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Temperature, Pressure, Density
and Vertical Motion
Adapted from Scott Denning’s presentation
for CSU CMMAP course Summer 2007
By Jim Barnaby Summer 2008
Temperature, Pressure, Density
and Vertical Motion
Temperature, Pressure and Density
Buoyancy and Static Stability (aka
vertical motion)
Present Atmospheric Composition
Present day atmosphere consists of nitrogen, oxygen, water
vapor, carbon dioxide, argon and trace gases
What is Air Temperature?
• Temperature is a measure of the kinetic (motion)
energy of air molecules
– So…temperature is a measure of air molecule speed
• The sensation of warmth is created by air molecules
striking and bouncing off your skin surface
– The warmer it is, the faster molecules move in a random
fashion and the more collisions with your skin per unit time
Pressure
• Pressure is defined as a
force applied per unit area
• Molecules bumping into an object also create a
force on that object, or on one another
• Air pressure results from the weight of the
entire overlying column of air!
At sea level 14.7 psi
At Fort Collins ~ 12.1 psi
On Longs Peak ~ 8.6 psi
Air Pressure
Air Pressure – (atmospheric pressure) is
the force of air pressing down on earth’s
surface. Air pressure equals weight of
air directly above that point.
Air pressure depends on the density of
the air. Denser air exerts more air
pressure than less dense air does.
Density
Density is mass divided by volume
Density is a measure of how closely
packed molecules are within a substance
(how condensed or crowded they are)
ex. Styrofoam vs. steel
Density (mass/volume)
• Same number of
molecules and mass
Sample 1
• Sample 1 takes up
more space
Sample 2
• Sample 2 takes up
less space
• Sample 2 is more
dense than sample 1
Pressure and Density
• Gravity holds most
of the air close to
the ground
• The weight of the
overlying air is the
pressure at any
point
Density is the Key to Buoyancy!
Changes in density drive vertical motion
in the atmosphere and ocean.
• Lower density air rises when it is
surrounded by denser air.
-Think of a hollow plastic ball submerged under
water. What happens when you release it?
Air Density
Density of the air (air pressure) is
affected by three factors.
• Temperature
• Water vapor
• Elevation
Air Density is Affected by:
Temperature – density of a fluid (gas or liquid)
decreases when the fluid is heated. Less dense air
exerts less air pressure.
Areas with high
temperatures usually have lower air pressures than
areas with lower temperatures.
Water vapor – At a given pressure, moist air is less
dense than dry air. A water molecule has less mass
than either a nitrogen or oxygen molecule.
Therefore, air with a large amount of water vapor
exerts less air pressure than drier air.
Elevation – as elevation increases, air becomes thinner
or less dense. Therefore, air pressure decreases
with increasing elevation.
Why is stability important?
• Vertical motions in the atmosphere are a critical part of
energy transport and strongly influence the hydrologic cycle
• Without vertical motion, there would be no precipitation, no
mixing of pollutants away from ground level - weather as we
know it would simply not exist!
• There are two types of vertical motion:
– forced motion such as forcing air up over a hill, over colder air, or from
horizontal convergence
– buoyant motion in which the air rises because it is less dense than its
surroundings - stability is especially important here
Vertical Motion and Temperature
Rising air
expands, using
energy to push
outward against its
environment,
adiabatically
cooling the air
A parcel of air
may be forced to
rise or sink, and
change
temperature
relative to
environmental air
High Pressure Creates Fair
Weather
Air pressure may become high when large
air masses come together in the upper
atmosphere. These air masses press
down on layers of air below.
This
pressure usually prevents warm, moist
air from rising into the upper
atmosphere. As a result, clouds do not
form; therefore, high pressure usually
means fair weather.
Low Pressure Creates Poor Weather
Air pressure may become low when large
air masses move apart in the upper
atmosphere. This reduces pressure on
the layers of warm air below. As a
result, the warm air rises. If the warm
air is moist, clouds will form in the upper
atmosphere, therefore, low pressure
may lead to cloudy, rainy weather.

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