AMS Weather Studies
Introduction to Atmospheric Science, 5th Edition
Chapter 5
Air Pressure
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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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Case-in-Point
Air Pressures on Mount Everest
 Mount Everest
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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
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Harsh conditions make survival at the summit difficult
Very thin air
Wind-chill factor
Most ascents take place in May
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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
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 Structures do not collapse under all the
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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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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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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 clockdriven drum chart
 Provides a continuous trace of air
pressure variations with time
 Easier to determine pressure
tendency
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Air Pressure Units
 Units of length
 Millimeters or inches
 Units of pressure
 Pascal – worldwide standard
 Sea-level pressure:
101,325 pascals (Pa) =
1013.25 hectopascals (hPa) =
101.325 kilopascals (kPa)
 Bars – US
 Bar is 29.53 in. of mercury
 Millibar (mb) standard on weather maps (mb = 1/1000 bar)
 Usual worldwide range is 970-1040 mb
 Lowest ever recorded – 870 mb (Typhoon Tip in 1979)
 Highest ever recorded – 1083.8 mb (Agata, Siberia)
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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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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
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
Average air pressure variation with
altitude expressed in mb.
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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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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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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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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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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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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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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
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 Density and surface pressure increase
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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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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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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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Expansional Cooling and
Compressional Warming
 Expansional cooling
 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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Expansional Cooling and
Compressional Warming
A. If the air is compressed, energy is used
to do work on the air.
B. If air expands, the air does work on the
surroundings.
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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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Adiabatic Processes
Dry adiabatic lapse rate describes the
expansional cooling of ascending of
unsaturated air parcels.
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Illustration of dry and moist adiabatic
lapse rates.
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