1. AMS Weather Studies Introduction to Atmospheric Science, 5th Edition
Chapter 5
Air Pressure
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2. Driving Question
What is the significance of horizontal and vertical variations in air pressure?
This chapter covers:
The properties of air pressure
Air pressure measurement
Spatial and temporal variations in air pressure
Gas Law
Expansional Cooling and compressional warming
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3. Case-in-Point Air Pressures on 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
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4. Defining Air Pressure
Air exerts a force on the surface of all objects it contacts
As a gas, air molecules in constant motion
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
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
Structures do not collapse under all the weight
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5. Air Pressure Measurement
Barometer – instrument used to measure air pressure and monitor changes
Mercury barometer – employs air pressure to support a column of mercury in a tube
Air pressure at sea level supports mercury to a height of 760 mm (29.92 in.)
Height of the mercury column changes with air pressure
Adjustments required for:
Expansion and contraction of mercury with temperature
Gravity variations with latitude and altitude
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6. Air Pressure Measurement
Aneroid barometer
More portable/less precise
Chamber with a partial vacuum
Changes in air pressure collapse or expand the chamber
Moves pointer on scale calibrated equivalent to mm or in. of mercury
New versions depend on the effect of air pressure on electrical properties of crystalline substance
Home-use aneroid barometers often have a fair, changeable, and stormy scale
These should not be taken literally
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7. Air Pressure Measurement
Air pressure tendency – change in air pressure over a specific time interval
Important for local forecasting
Barographs – Barometer linked to a pen that records on a clock- driven drum chart
Provides a continuous trace of air pressure variations with time
Easier to determine pressure tendency
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8. Air Pressure Units Units of length Units of pressure
Millimeters or inches
Units of pressure
Pascal – worldwide standard
Sea-level pressure: ,325 pascals (Pa) = hectopascals (hPa) = kilopascals (kPa)
Bars – US
Bar is in. of mercury
Millibar (mb) standard on weather maps (mb = 1/1000 bar)
Usual worldwide range is mb
Lowest ever recorded – 870 mb (Typhoon Tip in 1979)
Highest ever recorded – mb (Agata, Siberia)
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9. Variations in Air Pressure with Altitude
Overlying air compresses the atmosphere
The greatest pressure at the lowest elevations
Gas molecules closely spaced at Earth’s surface
Spacing increases with altitude
At 18 km (11 mi), air density is only 10% of sea level
Because air is compressible
Drop in pressure with altitude is greater in the lower troposphere,
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.
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10. Variations in Air Pressure with Altitude
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 m (18,000 ft)
99% of the mass is below 32 km (20 mi)
Denver, CO, average air pressure is 83% of Boston, MA
Average air pressure variation with altitude expressed in mb.
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11. © AMS

12. Horizontal Variations in Air Pressure
Horizontal variations much more important to weather forecasters than vertical
Local pressures at elevations adjusted to equivalent sea-level values
Shows variations of pressure in horizontal plane
Mapped by connecting points of equal equivalent sea-level pressure, producing isobars
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13. Horizontal Variations in Air Pressure
Horizontal changes in air pressure accompanied by changes in weather
In middle latitudes, continuous procession of different air masses brings changes in pressure and weather
Temperature has more pronounced affect on air pressure than humidity
In general, falling air pressure brings storms; rising air pressure brings clear or fair weather
Air pressure in Green Bay, WI, while under the
influence of a very intense low pressure system
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14. 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 due to increased kinetic energy
Molecules placed farther apart have a lower mass per unit volume (density)
The heated air is less dense and lighter
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15. Horizontal Variations in Air Pressure
Hot air balloons ascend within the atmosphere because the heated air within the balloons is less dense than the cooler air surrounding the balloon.
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16. Horizontal Variations in Air Pressure
Influence of temperature and humidity
Air pressure drops more rapidly with altitude in cold air
Cold air is denser, has less kinetic energy, 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 colder air than warmer
Increasing humidity decreases air density
Greater the concentration of water vapor, the less dense the air; due to Avogadro’s Law.
Muggy air reffered to as ‘heavy’ air, but it is lighter than dry air
Muggy air weighs heavily on personal comfort
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17. Horizontal Variations in Air Pressure
Influence of temperature and humidity
Cold, dry air masses are the densest
Generally produce higher surface pressures
Warm, dry air masses exert higher pressure than warm, humid air masses
Pressure differences create horizontal pressure gradients
Causes cold and warm air advection
Air mass modifications also produces changes in surface pressures
Conclusion: local conditions and air mass advection can influence air pressure.
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18. Horizontal Variations in Air Pressure
Influence of diverging and converging winds
Diverging winds blow away from a column of air
Converging winds blow towards a column of air
Causes
Horizontal winds blowing toward/away from a location
Wind speed changes in a downstream direction (Chap 8)
If more air diverges at the surface than converges aloft
Air density, surface air pressure decrease
If more air converges aloft than diverges at the surface
Density and surface pressure increase
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19. Highs and Lows Isobars on a map
U.S. convention at every 4-mb intervals (996 mb, 1000 mb, 1004 mb)
High – an area where pressure is higher than surrounding air
Usually fair weather systems
Sinking columns of air
Low – an area where pressure is lower than the surrounding air.
Usually stormy weather systems
Rising columns of air
Rising air necessary for precipitation formation
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20. The Gas Law Variables of state
Variability of temperature, pressure, and density
Magnitudes change from another across Earth’s surface, with altitude above Earth’s surface, and with time
Related through the ideal gas law, a combination of Charles’ law and Boyle’s law
Ideal gas law: pressure exerted by air is directly proportional to the product of its density and temperature
pressure = (gas constant) x (density) x (temperature)
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21. The Gas Law Conclusions from 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 air increases density because 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
At a fixed pressure, temperature is inversely proportional to density
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22. Expansional Cooling and Compressional Warming
When an air parcel expands, temperature of the gas drops
Compressional warming
When the pressure on an air parcel increases, parcel is compressed and 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 parcel
Change in internal energy directly proportional to change in temperature
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23. Expansional Cooling and Compressional Warming
If the air is compressed, energy is used to do work on the air.
If air expands, the air does work on the surroundings.
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24. Expansional Cooling and Compressional Warming
Adiabatic process
No heat is exchanged between an air parcel and surroundings
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 (averaged): 6 C°/1000 m (3.3 °F/1000 ft)
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25. Illustration of dry and moist adiabatic lapse rates.
Adiabatic Processes
Dry adiabatic lapse rate describes the expansional cooling of ascending of unsaturated air parcels.
Illustration of dry and moist adiabatic lapse rates.
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