Tuesday Sept. 13, 2011
click here to download today's notes in a more printer friendly format

You might not have liked the song selections this morning from  Eva Cassidy ("Fine and Mellow", "Blue Skies", and "Bridge Over Troubled Water"), but you must admit she sure can sing.

All 170 of the Practice Quizzes has been graded and were returned in class today.  The average (in both sections this semester) was 65%.  That's low but pretty typical for a Practice Quiz.

Semester
 MWF
class
 T Th
class
F11
 65%
 65%
S11
 62%
 ---
F10
 60%
 67%
S10
 62%
 61%
F09
 66%
 68%
S09
 60%
 ---
F08
 64%
 65%
S08
 64%
 66%

If you didn't take the Practice Quiz you should at least print out a copy to study for Quiz #1 on Sept. 22.  The answers are online.

The Optional Assignment turned in last Tuesday was also returned.  Not having a grade marked on your paper means you earned full credit (0.3 extra credit pts).

The source material for the Seasons 1S1P report topic is now online.  Because it took me a little longer to do this than planned I've extended the due date (for that topic only).  Check the 1S1P Report Topics and Assignments page for more details.

Unless noted otherwise, the Experiment #1 reports are due one week from today.  You should be finishing up the data collection and returning your materials this week so that you can pick up the supplementary information handout.  Experiment #2 materials should be handed out next week (probably Thursday next week).

An In-class Optional Assignment was handed out in class today.  If you'd like to download the assignment, answer the questions, and turn it in at the start of class on Thursday, you can earn at least partial credit.

Back to a topic that was mentioned at the end of class last Thursday. This figure tries to explain why the rate of pressure change as you move or down in the atmosphere depends on air density.  In particular why does pressure decrease more rapidly with increasing altitude in dense air than it does in lower density air?

I asked three questions (shown above in red) in class.

1. The rate of pressure decrease with increasing altitude is greatest in Layer A.  To determine the rate of pressure decrease you divide the pressure change (100 mb for both layers) by the distance over which that change occurs.  The 100 mb change takes place in a shorter distance in Layer A than in Layer B.  Layer A has the highest rate of pressure decrease with increasing altitude.

2.  There is a 100 mb drop in pressure in both air layers.  In order for this to be true both layers must have the same weight and therefore the same mass.  Both layers contain the same amount of air.

3.  Density is mass divided by volume.  The air in the Layer A is denser than the air in Layer B.  The same amount (mass) of air is squeezed into a thinner layer, a smaller volume, in the left layer.  This results in higher density air.

So both the most rapid rate of pressure decrease with altitude and the densest air are found in Layer A.

The fact that the rate of pressure decrease with increasing altitude depends on air density is a fairly subtle but important concept.  This concept will come up 2 or 3 more times later in the semester.  For example, we will use this concept to explain why hurricanes can intensify and get as strong as they do. 

The next bunch of material tries to explain 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 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.

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.  3 of the 10 strongest N. Atlantic hurricanes 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 potentially dangerous thing) 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.
Pressure at any level in the atmosphere depends on (is determined by) the weight of the air overhead.  We used a pile of bricks (each brick represents a layer of air) to help visualize and understand why pressure decreases with increasing altitude.  A pile of bricks can lead to the believe that air pressure exerts force in just a downward direction. 


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.

The air pressure in the four tires on your automobile pushes pushes upward with enough force to keep the 1000 or 2000 pound vehicle off the road.  The air pressure also pushes downward, you'd feel it if the car ran over your foot.

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.

This was 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.  The demonstration is summarized on p. 35 a in the ClassNotes.


Don't worry too much about the details above because there's a more detailed explanation is below.  At this point you should wonder why is it that the water in a balloon will fall while the water in the wine glass does not.


Here's a little bit more detailed and more complete explanation of what is going on.  First the case of a water balloon.



The figure at left shows air pressure (red arrows) pushing on all the sides of the balloon.  Because pressure decreases with increasing altitude, the pressure from the air at the top of the balloon pushing downward (strength=14) is a little weaker than the pressure from the air at the bottom of the balloon that is pushing upward (strength=15).  The two sideways forces cancel each other out.  The total effect of the pressure is a weak upward pressure difference force (1 unit of upward force shown at the top of the right figure). 

Gravity exerts a downward force on the water balloon.  In the figure at right you can see that the gravity force (strength=10) is stronger than the upward pressure difference force (strength=1).  The balloon falls as a result.  This is what you know would happen if you let go of a water balloon, it would fall.

