You can't use an
ordinary pan balance to weight the atmosphere. A
U-shaped tube filled with some kind of liquid that can
slosh back and forth would work. Such an instrument
is called a manometer and is often filled with mercury.

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To turn the manometer
into a true barometer, we'll extend the tube on the right
and close the top so that air isn't pushing down on the
mercury. We'll also use somewhat larger cylindrical
columns of air and mercury that completely fill the
insides of the tube.
The weight of a very tall cylindrical column of air is
balanced by a much shorter cylindrical column of
mercury. The height of the mercury column will change
as air pressure varies.
Mercury is a liquid. You need a liquid that can slosh
back and forth in response to changes in air pressure.
Mercury is also very dense which means the barometer won't
need to be as tall as if you used something like
water. A water barometer would need to be over 30 feet
tall. With mercury you will need only a 30 inch tall
column to balance the weight of the atmosphere at sea level
under normal conditions (remember the 30 inches of mercury
pressure units mentioned earlier). Mercury also has a
low rate of evaporation so you don't have much mercury gas
at the top of the right tube (there's some gas, it doesn't
produce much pressure, but it would be hazardous you if you
were to start to breath it).
Here is a more conventional barometer design.
The bowl of mercury is usually covered in such a way
that it can sense changes in pressure but sealed to keep
poisonous mercury vapor from filling a room.
2. Average and extreme sea level pressure values
Average sea level atmospheric pressure is about 1000 mb.
The figure above (p. 30 in the photocopied Class Notes) gives
1013.25 mb but 1000 mb is close enough in this class.
The actual pressure can be higher or lower than this average
value and usually falls between 950 mb and 1050
mb.
The figure also includes record high and low pressure
values. Record high sea level pressure
values occur during cold weather. The TV
weather forecast will often associate hot weather with high
pressure. They are generally referring to upper level
high pressure (high pressure at some level above the ground)
rather than surface pressure. You'll
sometimes here this upper level high pressure referred to as a
ridge, we'll learn more about this later in the semester.
Note that record high surface pressure values are observed
in the winter when it is cold.
There is some question about the accuracy of the 1085.7 mb
value above. The problem is that the pressure was
measured at over 5000 feet altitude and a calculation was
needed to figure out what the pressure would have been if the
location were at sea level. That calculation can
introduce uncertainty. But you don't really need to be
concerned with all that, I just wanted to give you an idea of
how high sea level pressure can get.
Most of the record low pressure values have all been set by
intense hurricanes.
Hurricane Wilma in 2005 set a new record low sea level
pressure reading for the Atlantic, 882 mb. Hurricane
Katrina had a pressure of 902 mb. The following table
lists some of the information on hurricane strength from p.
146a in the photocopied ClassNotes. 2005 was a very
unusual year, 3 of the 10 strongest N. Atlantic hurricanes
ever occurred in 2005.
You may remember Hurricane Patricia off the west coast of
Mexico last fall. Patricia set a new surface low
pressure record for the Western Hemisphere - 879 mb.
Sustained winds of 200 MPH were observed.
Most
Intense North Atlantic Hurricanes
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Most
Intense Hurricanes
to hit the US Mainland
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Wilma
(2005) 882 mb
Gilbert (1988) 888 mb
1935 Labor Day 892 mb
Rita (2005) 895 mb
Allen (1980) 899
Katrina (2005) 902
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1935
Labor Day 892 mb
Camille (1969) 909 mb
Katrina (2005) 920 mb
Andrew (1992) 922 mb
1886 Indianola (Tx) 925 mb |
What makes hurricanes so intense is the pressure gradient,
i.e. how quickly pressure changes with distance (horizontal
distance). Pressure can drop from near average values
(1000 mb) at the edges of the storm to the low values shown
above at the center of the storm. This large pressure
gradient is what causes the strong winds found in a hurricane.
The 850 mb pressure value measured in 2003 inside a strong
tornado in Manchester, South Dakota (F4 refers to the Fujita
scale rating, F5 is the highest level on the scale).
This is very difficult (and very dangerous) thing to try to
do. Not only must the instruments be built to survive a
tornado but they must also be placed on the ground ahead of an
approaching tornado and the tornado must then pass over the
instruments (also the person placing the instrument needs to
get out of the way of the approaching tornado).
You can experience much lower pressure values than shown above
(roughly 700 mb) by just driving up to Mt. Lemmon. Very
strong vertical changes in pressure are usually almost
balanced exactly by gravity.
