Friday Jan. 30, 2009
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supplied what I thought was an appropriate song (M79)
for the start of what looks to be a glorious weekend.
Next week's reviews (for the Practice Quiz) will both be held in FCS
225 from 4-5 pm on Monday and Tuesday afternoon.
this semester we have learned about the composition of the atmosphere
and about some of the main air pollutants. Today we will start
at how atmospheric characteristics such
air temperature, air pressure, and air density change with
altitude. In the case of air pressure we first need to understand
what pressure is and how it changes as you move vertically through the
An iron bar was passed around at the
beginning of class. You were supposed to guess how much it
We'll come back to this later in the period.
A pair of bottles, one containing water and the other
an equal volume of mercury, were also passed around in class. Feel the
difference in the weights of the two bottles. Mercury is much
denser than water.
Some of you may have seen Venus and the crescent moon in Thursday's
evening sky - it was quite spectacular. The sight motivated me to
try to write a tanka poem. Haiku poems have 3 lines, a tanka poem
has 5 lines (with 5, 7, 5, 7, and 7 syllables per line as you move from
to bottom). Here is what I had come up with so far:
I asked for some help coming up with the last line (sorry no extra
credit this time) while I was occupied handing out the bar and the
Before we can learn about
atmospheric pressure, we
need to review
the terms mass and weight. In some textbooks you'll find mass
defined as "amount of stuff" or "amount of a particular
material." Other books will define mass as
inertia or as resistance to change in motion (this comes from Newton's
2nd law of motion, we'll cover that later in the semester). The
illustrates both these definitions. A Cadillac and a volkswagen
have both stalled in an intersection. Both cars are made of
steel. The Cadillac is larger and has more steel, more stuff,
more mass. The Cadillac is also much harder to get moving than
the VW, it has
a larger inertia (it would also be harder to slow down once it is
It is possible to have two objects with the
volume but very
different masses. The bottles of water and mercury that were
passed around class were an example (thanks for being so
with the mercury).
is a force and depends on
both the mass of an object and the
strength of gravity. We tend to use
weight and mass
because we spend all our
lives on earth where gravity never changes.
Any three objects that all have the same mass
(even if they had different volumes and were made of different
necessarily have the same weight. Conversely:
objects with the
would also have the same mass.
The difference between mass and weight is clearer
(perhaps) if you
compare the situation on the earth and on the moon.
carry an object
earth to the moon, the mass
same (it's the same object, the same amount of stuff) but the weight
changes because gravity on the moon is weaker than on the earth.
the first example there is more mass (more dots) in the right box than
in the left box. Since the two volumes are equal the box at right
has higher density. Equal masses are squeezed into different
volumes in the bottom example. The box with smaller volume has
surrounds the earth has mass. Gravity pulls downward on the
atmosphere giving it weight. Galileo conducted (in the 1600s) a
experiment to prove that air has weight.
Pressure is defined as force divided by area. Air
pressure is the
of the atmosphere overhead divided by the area the air is resting
Atmospheric pressure is
determined by and tells you something about the weight of the air
Under normal conditions a 1 inch by 1 inch column of air
from sea level to the top of the atmosphere will weigh 14.7
pressure at sea level
is 14.7 pounds per square inch (psi, the units you use when you fill up
your car or bike tires with air).
Now here's where the steel bar
comes in. The steel bar also weighs exactly 14.7 pounds (many
people thought it was heavier than that). Steel is a lot denser
than air, so a steel bar only needs to be
52 inches tall to have the same weight as an air column that is 100
miles or more tall.
Here are some of the other commonly used pressure
Typical sea level
pressure is 14.7 psi or about 1000 millibars
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). If you ever find
yourself in France needing to fill your
automobile tires with air (I lived in France for a while and owned
remember that the air compressor scale is
probably calibrated in bars. 2 bars of pressure would be
equivalent to 30 psi.
The word "bar" basically means
pressure and is used in a lot of meteorological terms (the figure above wasn't shown in class).
at sea level is determined by the weight of the air overhead.
What about pressure at some level above sea level?
We can use a stack of bricks to try to
weighs 5 pounds. At the bottom of the 4 brick tall pile you would
measure a weight of 20 pounds. If you moved up a brick you would
measure a weight of 15 pounds, the weight of the three bricks still
above. To get the pressure you would need to divide by the
area. It should be clear that weight and pressure will decrease
as you move up the pile.
In the atmosphere, 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 numbered points on the figure below were added
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). At 3000 ft. about 10%
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. It is small horizontal changes that cause the
wind to blow however.
Point 4 shows a submarine at a depth of
about 33 ft. 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).
The person in the picture below (from a
Physics textbook) is 20 feet
underwater. At that depth there is a pretty large pressure
pushing against his body
from the surrounding water. The top of the snorkel is exposed to
the much lower air pressure at the top of the pool. If the
swimmer puts his mouth on the snorkel the pressure at the bottom of the
pull would collapse his lungs.
There is a lot going on in this
picture. 1000 mb at Point
1 is a typical value for sea level pressure. The fact that
pressures are equal at the bottoms of both
sides of the picture means that the weight of the atmosphere at the
bottom of the
picture on the left is the same as the weight of the atmosphere at the
bottom of the picture at right. The only way this can be true is
if there is the same total amount (mass) of air in both cases.
Point 2 - Moving upward from the ground we find that pressure
to 900 mb at the level of the dotted line in the picture at left.
This is what you expect, pressure decreases with increasing
altitude. In the figure at right you need to go a little bit
higher for the same 100 mb decrease.
The most rapid rate of pressure decrease with increasing
altitude is occurring in the picture at left because the 100 mb change
occurs in a shorter distance.
Point 3 -
Since there is a 100 mb drop in both the layer at left and
layer at right, both layers must contain the same amount (mass) of air.
Point 4 - The air in the picture at left is squeezed into a
layer than in the picture at right. The air density in the left
layer is higher than in the layer at right.
By carefully analyzing this figure we have proved to ourselves
rate of pressure decrease
with altitude is higher in dense air than in lower
This is a fairly subtle but important concept. This concept
will come up 2 or 3 more times later in the semester. For
example, this concept partly explains why hurricanes can intensify and
strong as they do.