Friday Nov. 18, 2011
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The El Tour de Tucson bicycle race/ride is Saturday so some bicycle racing instead of music before class this morning.  We saw the last few kilometers up the infamous Mont Ventoux from the 2000 Tour de France.  Marco Pantani just edged out Lance Armstrong at the finish line.  Here are the last couple of minutes of the race. 

Quiz #4 Stude Guide Pt. 2 is now online.  It is relatively short and lists the times and locations of the reviews.  Quiz #4 is the Wednesday after Thanksgiving (Nov. 30).
Here's another picture of the day.  Peppers from my vegetable garden, just in time for Thanksgiving.





The events leading up to the initiation of a summer air mass thunderstorm (something we looked at in detail in class on Wednesday) is summarized in the figure below.  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 (the dotted line at middle left in the picture).  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.  I've tried to show this with colors below.  Cool colors below the level of free convection because the air in the lifted parcel is colder and denser than its surroundings.  Warm colors above the dotted line indicate parcel air that is warmer and less dense than the surroudings.  Once the parcel is lifted above the level of free convection it becomes bouyant; this is the moment at which the air mass thunderstorm begins. 



Once a thunderstorm develops it then goes through 3 stages.


In the first stage you would only find updrafts inside the cloud (that's all you need to know about this stage, you don't even need to remember its name).

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 begins to interfere with the updraft.  This marks the beginning of the end for this thunderstorm. 


The downdraft eventually fills the interior of the cloud.  In this dissipating stage you would only find weak downdrafts throughout 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.  Convergence between the prexisting and the thunderstorm downdraft winds creates rising air that can initiate a new thunderstorm.

The picture below shows some of the features at the base of a thunderstorm.



 The cold downdraft air spilling out of a thunderstorm hits the ground and begins to move outward from underneather the thunderstorm.  The leading edge of this outward moving air is called a gust front.  You can think of it as a dust front because the gust front winds often stir up a lot of dust here in the desert southwest (see below).


The gust front in this picture (taken near Winslow, Az) is moving from the right to the left.  Visibility in the dust cloud can drop to near zero which makes this a serious hazard to automobile traffic.  Dust storms like this are sometimes called "haboobs".

There's lots of video on YouTube of an impressive dust storm this past summer.  Here's an example from Gilbert Arizona.  Another from South Mountain.

Just to hammer home the idea of what a gust front might look like I showed another (the last) of my homemade videos.



A large plastic tank was filled with water, the water represents air in the atmosphere.  Then a colored mixture of water and glycerin, which is a little denser than water, is poured into the tank.  This represents the cold dense air in a thunderstorm downdraft.  The colored liquid sinks to the bottom of the tank and then spreads out horizontally.  In the atmosphere the cold downdraft air hits the ground and spreads out horizontally.  These are the strong winds that can reach 100 MPH.

A narrow intense downdraft is called a microburst.  At the ground microburst winds will sometimes reach 100 MPH (over a limited area); most tornadoes have winds of 100 MPH or less.  Microburst winds can damage homes (especially mobile homes that aren't tied to the ground), uproot trees, and seem to blow over a line of electric power poles at some point every summer in Tucson. 

Microbursts are a serious threat to aircraft especially when they are close to the ground during landing or takeoff.  An inattentive pilot encountering headwinds at Point 1 might cut back on the power.  Very quickly the plane would lose the headwinds (Point 2) and then encounter tailwinds (Point 3).  The plane might lose altitude so quickly that it would crash into the ground before corrective action could be taken.  Microburst associated wind shear was largely responsible for the crash of Delta Airlines Flight 191 while landing at the Dallas Fort Worth airport on Aug. 2, 1985 (click here to watch a simulation of the final approach into the airport, I don't show it in class because it contains some of the actual cockpit communications).
 
Falling rain could warn of a (wet) microburst.  In other cases, dangerous dry microburst winds might be invisible (the virga, evaporating rain, will cool the air, make the air more dense, and strengthen the downdraft winds).


Here's a picture of a wet microburst, a narrow intense thunderstorm downdraft and rain. 

Here are three microburst videos from YouTube.  The first video shows a microburst from some distance away.  The second video was taken in the heavy rain and strong winds under a thunderstorm in the microburst.  You'll see a power pole snapped in half by the microburst winds at about 2:26 in the video.   Here's a third video of a microburst that hit Princeton KS in July 2009.  Someone watching the storm estimated the winds were at least 90 MPH.
The following picture shows a shelf cloud.


