Chapter 5
Air Pressure
Driving Question

What is the significance of horizontal
and vertical variations in air pressure?
Air Pressure

Air pressure is a measure of the force that air exerts
on a surface
 Weight per unit area of the column of air above that
location
– Weight = mass * acceleration due to gravity
 Average air pressure at sea level
– 1.0 kg/cm2
– 14.7 lb/in2
 The air pressure at any point is the same in all
directions
Air Pressure Measurement

Barometer
Aneroid 
 Mercury
Mercurial Barometer

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
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

More accurate than aneroid
Invented by Torricelli in 1643
One meter (39 inches) long glass tube sealed
on one end
Open end is inverted in pool of mercury
Height of mercury changes as pressure
changes
Adjustments are required for temperature and
latitude
Aneroid Barometer


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Flexible evacuated
chamber with a spring
inside
As pressure changes
allow the chamber to
flex
This causes movement
in gears which display
the pressure
Air Pressure Tendency

The change in air pressure with time
– Rising: continuing fair or clearing weather
– Falling: approaching inclement weather
– Steady: no change

Barograph
– An instrument that provides a continuous trace of
air pressure with time

Altimeter
– An aneroid barometer that is calibrated to
measure altitude or elevation
Air Pressure Units

Millibars, inches of Mercury, Pascals
Air Pressure
Meteorologists often express altitude in
terms of pressure (850mb map)
 Worldwide range in air pressure
averages between 970mb and 1040mb
 Lowest: 870mb (25.69in Hg) in Typhoon
Tip near Guam (Peak Winds of 190
mph)
 Highest: 1083.8mb (32.01in Hg) in
Siberia

Variations in Air Pressure With Altitude
The maximum air density is at the
surface
 Number density: the number of gas
molecules per unit volume

– Decreases with altitude
Thinning of the air with altitude is
associated with decline in air pressure
 50% atmosphere’s mass below 3 miles
 99% atmosphere’s mass below 20 miles

The Standard Atmosphere:
a model of the real
atmosphere averaged
across all latitudes for all
seasons characterized by
1. Sea level air temperature
of 15oC
2. Sea level pressure of
1013.25mb
Variations in Air Pressure With Altitude

Denver (mile high city) has an average
air pressure 83% less than Boston
– Altitude sickness
– Lexington is about 900 feet above sea
level, so actual pressure is less than “fixed”
pressure

In very sparse air (thermosphere)
temperature is very high, but heat
transfer is low
Horizontal Variations in Air
Pressure
On a surface weather map, variations in
pressure due to altitude are removed by
determining what the pressure would be
at that point if that point were at sea
level
 Easier to observe variations in pressure
from one place to another by day and
hour

Influence of Temperature and
Humidity
Generally, temperature has a greater
influence on density and pressure than
water vapor
 Air pressure drops more rapidly with
altitude in cold (more dense) air than
warm (less dense) air
 Dry air is more dense that humid air!!

Influence of Temperature and
Humidity
Cold, dry air masses are more dense
and produce higher surface pressures
than warm, humid air masses
 Change in air pressure is usually
accompanied by a change in air mass

Influence of Winds

Divergence (convergence) are caused by
winds blowing away (toward) a location
The Gas Law (Equation of State)

Variables of State: temperature, pressure,
and density
 p = ρRT
–
–
–
–
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Pressure (p)
Density (ρ)
Gas Constant (R)
Temperature (T)
In the atmosphere these 3 variables are
constantly changing
The Gas Law
P = ρRT
 Pressure is held constant

– If T increases then ρ decreases
– If ρ increases then T decreases

Temperature and Pressure are
inversely proportional
The Gas Law
P = ρRT
 Density is held constant

– If T increases then P increases
– If P increases then T increases

Temperature and Pressure are directly
proportional
The Gas Law
P = ρRT
 Temperature is held constant

– If P increases then ρ increases
– If ρ increases then P increases

Density and Pressure are directly
proportional
Basic Understandings (Ch. Review)
Important changes in weather often
accompany relatively small changes in
air pressure at the Earth’s surface.
 High or rising pressure indicates fair
weather
 Low or falling pressure indicates
inclement weather
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