Air Pressure
A. Introduction:
Weather reports include measurements of air temperature, wind speed, wind direction,
relative humidity, dew point, and barometric pressure. Meteorologists sometimes also include
pollen counts, wind chill factors, heat indices, ultra-violet indices, and other measurements in
weather reports.
Weather maps in newspapers and on televised weather reports always indicate the
location of high pressure and low pressure systems using the symbols H and L. They also
include the location of warm fronts, cold fronts, stationary fronts, and occluded fronts.
However, for most people, this is an information overload. For this reason, most
televised weather reports provide some of the basic weather data and then repeat the forecast
for the next several days at least three times.
This activity will help you develop an understanding of how air pressure is measured and
some interesting effects of differences in air pressure.
B. A Water Barometer Demonstration
Have your ever dipped a straw into water, placed your finger on top of the straw, and
then lifted the straw out of the liquid? The liquid stays in the straw unless you lift your finger off
of the top of the straw.
Question 1: What force pulls down on the liquid when it is in the straw?
Question 2: What evidence is there that the force of air pressure balances the force pulling
down on the liquid?
The following procedure demonstrates how tall of a column of water can be supported by
air pressure.
1. A long plastic tube has been filled with water. Record the length of the column of water in
centimeters.
2. One end of the tube has been closed with a clamp.
3. The other end of the tube will be placed in a bucket of water at ground level.
4. I will hold one end of the tube and the rest of the tube will be lowered out of the third story
window at the end of the hallway.
5. The water level will fall a short distance at the upper end of the tube, which has been
clamped shut. Record the distance that the water falls in centimeters.
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Question 3: What is inside of the tube in the space above the water level? Remember that
the upper end of the tube has been clamped shut.
Question 4: Why will water leave the tube when the clamp is opened?
Question 5: What height of a water column was supported by air pressure in this water
barometer?
a. in centimeters of water
b. in inches of water
Question 6: What are some disadvantages of using a tube of water to measure air pressure.
6. The actual atmospheric pressure can be obtained by calling 617.936.1234 or by logging
onto a weather web site. The barometric pressure will be measured in inches of mercury.
Question 7: What is the atmospheric pressure today (in inches of mercury)?
C. Mercury Barometers
Water barometers need to be very tall. Water also freezes at a temperature of 0° C. A
much shorter barometer can be constructed using liquid mercury. The following web sites can
be used to answer Questions 8 through 10. The second web site does not have a typo.
http://www.geocities.com/Athens/Acropolis/6914/toreng.htm
http://www.sciencemuseum.orh.uk/collections/exhiblets/weather/barometer.asp
Question 8: What steps are required to construct a mercury barometer?
Question 9: Why does the column of mercury stop falling after is has been turned upside
down and placed in a pool of mercury?
Question 10: What would happen to the column of mercury in the gall tube if the sealed end
of the tube were opened? Explain your answer.
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D. Air Pressure Standards
Meteorologists use barometers to measure air pressure in inches of mercury. Chemists
use manometers to measure gas pressure and arbitrarily have decided that standard
atmospheric pressure is equal to 760 mm of mercury. The variety of pressure units is shown in
the following table.
Unit
1 Atmosphere
inches of mercury
millimeters of water
millimeters of mercury
millibars
Torr
Abbreviation
atm
in. Hg
mm H2O
mm Hg
mb
torr
Unit equivalent to 1 atm
1 atm
29.92 in. Hg
10,336 mm
760 mm Hg
1013.2 mb
760 torr
pounds per square inch
Pascal
kiloPascals
14.7 lb / in2
101,325 Pa
101.325 kPa
lb / in2 (psi)
Pa
kPa
Question 11: How would you convert a pressure measured in inches of water in a water
barometer into inches of mercury? Show that calculation using the data from the water
barometer demonstration.
Question 12: What is the percent difference between the value calculated in Question 11 and
the barometric pressure obtained from a weather station?
Question 13: What are some sources of error in the water barometer value?
