Monday Nov. 16, 2009
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A couple of songs from Madeleine Peyroux ("Instead" and "Love and Treachery") and a part of a song from Tracy Chapman ("Give me One Reason") before class today.

The Experiment #3 reports were returned last Friday.  Revisions are due before Thanksgiving (class on Wednesday afternoon has been cancelled but you can bring the reports by my office).  Please return your original report with your revised report. 

The 1S1P Assignment #2 reports are due on Wednesday this week.  A list of people that have reached or are very close to 45 pts (the maximum number of points you can earn on 1S1P reports) in now online.

Here are the results of the great Toilet Flushing Experiment:
57 people reported clockwise spinning toilets
43 people observed counterclockwise spinning
There are a few people that still think toilets should all spin in one direction in the northern hemisphere and in another direction in the southern hemisphere. That isn't correct.  Toilets can spin in either direction in either hemisphere.  The direction of spin is determined by the design of the toilet itself.


We reviewed some material about ocean currents and the SW monsoon that was tacked onto the end of the Friday Nov. 13 online notes.

Between now and the end of the semester we will be covering Thunderstorms, Tornadoes, Lightning, and Hurricanes.  We just got started on thunderstorms today.  Here's a brief introduction.




Thunderstorms come in different sizes and levels of severity.  We will mostly be concerned with ordinary single-cell thunderstorms (also referred to as air mass thunderstorms).  Most summer thunderstorms in Tucson are this type.  At the other end of the spectrum are supercell thunderstorms.  We'll watch a short video at some point that shows a computer simulation of the complex air motions inside a supercell thunderstorm.

The following somewhat tedious material was intended to prepare you to better appreciate a time lapse video movie of a thunderstorm developing over the Catalina mountains.  I don't expect you to remember all of the details given below.  The figures below are more carefully drawn versions of what was done in class.


Refer back and forth between the lettered points in the figure above and the commentary below.

The numbers in Column A show the temperature of the air in the atmosphere at various altitudes above the ground (note the altitude scale on the right edge of the figure).  On this particular day the air temperature was decreasing at a rate of 8 C per kilometer.  This rate of decrease is referred to as the environmental lapse rate (lapse rate just means rate of decrease with altitude).  Temperature could decrease more quickly than shown here or less rapidly.  Temperature in the atmosphere can even increase with increasing altitude (a temperature inversion).

At Point B, some of the surface air is put into an imaginary container, a parcel.  Then a meterological process of some kind lifts the air to 1 km altitude (in Arizona in the summer, sunlight heats the ground and air in contact with the ground, the warm air becomes bouyant - that's called free convection).  The rising air will expand and cool as it is rising.  Unsaturated (RH is less than 100%) air cools at a rate of 10 C per kilometer.  So the 15 C surface air will have a temperature of 5 C once it arrives at 1 km altitude. 

"Mother Nature" lifts the parcel to 1 km and "then lets go."  At Point C note that the air inside the parcel is slightly colder than the air outside (5 C inside versus 7 C outside).  The air inside the parcel will be denser than the air outside and the parcel will sink back to the ground. 

By 10:30 am the parcel is being lifted to 2 km as shown at Point D.  It is still cooling 10 C for every kilometer of altitude gain.  At 2 km, at Point E  the air has cooled to its dew point temperature and a cloud has formed.  Notice at Point F, the air in the parcel or in the cloud (-5 C) is still colder and denser than the surrounding air (-1 C), so the air will sink back to the ground and the cloud will disappear.  Still no thunderstorm at this point.


At noon, the air is lifted to 3 km.  Because the air became saturated at 2 km, it will cool at a different rate between  2 and 3 km altitude.  It cools at a rate of 6 C/km instead of 10 C/km.  The saturated air cools more slowly because release of latent heat during condensation offsets some of the cooling due to expansion.  The air that arrives at 3km, Point H, is again still colder than the surrounding air and will sink back down to the surface.

By 1:30 pm the air is getting high enough that it becomes neutrally bouyant, it has the same temperature and density as the air around it (-17 C inside and -17 C outside).  This is called the level of free convection, Point J in the figure.

If you can, somehow or another,  lift air above the level of free convection it will find itself warmer and less dense than the surrounding air as shown at Point K and will float upward to the top of the troposphere on its own.  This is really the beginning of a thunderstorm.  The thunderstorm will grow upward until it reaches very stable air at the bottom of the stratosphere.

The top portion of this figure summarizes what we just covered: it takes some effort and often a good part of the day before a thunderstorm forms.  The air must be lifted to just above the level of free convection.  Once air is lifted above the level of free convection it finds itself warmer and less dense that the air around it and floats upward on its own.  The is the moment at which the air mass thunderstorm begins. 

The thunderstorm then goes through 3 stages.


In the first stage you would only find updrafts inside the cloud.



Once precipitation has formed and grown to a certain size, it will begin to fall and drag air downward with it.  This is the beginning of the mature stage where you find both an updraft and a downdraft inside the cloud.  The falling precipitation will also pull in dry air from outside the thunderstorm (this is called entrainment).  Precipitation will mix with this drier air and evaporate.  The evaporation will strengthen the downdraft (the evaporation cools the air and makes it more dense).  The thunderstorm is strongest in the mature stage.  This is when the heaviest rain, strongest winds, and most of the lightning occur.

Eventually the downdraft spreads horizontally throughout the inside of the cloud and interferes with or cuts off the updraft.  This marks the beginning of the end for this thunderstorm.


In the dissipating stage you would only find weak downodrafts throughout the interior of the cloud.

Note how the winds from one thunderstorm can cause a region of convergence on one side of the original storm and can lead to the development of new storms.  Preexisting winds refers to winds that were blowing before the thunderstorm formed.