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AMS Weather Studies
Introduction to Atmospheric Science, 4th Edition
Chapter 5
Air Pressure
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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
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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
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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
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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
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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
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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
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 Units of length
– Millimeters or inches
 Inches typical for TV
Air Pressure
Units
 Units of pressure
– Pascal – worldwide standard
 Metric scale
 Sea-level pressure =
– 101,325 pascals (Pa)
– 1013.25 hectopascals (hPa)
– 101.325 kilopascals (kPa)
– Bars – U.S.
 A bar is 29.53 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 - 870 mb (Typhoon Tip in 1979)
– Highest ever recorded – 1083.8 mb (Agata, Siberia)
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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
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Average Air
Pressure
Variation with
Altitude
Expressed in mb
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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
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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
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Air pressure varies continuously
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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
– 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.
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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
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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
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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)
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Influence of Temperature and
Humidity
 When air is heated, air
density usually decreases
as a result in the increased
activity of the heated
molecules.
 Air pressure drops more
rapidly with altitude in cold
air than in warm air
 Increasing humidity also
decreases the density of
air, because water vapor
has a lower molecular
weight than dry air
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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
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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.
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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)
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The Gas Law
 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
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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
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change in temperature
Conservation of Energy
A. If the air is
compressed, energy
is used to do work on
the air
B. If we allow the air to
expand, the air does
work on the
surroundings
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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
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Adiabatic Processes
Dry adiabatic lapse rate describes the expansional
© AMSof ascending unsaturated air parcels
cooling
Illustration of dry and moist
adiabatic lapse rates
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