Thursday Sep. 10, 2009
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A couple of songs ( "Stairway to Heaven" and "Diablo Rojo" ) from Rodrigo y Gabriela before class today.  A local group, Calexico, will be featured next week, all week.

The first of this semester's 1S1P Assignments is now online.  The Bonus Report is due next Thursday, Sep. 17.  If you decide to do a report on Topic #1 or #2 (or both), those reports are due on Tue., Sep 29.

So far this semester we have learned about the composition of the atmosphere and about some of the main air pollutants.  Today we will start looking at how atmospheric characteristics such as 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 why it changes as you move vertically through the atmosphere.

An iron bar was passed around at the beginning of class.  You were supposed to guess how much it weighed.



We came 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.



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 next picture 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 moving).


Differences in volumes account for the differences in mass in the example above.  It is possible to have two objects with the same volume but very different masses.  The bottles of water and mercury that were passed around class were an example (thanks for being so careful with the mercury).

Weight is a force and depends on both the mass of an object and the strength of gravity.  We tend to use weight and mass interchangeably because we spend all our lives on earth where gravity never changes.



On the earth where the pull of gravity never changes, any three objects that all have the same mass (even if they had different volumes and were made of different materials) would always have the same weight. Conversely:


When gravity is always the same, three objects with the same weight 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.

If you carry an object from the earth to the moon, the mass remains the 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.


In 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 higher density.


The air that surrounds the earth has mass.  Gravity pulls downward on the atmosphere giving it weight.  Galileo conducted (in the 1600s) a simple experiment to prove that air has weight.  That experiment wasn't mentioned in class.

Pressure is defined as force divided by area.  Air pressure is the weight of the atmosphere overhead divided by the area the air is resting on.  Atmospheric pressure is determined by and tells you something about the weight of the air overhead.  This is one way, a sort of large scale representation, of understanding air pressure.

Under normal conditions a 1 inch by 1 inch column of air stretching from sea level to the top of the atmosphere will weigh 14.7 pounds.  Normal atmospheric 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 think it is 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 units. 


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).  If you ever find yourself in France needing to fill your automobile tires with air (I lived in France for a while and owned a  Peugeot 404) 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.

And that's as far as we got in class on Thursday.  The remainder of the period was devoted to the Practice Quiz.