horizontal Variations in Air Pressure

19. 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.

20. 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)

21. The Gas Law, Continued 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

22. Expansional Cooling and Compressional Warming
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

23. 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

24. Adiabatic Processes Dry adiabatic lapse rate describes the expansional
cooling of ascending unsaturated air parcels
Illustration of dry and moist
adiabatic lapse rates