Chapter 6
Air Pressure
Gale Force
Winds Create
Waves that
Batter Coast
in UK in
2002
What are the Basic Elements
of Weather & Climate
(list 6)
Temperature
Humidity
Clouds
Precipitation
Pressure
Wind
Objectives (pg. 169)
1. Define air pressure.
2. The units of force used to measure it.
3. Discuss how a mercury barometer and
an aneroid barometer measure air pressure.
Bell work
How many pounds of pressure do you exert on Earth?
1. Measure the sole of your shoe: L x W to get the
AREA of your shoe in inches2
2. Multiply x 2 shoes.
3. Divide your weight in lbs.
area of your shoes
4. How many pounds of pressure do you exert on Earth?
__________ lbs./in2
Air pressure is…(pg. 168)
• The pressure exerted by the weight of the
air above.
• Weight is a FORCE, a push or pull.
You don’t “feel” it because the pressure on you
is balanced.
You feel it when it changes and your ears ‘pop’
as in a rapidly dropping airplane or elevator.
Average Air pressure at sea level
cm3
Column of water 33 ft. high
Ocean of Air around us
We are surrounded by an “ocean” of air.
Gas molecules are free to move – filling all
the space available
At the surface = 1 ATM
Pressure
5 km altitude = 50%
ATM
10 km = 25% ATM
Denver, CO the ‘mile
high city’ = 82% ATM
1. Which has MORE PARTICLES?
2. Which is more DENSELY packed?
3. Which has more COLLISIONS?
4. Which is exerting HIGHER PRESSURE?
5. Which has a colder TEMPERATURE?
• When gas
molecules collide,
they bounce off
each other,
exerting an
outward pressure
The force exerted
against a surface by a
continuous collision
of gas molecules.
Measuring Air Pressure
• Unit for force in physics is Newton (N)
• At sea level the atmosphere exerts a force of
101,325 newtons/meter2
• U.S National Weather Service 1940,
• Millibar: (mb) = 100 newtons/m2
• Standard sea-level pressure is 1013.25 mb
Measuring
Atmospheric Pressure
Mercury Barometer
1643 Torricelli
When air pressure
increased, the
mercury in the
tube rose up increased
Comparing Atmospheric Pressures
1013.25 mb
29.92 in
Aneroid Barometer
(without air)
High pressure readings (>30)
indicates _________ weather,
and low pressure readings (<30)
indicates ________ weather
Barograph
Associated Weather…
…with
High Pressure
…with
Low Pressure
Adjusting to Sea-Level Pressure by adding the pressure that would
be exerted by an imaginary column of air to the stations pressure
reading.
Isotherms: lines of same TEMPERATURE
Isobar- lines connecting areas of equal PRESSURE
Locate the areas of the highest and lowest pressure
H
H
1012
L
1018
Draw isobars for each of the following measurements 1004, 1008, 1012, 1016, 1020
Draw Isobar lines at 1008, 1012, 1016, 1020, 1024