PHSC 3033: Meteorology
Pressure
• Lecture Notes Pressure.pdf
Pressure
• The steady drumbeat of countless numbers of atoms
and molecules, exchanging momentum with the
walls of a container and providing the “Pressure”.
Pressure Units
Units of Pressure
• Force / Area = Newton/m2 = Pascal
Weather Related
Pascals
Millibars
Atmospheres
Inches of Mercury
PSI
Sea Level
101,325
1013.25
1.0
30.0
14.7
5,000 feet
10,000 feet
500
0.5
15.0
7.4
250
0.2
7.5
3.0
Gas Law
• The pressure is related to the density and
temperature of the gas through its internal energy.
– Kinetic Energy = 1/2 m v2
– Exchange of Momentum, the drumbeat of atoms and
molecules.
• Equation of State
– PV = NkT
– PV = nRT
N = number
k = Boltzman's constant = 1.38 x 10-23 J/K
n = grams per mole / molar mass
R = Gas constant = 8.31 Joules/mole/K
Partial Pressures
Pressure = PN2 + PO2 + + PAr + PH20 + PCO2 + ...
Pressure = Force / Area
(Snow shoes on thin ice, high heels on asphalt)
Force = Weight of overlying air = mass x gravity
Pressure Records
Fluid Pressure at Depth
Since pressure (P) = Force / Area and the force
is the weight (W = mg) of the overlying column...
P = mg/A
Multiply the top and bottom by the volume (V)
P = Vmg/VA
m/V is the density and V/A is just the height for a
Therefore,
P= gh
Weight
Pressure = Force / Area
Force = Weight of overlying column of air = mass x gravity
= gh
Pressure and Columns
At what depth do we encounter a pressure = 1 atm.
In a) water, b) gasoline, c) mercury?
Pressure and Columns
At what depth do we encounter a pressure = 1 atm.
In a) water, b) gasoline, c) mercury?
1 atm = 14.7 psi = 101325 Pascals
P = gh
ha = P/ ag
hb = P/ bg
hc = P/ cg
Pressure and Columns
At what depth do we encounter a pressure = 1 atm.
In a) water, b) gasoline, c) mercury?
1 atm = 14.7 psi = 101325 Pascals
P = gh
ha = P/ ag = 101325/1000*9.8
hb = P/ bg = 101325/680*9.8
hc = P/ cg = 101325/13600*9.8
Inches of Mercury
At what depth do we encounter a pressure = 1 atm.
In a) water, b) gasoline, c) mercury?
1 atm = 14.7 psi = 101325 Pascals
P = gh
ha = P/ ag = 10.3 meters
hb = P/ bg = 15.2 meters
hc = P/ cg = 0.76 meters
1 atm ~ 30 inches mercury (0.76 meters)
Pressure and Altitude
• There is a large difference in pressure with altitude.
Pressure Changes
Horizontal: Changes ~ 1 mb over 10000s of meters
Vertical: 1 mb over 10s of meters
Barometric Pressure must be corrected for Altitude
(and Temperature).
Vertical atmospheric motions are more important
than horizontal ones because of temperature and
pressure changes in this direction are drastically larger
than similar distances in the horizontal direction.
Pressure and Altitude
Station pressure:
Local Reading
Local Values
must be corrected
for elevation and
reduced to
sea level for
comparison.
Pressure Lapse Rate
In an average standard atmosphere with a lapse rate
of ~ 6.5 oC/1000 m, atmospheric pressure decreases are
approximated by ~ 100 mb/1000 m.
Station pressure at its elevation is then corrected to sea level
with this standard gradient.
Example:
Station Pressure 894 mb, Elevation +1100 m
Equivalent Sea Level Pressure = 894 + 1100*(100/1000)
= 1004 mb
PHSC 3033: Meteorology
• Return to Meteorology