Friday Aug. 31, 2007

The first 1S1P assignment of the semester was mentioned and briefly discussed.

Meanwhile local weather forecasters are keeping an eye on Tropical Storm Henriette (the storm may have dissipated and disappeared from the National Hurricane Center by the time you click on the link at left).  Some models show moisture from that storm system making its way up into southern Arizona toward the end of next week.  This is a possibility not certainty or even likely at this point.
We first finished the plot of temperature versus altitude and the discussion of the troposphere and stratosphere that was started in class last Wednesday.  You'll find all of that in the Wed., Aug. 29 class notes.

Sulfur dioxide was discussed briefly in class on Wednesday.  Sulfur dioxide is one of the pollutants that can react with water in clouds to form acid rain.  The formation and effects of acid rain are discussed on p. 12 in the photocopied Class Notes.

Note that clean unpolluted rain has a pH less than 7 and is slightly acidic.  This is because the rain contains dissolved carbon dioxide gas.  Acid rain is often a problem in regions that are 100s even 1000s of miles from the source of that sulfur dioxide that forms the acid rain.

Some of the problems or consequences of acid rain. 

A short colorful and calming acid rain demonstration was done in class.  Carbon dioxide gas was used instead of sulfur dioxide.
What follows is a little more detailed discussion of the basic concepts of mass, weight, and density (found on p. 23 in the photocopied Class Notes) than was done in class. 


Before we can learn about atmospheric pressure, we need to review the terms mass and weight.  In some textbooks you'll find mass defined at the "amount of stuff."  Other books will define mass as inertia or as resistance to change in motion.  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 if it were already moving).

It is possible to have two objects with the same volume but very different masses.  Here's an example:

Bottles containing equal volumes of water and mercury were passed around in class (thanks for being careful with the bottles of mercury).  The bottle of mercury was quite a bit heavier than the bottle of water.

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.


Any three objects that all have the same mass would necessarily have the same weight.  Conversely

Three objects with the same weight would 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 (its the same object, the same amount of stuff) but the weight changes because gravity on the moon is weaker than on the earth.

Mercury atoms are built up of many more protons and neutrons than a water molecule (also more electrons but they don't have nearly as much mass as protons and neutrons).  The mercury atoms have 11.1 times as much mass as the water molecule.  This doesn't quite account for the 13.6 difference in density.  Despite the fact that they contain more protons and neutrons, the mercury atoms must also be packed closer together than the molecules in water.

Definition and illustrations of high and low density.


The air that surrounds the earth has mass.  Gravity pulls downward on the atmosphere giving it weight.  Galileo used a simple experiment to prove that air has weight.

Pressure is defined as force divided by area.  Air pressure is the weight of the atmosphere overhead divided by area the air is resting on.  Atmospheric pressure is determined by and tells you something about the weight of the air overhead.

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).

We'll return to this figure in class next Wednesday or Friday and learn about the millibar units most commonly used by meterologists for atmospheric pressure.