Warm moist air if lifted by the cold air behind the gust front which is moving from left to right in this picture.  The shelf cloud is very close to the ground, so the warm air must have been very moist because it didn't have to rise and cool much before it became saturated and a cloud formed.  Here are a couple of pretty good videos (Grand Haven, MI and Massillon, OH)



The United States has more tornadoes in an average year than any other country in the world (over 1000 per year).  The central US has just the right mix of meteorological conditions.  Note the author T.T. Fujita "Mr. Tornado."



I got a little carried away with the colored pencils on this picture.   Without any mountains in the way, cold dry air can move in the spring all the way from Canada to the Gulf Coast.  There is collides with warm moist air from the Gulf of Mexico to form strong cold fronts and thunderstorms.


Tornadoes have been observed in every state in the US, but tornadoes are most frequent in the central plains, a region referred to as "Tornado Alley" (highlighted in red, orange, and yellow above).  You'll find this map on p. 161 in the photocopied ClassNotes.





Here are some basic tornado characteristics (the figure above is also on p. 161)

1.  About 2/3rds of tornadoes are F0 or F1 tornadoes (we'll learn moare about the Fujita scale used to rate tornado intensity later today and next week) and have spinning winds of about 100 MPH or less.  Microburst winds can also reach 100 MPH.  Microbursts are much more common in Tucson in the summer than tornadoes and can inflict the same level of damage. 

2.  A very strong inwardly directed pressure gradient force is needed to keep winds spinning in a circular path.  The pressure in the center core of a tornado can be 100 mb less than the pressure in the air outside the tornado.  This is a very large pressure difference in such a short distance.  The PGF is much stronger than the Coriolis Force (CF) and the CF can be neglected.

3.  Because the Coriolis force doesn't play a role, tornadoes can spin clockwise or counterclockwise, though counterclockwise rotation is more common.  This might be because larger scale motions in the cloud (where the CF is important, might determine the direction of spin in a tornado).

4, 5, 6.  Tornadoes usually last only a few minutes, leave a path on the ground that is a few miles long, and move at a few 10s of MPH.  There are exceptions, we'll look at one shortly.

7, 8.  Most tornadoes move from the SW toward the NE.  This is because tornado-producing thunderstorms are often found just ahead of a cold front.  Winds ahead of a cold front often blow from the SW.   Most tornadoes have diameters of tens to a few hundred yards but tornadoes with diameters over a mile have been observed.

9, 10.  Tornadoes are most frequent in the Spring.  The strongest tornadoes also occur at that time of year.  Tornadoes are most common in the late afternoon when the atmosphere is most unstable.

We didn't have time to cover the remaining material in class on Friday.  We'll start here next Monday.  I'll include it here just to finish with the material on p. 161.


At the present time about 75 people are killed every year in the United States.  This is about a factor of ten less than a century ago due to improved methods of detecting tornadoes and severe thunderstorms.  Modern day communications also make easier to warm people of dangerous weather situations.  Lightning and flash floods (floods are the most serious severe weather hazard) kill slightly more people.  Hurricanes kill fewer people on average than tornadoes.





This figure traces out the path of the 1925 "Tri-State Tornado" .  The tornado path (note the SW to NE orientation) was 219 miles long, the tornado lasted about 3.5 hours and killed 695 people.  The tornado was traveling over 60 MPH over much of its path. It is the deadliest single tornado ever in the United States.  The Joplin Missouri tornado this past spring (May 22) killed 162 people making it the deadliest since 1947 and the 7th deadliest tornado in US history.



Tornadoes often occur in "outbreaks."  The paths of 148 tornadoes during the April 3-4, 1974 "Jumbo Tornado Outbreak" are shown above.  Note the first tornadoes were located in the upper left corner of the map.  The tornadoes were produced by thunderstorms forming along a cold front (see the weather map below).  During this two day period the front moved from the NW part toward the SE part of the figure.  Note that all the tornado paths have a SE toward NE orientation.

The April 25-28, 2011 outbreak this year is now apparently the largest tornado outbreak in US history (336 tornadoe, 346 people killed)