3. Changes in atmospheric pressure with altitude
If you remember and understand the statement
atmospheric
pressure at any level in the atmosphere
depends on (is determined by)
the
weight of the air overhead
You can quickly and easily figure out what happens to air
pressure as you move upward in the atmosphere. A
pile of bricks can also help; here's
a picture of 5 bricks stacked on top of each
other.
Each of the bricks weighs 5 pounds, there's a
total of 25 pounds of weight. At
the bottom of the pile you would measure a weight of
25 pounds. If you moved up a brick you would
measure a weight of 20 pounds, the weight of the four
bricks that are still above. The pressure would
be less. Weight and pressure will decrease as
you move up the pile.
Layers
of air in the atmosphere is not too
much different from a pile of
bricks. Pressure at any level is
determined by the weight of the air
still overhead. Pressure
decreases with increasing altitude
because there is less and less air
remaining overhead.
At sea level altitude, at Point 1, the
pressure is normally about 1000 mb. That is
determined by the weight of all (100%) of the air in
the atmosphere.
Some parts of Tucson, at Point 2, are 3000
feet above sea level (most of central Tucson is a
little lower than that around 2500 feet). At
3000 ft. about 10% of the air is below, 90% is still
overhead. It is the weight of the 90% that is
still above that determines the atmospheric pressure
in Tucson. If 100% of the atmosphere produces a
pressure of 1000 mb, then 90% will produce a pressure
of 900 mb.
Pressure is typically about 700 mb
at the summit of Mt. Lemmon (9000 ft. altitude at Point 3) because
70% of the atmosphere is overhead..
Pressure decreases rapidly
with increasing altitude. We will find that
pressure changes more slowly if you move
horizontally. Pressure changes about 1 mb for
every 10 meters of elevation change. Pressure
changes much more slowly normally if you move
horizontally: about 1 mb in 100 km. Still the
small horizontal changes are what cause the wind to
blow and what cause storms to form.
Point
4 shows a submarine at a depth of about 30
ft. or so. The pressure there is determined by
the weight of the air and the weight of the water
overhead. Water is much denser and much heavier
than air. At 30 ft., the pressure is already
twice what it would be at the surface of the ocean
(2000 mb instead of 1000 mb).
I'll try to show a short video segment about what would happen
to a human head if it were taken down to a depth of 10,000
feet in the ocean where the surrounding pressure is
enormous.
I learned about a relatively new sport called free diving
as semester or two ago. Basically divers see how deep
they can go while holding their breath. They must
descend and return to the surface on just a single lungful of
air. It is a very hazardous sport. Here is a link
to an article about a diver that made it to a depth of 236
feet but died upon reaching the
surface. Death was caused by
the high pressure deep under water forcing fluid from
the blood into the diver's lungs.
4. The downward force of air pressure
Sea level pressure, 14.7 psi, might not sound like
much. But when you start to multiply 14.7 by all the
square inches on your body it turns into a lot of pounds of
force.
The yellow box on the person's chest in the picture is a brick
size, 4" x 8" = 32 square inch, area. If you multiply
14.7 psi by 32 sq. in. you get 470 pounds! It would take
a stack of 90 to 100 bricks to produce that much weight.
Imagine lying on the beach with 90 bricks stacked up on your
chest.
Why isn't the person in the picture
above crushed by the weight of the atmosphere
above. The answer is that the person's body
pushes back with the same amount of force. Air
does the same thing. This is the topic we will
explore next.
5. The upward (and sideways) force of air pressure
Air pressure is a force that
pushes downward, upward, and sideways. If you fill a
balloon with air and then push downward on it, you can feel
the air in the balloon pushing back (pushing upward).
You'd see the air in the balloon pushing sideways as
well.
We were able to see this by placing a
brick on top of a balloon. The balloon gets squished
(pushed out sideways) but not flattened. It eventually
pushes upward with enough force to support the brick.
The squished balloon is what air at the bottom of the
atmosphere looks like. And it is supporting more than
just one brick, it is supporting a pile 90 to 100 bricks
tall (just like the yellow box on the chest of the guy at
the beach).
Another helpful
representation of air in the atmosphere might be a
people pyramid.
The people in the figure are like
layers of air in the atmosphere all stacked on top of each
other.
If the bottom person in the stack above were standing on a
scale, the scale would measure the total weight of all the
people in the pile. That's analogous to sea level
pressure being determined by the weight of the all the air
above.
The bottom person in the picture above must
be strong enough to support the weight of all the people
above. The bottom layer of the atmosphere pushes
upward with enough pressure to support the weight of the
air above.
