Tuesday Jan. 31, 2006

The lectures in this class are all being video recorded.  These recordings can be accessed at www.svl.arizona.edu.  Click on the Search Class/Title drop down arrow near the top right of the screen.  Then select NATS 101: Weather and Climate.  Click on the Find Videos button near the bottom center of the screen.

The Assignment #1 1S1P reports were collected in class today.  It takes a while to grade these.  You should expect to start receiving your graded reports back sometime next week.

Photocopies of the Practice Quiz Study Guide were distributed in class (the Practice Quiz is this coming Thursday).  Note the location of the reviews (Tue. and Wed. afternoons 4 - 5 pm) will be Chavez (Econ) 301.  There won't be any questions from the Stratospheric Ozone and the Ozone Hole section at the end of the study guide.

The Experiment #1 reports are due next Tuesday. You should bring back your materials this week so that you can pick up the supplementary information sheet.

The Optional Assignment is due on Thursday.
You'll find the following material discussed on p. 29 in the photocopied notes.  In 3 simple steps you can understand how a barometer works.  It is probably worth mentioning that barometers are used to measure pressure (atmospheric pressure).  It is not coincidence that the word bar that appears in barometer is the same bar that appears in millibars.  We will be learning about weather maps next week and will come across isobars, contours of pressure.
a manometer can be used to measure pressure difference, here P1 is equal to P2
A manometer can be used to measure pressure difference.  The manometer is just a u-shaped tube usually made of glass so that you can see the liquid that is inside.  The liquid can slosh back and forth just like the pans on a balance can move up and down.  A manometer really behaves just like a pan balance.

In this picture the fact that the liquid levels are the same in the right and left tubes means P1 and P2 are the same (note you really don't know what P1 and P2 are, just that they are equal).
the manometer here shows that P1 is higher than P2
Now the situation is a little different, the liquid levels are no longer equal.  The orange shaded portion of the liquid is the balance that we had in the previous picture.  The pressure P1 is equal to Phere, the pressure part way up the right column.  P2 is not able by itself to balance P1, P2 is lower than P1.  P1 plus the pressure produced by the column of extra liquid on the right balances P1.  The height of the column of extra liquid provides a measure of the difference between P1 and P2.
with slight modification the manometer becomes a barometer
We have changed the manometer by lengthening the right tube and sealing it off at the top.  Air pressure can't get into the right tube any more.  The balance is again shaded in orange at the bottom of the barometer.  Pressures at the two points indicated are equal.  Pair is equal to the pressure produced by the column h inches tall on the right.  If Pair changes, h will change.  You now a way of measuring and monitoring the atmospheric pressure.

Barometers like this are usually filled with mercury.  Mercury is a liquid.  You need a liquid that can slosh back and forth in response to changes in air pressure.  Mercury is also 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 in class a week or so ago).  Mercury also has a low rate of evaporation so you don't have much mercury gas at the top of the right tube.
invention of the mercury barometer
Galileo invented the thermometer (I believe).  The barometer was invented in 1643.  Not long after that a couple of French researchers verified that pressure decreased with increasing altitude. 
average and range of sea level pressure values
Under normal conditions sea level pressure is about 1000 mb (about 30 inches of mercury).  It can be higher and lower than this but usually falls in the range 950 mb to 1050 mb.  Record high and low sea level pressure values are shown in the chart.  Note the record low values have all be observed in hurricanes.
Now on to a second way of thinking about air pressure, a sort of microscopic view.  But first
gas molecules in a balloon
What keeps an air filled balloon inflated?  Is it the fact that the air molecules are like clear glass marbles that just take up all of the room inside the balloon?  If that were true it would be hard to reduce the volume of the balloon without taking out any of the air (and we will reduce the volume of a balloon without removing any air later in the class).  Or is the picture on the right, miniscule air molecules moving around inside the balloon at 100s of MPH and occasionally colliding with the walls of the balloon and pushing outward, more accurate?

Click on this molecules in a gas animation link to see a computer graphics representation of molecules moving around inside a sealed box.

Next we will see what factors affect the strength of the pressure produced by moving air molecules inside a balloon.
pressure inside a balloon depends on the number of air molecules

Pressure first depends on the number of air molecules, N. 
pressure is inversely proportional to volume

Pressure depends (inversely) on the volume of the balloon.  In the Oxygen concentration of the air experiment, the Number of air molecules in an air sample and the Volume of the air sample will change together in a way that keeps the air sample Pressure constant.
pressure is proportional to the air temperature

Pressure depends on the air's temperature.  You shouldn't thrown an aerosol can into a fire, the fire would heat the gas inside the can, increase the pressure, and possibly cause the can to explode.

pressure does not depend on the mass of the gas molecules
Surprisingly the pressure does not depend on the mass of the molecules.  Pressure doesn't depend on the composition of the gas.  The massive slow moving molecules hit the walls of the balloon with the same force as the lighter but faster moving molecules.

two forms of the ideal gas law

Here are the two ideal gas law equations.  You can ignore the constants k and R if you are just trying to understand how a change in one of the variables would affect the pressure.  You only need the constants when you are doing a calculation involving numbers.

(1) Pressure  = (Number of air molecules) multiplied by temperature divided by volume
or
(2) Pressure = (density) multiplied by (temperature)
warming and cooling the air in a flash
We used a manometer to show how warming the air in a flash (by touching it with your hands) caused the air pressure in the flask to increase.  Conversely  by cooling the air with a little liquid nitrogen causes the air pressure in the flask to decrease.
warming or cooling air causes density to change in a way that keeps pressure constant

Some of the colors in these figures have been changed slightly from those in class.

Air in the atmosphere behaves more like the air in a balloon than air in a flask.  A flask doesn't change volume, a balloon can. 

We start in the upper left hand corner with air inside a balloon that is exactly the same as the air outside.  The air inside and outside have been colored green.  The arrows show that the pressure of the air  inside pushing outward and the pressure of the air surrounding the balloon pushing inward are all the same.

Next week warm the air in the balloon.  The pressure of the air increases (just like it did in the flash).  The increase is momentary though.  Because the pressure inside is now greater than the pressure outside the balloon will expand.  An increase in volume will reduce the pressure of the air inside however.  Eventually the balloon will expand enough that the pressures inside and outside are again in balance.  You end up with a balloon of warm low density air that has the same pressure as the air surrounding it.

You can use the same reasoning to understand that cooling a balloon will cause its volume to decrease.  You will end up with a balloon filled with cold high density air.  The pressures inside and outside the balloon will be the same.

These associations: warm air = low density air and cold air = high density air are important and will come up a lot during the remainder of the semester

changing the density of the air inside a balloon determines whether it will rise or sink
Now that you know how temperature causes air density to change (in order to keep pressure constant), we can look at the forces acting on a parcel or balloon of air.  There are two forces: gravity and an upward pointing pressure difference force.  When the air inside a parcel of balloon is exactly the same as the air outside the two forces are of equal strength.  Because they point in opposite directions, they cancel each other out.

If the balloon is filled with warm, low density air the gravity force will weaken. The upward pressure difference force (which depends on the surrounding air) will not change.  The upward force will be stronger than the downward force and the balloon will rise. 

Conversely if a balloon is filled with cold low density air, gravity will strengthen and the balloon will sink.
Charles's Law demonstration, rising parcels of air

The two demonstrations performed near the end of class are described on p. 53 in the photocopied notes.