Thu., Sep. 12, 2013

One of my favorite groups, Calexico, featured before class today.  And they're local.  This is the first time I've heard "Algiers" and I liked it.  Then you saw "Ballad of Cable Hogue" which featured Mariachi Luz de Luna (also a local group that often plays with Calexico) and a French singer Francoiz Breut.  That song was recorded at a concert in London.  Finally "Alone Again Or" recorded live in Austin (Austin City Limits?).

The 1S1P Assignment #1 reports are due next Tuesday.  The Bonus Assignment report (if you do it) is due a week from today.  The Experiment #1 reports are due a week from next Tuesday, i.e. Tue., Sep. 24.  Try to collect your data now so that you can return the materials next week (if you haven't already done so) and pick up a copy of the supplementary information handout.  There are Expt. #1 materials available for the remaining people on the waiting list.

If you weren't able to make it to class to take the Practice Quiz, you can download a copy.  I would suggest you do that just so that you can become familiar with the format of the quizzes.  Answers are also available online.

Keep an eye out for the first of this semester's Optional Assignments.  It will most likely appear online on Friday.  You don't have to do Optional Assignments, but they're the way you earn extra credit points in this class.

Because today's quiz is just for practice and doesn't count, we spent about half the period on some new material.



Sea level pressure, 14.7 psi, might not sound like much.  But when you start to multiply 14.7 by all the square inches on your body it turns into 1000s of pounds of weight (force).  As the picture above shows, when you're lying on the beach there are 470 pounds of air pressing down on a 4" x 8", brick size, area of your chest.  That's the weight of a pile almost 100 bricks tall.  That's a lot of weight.

Why isn't the person in the picture above crushed by the weight of the atmosphere above.  The answer is that the person's body pushes back with the same amount of force.  Air does the same thing.  This is a topic we will start to look at today.  We'll come back to it briefly next Tuesday.

Pressure at any level in the atmosphere depends on (is determined by) the weight of the air overhead.  You might be left with the idea that pressure just pushes downward.


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. 


We were able to see this by placing a brick on top of a balloon.  The balloon gets squished but not flattened.  It eventually pushes back with enough force to support the brick.  If I could figure out a way of keeping the balloon from moving too far sideways I could have gotten on the table and stood on the balloon.  With only a little compression it would have been able to support all 150 pounds of my weight.

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 pushing upward with enough pressure to support the weight of the air above.

Here's another example of air pressure pushing upward - my automobile.



The car sits on 4 tires, which are really nothing more than balloons.  The air pressure in the four tires pushes upward with enough force to keep the 1000 or 2000 pound vehicle off the ground.  The air pressure also pushes downward, you'd feel it if the car ran over your foot.  The air also pushes sideways with a lot of force; tires need to be strong to keep from exploding or coming off the wheel.

If it weren't quiz day, this would be a logical point to do a demonstration.  A demo that tries to prove that air pressure really does push upward as well as downward.  Not only that but that the upward force is fairly strong.  We'll do the demonstration next Tuesday.

Next, a short explanation of how a mercury barometer works.  A mercury barometer is used to measure atmospheric pressure and is really just a balance that can be used to weigh the atmosphere.  You'll find a messier version of what follows on p. 29 in the photocopied Class Notes. 









The instrument in the left figure above ( a u-shaped glass tube filled with a liquid of some kind) is actually called a manometer and can be used to measure pressure difference.  The two ends of the tube are open so that air can get inside and air pressure can press on the liquid.  Given that the liquid levels on the two sides of  the manometer are equal, what could you about PL and PR?

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 (pictured above at right) or a teeter totter (seesaw).  Because the two pans are in balance, the two columns of air have the same weight.   PL and PR are equal (but note that you don't really know what either pressure is, just that they are equal).


Now the situation is a little different, the liquid levels are no longer equal.  You probably realize that the air pressure on the left, PL, is a little higher than the air pressure on the right, PR.  PL is now being balanced by PR + P acting together.  P is the pressure produced by the weight of the extra fluid on the right hand side of the manometer (the fluid that lies above the dotted line).  The height of the column of extra liquid provides a measure of the difference between PL and PR.

Next we will just go and close off the right hand side of the manometer.


Air pressure can't get into the right tube any more.  Now at the level of the dotted line the balance is between Pair and P (pressure by the extra liquid on the right).  If Pair changes, the height of the right column, h,  will change.  You now have a barometer, an instrument that can measure and monitor 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 very 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 earlier).  Mercury also has a low rate of evaporation so you don't have much mercury gas at the top of the right tube (there's some gas, it doesn't produce much pressure, but it would poison you if you were to start to breath it).



Here is a more conventional barometer design.  The bowl of mercury is usually covered in such a way that it can sense changes in pressure but is sealed to keep poisonous mercury vapor from filling a room.


Average sea level atmospheric pressure is about 1000 mb.  The figure above (p. 30 in the photocopied Class Notes) gives 1013.25 mb but 1000 mb is close enough in this class.  The actual pressure can be higher or lower than this average value and usually falls between 950 mb and 1050 mb. 

The figure also includes record high and low pressure values.  Record high sea level pressure values occur during cold weather.  The TV weather forecast will often associate hot weather with high pressure.  They are generally referring to upper level high pressure (high pressure at some level above the ground) rather than surface pressure.  There is some debate about the 1085 mb pressure value measured in Mongolia.  The problem is that the pressure was measured at over 5000 feet altitude and a calculation was needed to figure out what the pressure would have been if the location were at sea level.  That calculation can introduce uncertainty.  But you don't really need to be concerned with all that, I just wanted to give you an idea of how high sea level pressure can get.

Most of the record low pressure values have all been set by intense hurricanes (the extreme low pressure is the reason these storms are so intense).  Hurricane Wilma in 2005 set a new record low sea level pressure reading for the Atlantic, 882 mb.  Hurricane Katrina had a pressure of 902 mb.  The following table lists some of the information on hurricane strength from p. 146a in the photocopied ClassNotes.  2005 was a very unusual year, 3 of the 10 strongest N. Atlantic hurricanes ever occurred in 2005. 


Most Intense North Atlantic Hurricanes
 Most Intense Hurricanes
to hit the US Mainland
Wilma (2005) 882 mb
Gilbert (1988) 888 mb
1935 Labor Day 892 mb
Rita (2005) 895 mb
Allen (1980) 899
Katrina (2005) 902
 1935 Labor Day 892 mb
Camille (1969) 909 mb
Katrina (2005) 920 mb
Andrew (1992) 922 mb
1886 Indianola (Tx) 925 mb

Note that a new all time record low sea level pressure was measured in 2003 inside a strong tornado in Manchester, South Dakota (F4 refers to the Fujita scale rating, F5 is the highest level on the scale).  This is very difficult (and very dangerous) thing to try to do.  Not only must the instruments be built to survive a tornado but they must also be placed on the ground ahead of an approaching tornado and the tornado must then pass over the instruments (also the person placing the instrument needs to get out of the way of the approaching tornado).

You can experience much larger changes in pressure if you move vertically in the atmosphere than you would ever be able to do at sea level.  Pressure in Tucson at 2500 feet altitude is routinely about 920 mb; it's even lower, about 700 mb, at the top of Mt. Lemmon.  The only place to experience 920 mb pressure at sea level would be in the middle of a strong hurricane.  Pressure never drops to 700 mb at sea level.