Here's another pretty amazing example of air
pressure pushing upward.
This is my present day car (a 1980 Toyota Celica)
sits on 4 tires, which are really nothing more than
balloons. The air pressure in the four tires
pushes upward with enough force to keep the 1000 or 2000 pound
vehicle off the ground. The air pressure also pushes
downward, you'd feel it if the car ran over your foot.
The air also pushes sideways with a lot of force; tires need
to be strong to keep from exploding or coming off the wheel.
6. Upward Air Pressure force demonstration
This is a logical point to do a demonstration. A
demo that tries to prove that air pressure really does push
upward as well as downward. Not only that but that the
upward force is fairly strong. The demonstration is
summarized on p. 35a in the ClassNotes.
It's pretty obvious that if you fill a balloon with a
little water and let go it will drop. And most
everyone in the class knows why (see below - I broken the
figure on p. 35b into pieces for clarity).

Gravity
exerts a downward force on the balloon. I just
made up a number, 10, to give you some idea of its
strength.
But the picture above isn't quite complete.
The water balloon is surrounded by air that
is pushing upward, downward, and sideways on the
balloon. These pressure
forces are strong but mostly cancel each
other out. The sideways forces do cancel out
exactly.
The up and down forces aren't quite equal because
pressure decreases with increasing altitude. The
upward pointing force at the bottom is stronger (15
units) than the downward force at the top (14
units). They don't cancel and there is a weak upward pressure difference force
(1 unit strong). I'm pretty sure that most people
in the class don't know about this pressure difference
force.
This picture includes
all the forces (gravity and pressure
difference). The downward gravity force is
stronger than the upward pressure difference
force and the balloon falls.
It seems like we could change things
a little bit and somehow keep the upward and
downward pressure forces from working against each
other. That's
what we do in the demonstration.
In
the demonstration a wine glass is filled
with water (about the same amount of
water that you might put in a small
water balloon).
A small plastic lid is used to cover the wine glass (you'll
need to look hard to see the lid in the photo above).
The wine glass is then turned upside and the water does not
fall out.
All the same forces are shown again in the left most
figure. We'll split that into two parts - a water and
lid part and an empty glass part.
The 14 units of pressure force is pushing on the glass
now and not the water. I was holding onto the glass,
I'm the one that balanced out this downward pressure force.
Gravity still pulls downward on the water with the same
10 units of force. But with 15 units, the upward
pressure force is able to overcome the downward pull of
gravity. It can do this because all 15 units are used
to overcome gravity and not to cancel out the downward
pointing pressure force.
7. The Magdeburg hemispheres
experiment (sideways pressure force)
Air pressure pushes downward with hundreds of pounds of
force on someone lying on the beach.
The pressure of the air in tires
pushes upward with enough force to keep a 1 ton automobile
off the ground.
What about the sideways air pressure
force?
Here's a description of a
demonstration that really needs to be done in Arizona
Stadium at half time during a football game. It
involves Magdeburg hemispheres and two teams of horses
(the following quote and the figure below are from an
article in Wikipedia):
" ... Magdeburg hemispheres
are a pair of large copper hemispheres with mating rims,
used to demonstrate the power of atmospheric pressure.
When the rims were sealed with grease and the air was
pumped out, the sphere contained a vacuum and could not
be pulled apart by teams of horses. The Magdeburg
hemispheres were designed by a German scientist and mayor
of Magdeburg, Otto von Guericke in 1656 to demonstrate the
air pump which he had invented, and the concept of
atmospheric pressure."

Gaspar Schott's sketch of Otto von
Guericke's Magdeburg hemispheres experiment (from the
Wikipedia article referenced above)
It is the pressure of the air
pushing inward against the outside surfaces of the
hemispheres that keeps them together. The
hemispheres appear to have had pretty large surface
area. There would be 15 pounds of force pressing
against every square inch (at sea level) of the
hemisphere which could easily have been several
thousand pounds of total force.
Suction cups work the same way
The suction cup has been pressed against smooth
surface. The cup is flexible and can be pulled away
from the wall leaving a small volume between the wall and
the cup where there isn't any air (a vacuum). There's
no air pressure pushing outward, away from the wall, in the
space between the wall and the suction cup. There's
just pressure from the air surrounding the suction cup that
is pushing and holding it against the wall.
I suspect that if I were to attach the suction cup I had
in class to a white board mounted to a wall and were to ask
a couple of strong people to come down and try to pull it
off the white board they would end up pulling the white
board off the wall. The Facilities Management people
wouldn't appreciate that very much.