Thu., Sept. 6, 2007

The first 1S1P assignment of the semester was mentioned briefly.  You can choose from the topics listed and write up to 2 reports.  Reports are due on Tue., Sept. 18.

The first optional assignment of the semester was handed out in class.  Assignments are due at the beginning of class on Thu., Sept. 13.

We had a quick look at satellite imagery and the predicted path of Tropical Depression Henriette.  Henriette was bringing cloudy skies, cooler temperatures, and scattered showers to southern Arizona.  The center of the remnants of Henriette was expected to pass to the east of Tucson, that is where the heavier rains were expected.

Note a new reading assignment was posted Friday after class on Thursday.
In this class we will sometimes "beat topics to death."   We will do that today in trying to understand how mercury barometers work.  You will find most of what follows on p. 29 in the photocopied Class Notes.  The pictures below are hopefully clearer and more carefully drawn versions of what were drawn in class.

The instrument above ( a u-shaped glass tube filled with a liquid of some kind) is 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. 

PL and PR are equal (note 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 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 go an extreme 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 theatmospheric 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 earlier).  Mercury also has a low rate of evaporation so you don't have much mercury gas at the top of the right tube.

Finally 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 not evaporate and fill the room with poisonous mercury vapor.

The figure above (p. 30 in the photocopied Class Notes) first shows average sea level pressure values (1000 mb or 30 inches of mercury are close enough in this class).  Sea level pressures usual fall between 950 mb and 1050 mb.  Record high sea level pressure values occur during cold weather.  Record low pressure values have all been set by intense hurricanes (the record setting low pressure is the reason these storms were so intense)
.
Next we looked at some of the historical events listed on pps 31 and 32 in the photocopied class notes.  The barometer was invented in the mid-1600s following the discovery by Galileo that air had weight.  Note (at the botom of p. 31) the amount clothing that Hawthorne Charles Grey wore on this trip to 28,510 feet altitude (in an open balloon cabin).

Grey would die on a later trip to 42,470 feet when he ran out of oxygen.  We watched a short video recalling Auguste Piccard's first flight into the stratosphere by balloon (May 27, 1931).  The video segment was from a PBS program titled "The Adventurers."
The remainder of the period was devoted to the Practice Quiz.  Graded quizzes will be returned next Tuesday.