Typical sea level
pressure is 14.7 psi or about 1000 millibars
(the
units used by meterologists and the units that we will use in this
class most of the time) or about 30 inches of mercury (refers to
the reading on a mercury barometer).
We'll talk briefly about mercury barometers next week.
They're used to measure atmospheric pressure.
Mercury is more dense than steel and you only need a
30 inch
column of mercury to balance the weight of a column of air. And
that's basically what a mercury barometer does. It basically
balances the weight of a tall column of air with a shorter column of
mercury.
Bar means pressure. Isobars are contours of pressure
drawn on weather maps.
Pressure
at sea level is determined by the weight of the air overhead.
What happens to pressure as you move
upward in the atmosphere. We can use a pile of bricks (which are
easier to visualize than invisible layers of air) to help
answer this question.
Bricks weigh about 5 pounds each. At
the bottom of the 5 brick tall pile you would
measure a weight of 25 pounds (if you wanted to find the pressure you'd
divide 25 lbs by the area on the bottom of the brick). If you
moved up a brick you would
measure a weight of 20 pounds, the weight of the four bricks still
above. It should be clear that weight and pressure will decrease
as you move up the pile.
The atmosphere is really no different. 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. The figure below is a more
carefully drawn version of what was done in class.
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 the valley 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) and
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
33
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 33 ft., the pressure is already twice what it would be at
the surface of the ocean (2000 mb instead of 1000 mb).
What
difference
does it make if pressure decreases with
increasing altitude? That's a fair question.
Here's one answer to that question.
Hot air balloons can go up and come back down. I'm pretty
sure you know what would cause the balloon to
sink. I suspect you don't know what causes it to float upward.
Gravity pulls downward on the balloon. There is an upward
force caused by the pressure difference between the top and bottom of
the balloon. Pressure decreases with increasing altitude.
The pressure at the bottom is a little higher than at the top. A
pressure difference force points from high toward low pressure.
Next we'll try to figure out what happens to air density as you
move upward in the atmosphere. You probably already know the
answer.
Because air is compressible, a stack of mattresses might be a more
realistic representation of layers of air than a pile of bricks.
Four mattresses are stacked on top of each
other. Mattresses
are reasonably heavy, the mattress at the
bottom of the
pile is compressed by the weight of the three mattresses
above. This is shown at right. The mattresses higher up
aren't squished as much because
their
is less weight remaining above. The same is true with layers of
air in the atmosphere.
There's a lot of information in
this figure (p. 34 in the photocopied ClassNotes). It is worth
spending a minute or two looking at it and thinking about it.
1. You can first notice and remember that pressure
decreases
with increasing altitude. 1000 mb at the bottom decreases to 700
mb at the top of the picture.
Each layer of air contain the same amount (mass) of air.
This is a fairly subtle point. You can tell because there is an
equal 100 mb pressure drop as you move upward through each layer.
Pressure depends on weight. So
if all the pressure changes are equal, the
weights of each of the layers must be the same. Each of the
layers
must contain the same amount (mass) of air (each layer contains
10% of the air in
the atmosphere).
2. The densest air is found in the bottom
layer. The bottom layer is compressed the most. Since each
layer has the same amount of air
(same mass) and the bottom layer has
the
smallest volume it must have the highest density. The top layer
has the same amount of air
but about twice the volume. It therefore has a lower density.
3. The rate of pressure change with altitude depends on air
density. The most rapid rate of pressure decrease with increasing
altitude is in the densest air at the bottom of the picture.
Pressure
at
any
level in the
atmosphere depends on (is determined by) the weight of the air
overhead. We used a pile of bricks (each brick represents a layer
of air)
to help visualize and understand why pressure decreases with
increasing altitude. A pile of bricks can lead to the believe
that
air pressure exerts force in just a downward direction.
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.
The air
pressure in the four tires on your automobile pushes
pushes upward
with enough force to keep the 1000 or 2000 pound vehicle off the
road.
Another helpful representation of air in the atmosphere might be a
people pyramid.
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. That is equivalent to the bottom layer of the
atmosphere having enough pressure to support the weight of the air
above.
Coming on Monday proof that air pressure pushes upward.