In the demonstration a wine glass is filled with water.  A small plastic lid is used to cover the wine glass.  The wine glass is then turned upside and the water does not fall out.



All the same forces are shown again in the left most figure.  In the right two figures we separate this into two parts.  First the water inside the glass isn't feeling the downward and sideways pressure forces (because they're pushing on the glass, they're included on the right figure ).  Gravity still pulls downward on the water but the upward pressure force is able to overcome the downward pull of gravity.  It can do this because all 15 units are used to overcome gravity and not to cancel out the downward pointing pressure force.  The net upward force is strong enough to keep the water in the glass.

The demonstration was repeated using a 4 Liter flash (more than a gallon of water, more than 8 pounds of water).  The upward pressure force was still able to keep the water in the flask (much of the weight of the water is pushing against the sides of the flask which the instructor was supporting with his arms).

We spent the last portion of the period looking at how temperature changes with increasing altitude in the atmosphere.  Temperature can increase, decrease, even remain constant with increasing altitude.  The figures below are more clearly drawn versions of what was done in class.


The atmosphere can be split into layers depending on whether temperature is increasing or decreasing with increasing altitude.  The two lowest layers are shown in the figure above.  There are additional layers (the mesosphere and the thermosphere) above 50 km but we won't worry about them. 

1. We live in the troposphere.  The troposphere is found, on average, between 0 and about 10 km altitude, and is where temperature usually decreases with increasing altitude.  [the troposphere is usually a little higher in the tropics and lower at polar latitudes]

The troposphere contains most of the water vapor in the atmosphere (the water vapor comes from evaporation of ocean water and then gets mixed throughout the troposphere by up and down air motions) and is where most of the clouds and weather occurs.  The troposphere can be stable or unstable (tropo means to turn over and refers to the fact that air can move up and down in the troposphere).

2a. The thunderstorm shown in the figure with its strong updrafts and downdrafts indicates unstable conditions.  When the thunderstorm reaches the top of the troposphere, it runs into the bottom edge of the stratosphere which is a very stable layer.  The air can't continue to rise into the stratosphere so the cloud flattens out and forms an anvil (anvil is the name given to the flat top of the thunderstorm).   The flat anvil top is something that you can go outside and see and often marks the top of the troposphere.

2b.  The summit of Mt. Everest is a little over 29,000 ft. tall and is close to the average height of the top of the troposphere.

2c.   Cruising altitude in a passenger jet is usually between 30,000 and 40,000, near or just above the top of the troposphere, and at the bottom of the stratosphere.

3.  Temperature remains constant between 10 and 20 km and then increases with increasing altitude between 20 and 50 km.  These two sections form the stratosphere.  The stratosphere is a very stable air layer.  Increasing temperature with increasing altitude is called an inversion.  This is what makes the stratosphere so stable.

4.   A kilometer is one thousand meters.  Since 1 meter is about 3 feet, 10 km is about 30,000 feet.  There are 5280 feet in a mile so this is about 6 miles (about is usually close enough in this class). 

5.    The ozone layer is found in the stratosphere.  Peak ozone concentrations occur near 25 km altitude.

Here's the same picture drawn again (for clarity) with some different information.  We need to explain why when temperature decreases all the way up to the top of the troposphere, it can start increasing again in the stratosphere.


6.   Sunlight is a mixture of ultraviolet (7%), visible (44%, colored green in the picture above) and infrared light (49%, colored red).  We can see the visible light.

6a. On average about 50% of the sunlight arriving at the top of the atmosphere passes through the atmosphere and is absorbed at the ground (20% is absorbed by gases in the air, 30% is reflected back into space).  This warms the ground.  The air in contact with the ground is warmer than air just above.  As you get further and further from the warm ground, the air is colder and colder.  This explains why air temperature decreases with increasing altitude in the troposphere.

5b. How do you explain increasing temperature with increasing altitude in the stratosphere? 

     Absorption of ultraviolet light by ozone warms the air in the stratosphere and explains why the air can warm (oxygen also absorbs UV light).  The air in the stratosphere is much less dense (thinner) than in the troposphere.  So even though there is not very much UV light in sunlight, it doesn't take as much energy to warm this thin air as it would to warm denser air closer to the ground.

7. I don't think I mentioned this last point in class.  That's a manned balloon; Auguste Piccard and Paul Kipfer are inside.  They were the first men to travel into the stratosphere (see pps 31 & 32 in the photocopied Class Notes)  We might have a look at a short segment of video at some point that describes their voyage.  It really was quite a daring trip at the time at the time, and they very nearly didn't survive it.