Question 14: What is the weather station’s barometric pressure today:
a. in millimeters of mercury
b. in kiloPascals
Air Pressure
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E. Your Middle Ear Is An Aneroid Barometer
Aneroid barometers are used to measure atmospheric pressure. The aneroid
barometers are closed containers of air. The containers are flexible and expand and contract
as air pressure outside the barometer changes. The following web site can be used to answer
Questions 15, 16, and 17.
http:/www.bartleby.com/61/imagepages/A4aneroi.html
Question 15: What is the difference between a mercury barometer and an aneroid
barometer?
Question 16: What will happen to the volume of the flexible chamber inside an aneroid
barometer if the air pressure is:
a. high?
b. low?
Question 17: What causes the needle of the aneroid barometer to move?
Your middle ear is usually a closed container of air. Your ear drum is pushed out as air
pressure around you decreases. The opposite happens as the air pressure around you
increases. Eustachian tubes that connect your middle ear chamber to your mouth chamber can
be opened to balance the pressure on either side of your ear drum.
The following web site provides an illustration of the structure of your ear and includes
the eustachian tube.
http://www.earaces.com/anatomy.htm
Question 18: What happens to some of the air in your middle ear when you drop your jaw to
open the eustachian tube as you are:
a. dramatically increasing your altitude?
b. dramatically decreasing your altitude?
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F. Use Differences in Air Pressure to Crush A Can
In this procedure, you will be adding thermal energy to a parcel of air inside an open can.
Changing the amount of energy in a system produces a non-adiabatic change. The result of
this non-adiabatic change will be a dramatic illustration of what happens when there is a
difference in air pressure.
Be Careful! Hotplates can cause skin burns and damage clothes. Place nothing other
than the metal cans on the hotplates!
1. Be sure that the hotplate is on its highest setting.
2. Place approximately 10 ml of water in a soda can. Place the can on the hot plate.
3. Let the can sit on the hotplate for approximately 10 minutes.
Question 19: Why would it be very dangerous to be heating a closed can of air?
Question 20: What do you think is happening to the number of air molecules in the can as the
can is being heated?
4. After 10 minutes, use tongs to grab the lower end of the can. Quickly turn the can upside
down and place the open end of the can in the pan of cold water.
5. Turn the can upright as you remove the can from the pan of water
6. Record your observations of what happened to the can.
7. Determine what has happened to the amount of water in the can?
Question 21: What evidence was there that the air pressure inside the can was less than the
air pressure outside the can as it was being cooled?
Question 22: What evidence was there that the air pressure inside the can eventually was the
same as the air pressure outside the can?
Air Pressure Activities
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G. Use Differences in Air Pressure to Push Water Uphill
The following procedure demonstrates an effect caused by a difference in air pressure. Be
sure to read directions carefully. As you answer questions, remember that air molecules can
only push against a surface when they collide with that surface. Air molecules can not pull on a
material or object.
Be Careful: Hotplates can cause skin burns and damage other materials. Place nothing
other than Flask B onto the hotplate.
1. Flask A is the flask with 2 glass tubes
in the stopper. Flask A must be approximately
2/3 full of colored water.
2. Flask B is the flask with only 1 glass
in the stopper. Flask B must have
1 centimeter of colored water in it.
3. Ask for help if the incorrect amounts
of water are in the flasks.
Question 23: What evidence is there that the air pressure inside of Flask B is the same as the
air pressure outside of the Flask B before you heat Flask B?
4. Carefully lower Flask B onto the HOT hotplate. Be sure that the clamp is secured to the
ring stand.
5. Heat the water in Flask B until it boils. Do Not boil away all of that water in Flask B.
Question 24: What evidence is there in Flask A that air molecules are leaving Flask B when
you are heating Flask B?
6. Raise Flask B up from the hot plate and be sure that the clamp is secured to the ring stand
so that the Flask does not fall.
7. Move the hot plate away and carefully observe what happens very carefully.
Question 25: What evidence is there that a Low Pressure System developed inside of Flask B
when you raise it off of the hot hotplate?
I. Highs and Lows
If meteorologists know the location of low pressure or high pressure systems, they can
make weather predictions more easily. The skies are cloudy in the vicinity of a low pressure
system. Skies are clear in the vicinity of a high pressure system. In addition, winds move in a
clockwise direction around a high pressure system in the Northern Hemisphere. Winds move in
the opposite direction around a low pressure system.