Tue., Nov. 19, 2013

A potpourri of musical selections before class today: Eilen Jewell "Everywhere I Go", The Be Good Tanyas "When Doves Cry" (which I wasn't able to find online), Eva Cassidy "Cheek to Cheek", and Melody Gardot "Who Will Comfort Me".

The grade summaries that I hoped to have available today will be handed out on Thursday.  I seriously underestimated the amount of grading that needed to be done over the weekend.

The revised Expt. #2 reports, and the Expt. #4, Scientific Paper, and Book reports have all been graded and were returned today.  Revisions of the Experiment #3 reports are due next Tuesday.  Everything else is due Dec. 3, the Tuesday after Thanksgiving.  Though if you could get them in before Thanksgiving that would be a big help.

Most of the 1S1P Assignment #2 reports have been graded.  The exception is the Koppen Climate Classification topic.  Assignment #3 reports are due this week.  And that may then well be it for 1S1P reports.  Perhaps a short Bonus Assignment but that's not a certainty.

Probably also the final Optional Assignment of the semester is now online.  It's due next Tuesday.  You can earn extra credit (up to 0.5 pts) and also a green card if you do a good job on the assignment.  The Controls of Temperature assignment turned in quite some time ago has been graded and was returned.  You'll find answers online.

The Quiz #4 Study Guide pt. 1 has made its appearance online.  Quiz #4 isn't until Thu., Dec. 5

Among the many emails sent to report observations from the Toilet Flushing Experiment was the following:
"I was able to analyze the direction of the water in the toilet bowl at the Eller building on campus and found that the water moves in a counterclockwise direction. 

My family and I went Kenya, Africa a few years ago and were able stand on both sides of the hemisphere at one time. In other words we were at the equator. The tour guides did a demonstration using water in a bucket. On one side of the equator the water was going clockwise and on the other side the water was going counter clockwise. Additionally, on the equator it self the water was still."

I showed a short segment from a television show that appeared on PBS several years ago.  It was called "Pole to Pole" and featured a demonstration on the equator like the one described above.  I showed a video of the demonstration in class today.  I don't normally show the video because it erroneously leads people to believe water does spin in opposite directions in the northern and southern hemispheres.

I want to be sure that everyone understands

The Coriolis force does NOT play a role in determining what direction water draining from a sink or toilet spins.  Either direction should be equally likely in both the northern and southern hemispheres.

And the Toilet Flushing experiment bore that out this semester.  There almost equal numbers of reports of clockwise spin (35) as there were counterclockwise spin (29).

The events leading up to the initiation of a summer air mass thunderstorm are summarized in the figure below.  This is something we looked at in more detail last Thursday.  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 surroundings.  Once the parcel is lifted above the level of free convection it becomes buoyant; this is the moment at which the air mass thunderstorm begins. 





Once a thunderstorm develops it then goes through a 3-stage life cycle


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 the name of the stage).

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, hail, 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.

Here's a sketch of 4 thunderstorm clouds and a question: what information could you add to each picture.



You should be able to say something about the first three.  The 4th cloud might be a bit of a puzzle.  You'll find the answer to the question at the end of today's notes.

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 a summer or two ago.  Here's an example from Gilbert Arizona.  Another from South Mountain (same storm seen from a different location).  You can see day literally turn to night when the dust cloud is overhead.



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 (caution the audio contains some of the actual cockpit communications).
 
Falling rain could warn of a wet microburst (see photo below).   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 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 into the video.  I showed portions of a 3rd video in class.  It was taken in or near San Tan AZ.  The microburst doesn't look too impressive at the start of the footage but the storm winds soon get pretty violent and cause some damage.

I had planned on showing another of my "homemade videos", a microburst demonstration, but wasn't able to find the tape.




In the video 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.  This is sketched below.


Note the characteristic curling motion at the outer edge of the gust front.






A sketch and a photograph of a shelf cloud.  Warm moist air if lifted by the cold air behind the gust front which is moving from left to right.  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 very far before it had cooled enough to become saturated and form a cloud.  Here are a couple of pretty good videos of shelf clouds (Grand Haven, MI and Massillon, OH)


Next we need to look at some of the conditions that can lead to severe thunderstorm formation and some of the characteristics of these storms.  Severe thunderstorms last longer, grow bigger, and become stronger than ordinary air mass thunderstorms.  They can also produce tornadoes.



Severe storms are more likely to form when there is vertical wind shear (the picture above is on p. 154a in the ClassNotes).  Wind shear (Point 1) is changing wind direction and/or wind speed with distance.  In the case shown above, the wind speed is increasing with increasing altitude, this is vertical wind shear.

A thunderstorm that forms in this kind of an environment will move at an average of the speeds at the top and bottom of the cloud (pt. 2).  The thunderstorm will move to the right more rapidly than the air at the ground which is where the updraft begins.  Rising air that is situated at the front bottom edge of the thunderstorm will find itself at the back edge of the storm when it reaches the top of the cloud. 

This produces a tilted updraft (pt. 3).  The downdraft is situated at the back of the ground.  The updraft is continually moving to the right and staying away from the downdraft.  The updraft and downdraft coexist and do not "get in each others way."  If you remember in air mass thunderstorms, the downdraft gets in the way of the updraft and leads to dissipation of the storm.

Sometimes the tilted updraft will begin to rotate.  A rotating updraft is called a mesocyclone (pt. 4).  Meso refers to medium size (thunderstorm size) and cyclone means winds spinning around low pressure (tornadoes are sometimes called cyclones).  Low pressure in the core of the mesocyclone creates an inward pointing pressure gradient force needed to keep the updraft winds spinning in circular path.

The cloud that extends below the cloud base and surrounds the mesocyclone is called a wall cloud (pt. 5).  The largest and strongest tornadoes will generally come from the wall cloud.  We'll see some pretty dramatic videos of wall clouds on Thursday.

Note (pt. 6) that a tilted updraft also provides a way of keeping growing hailstones inside the cloud.  Hailstones get carried up toward the top of the cloud where they begin to fall.  But they then fall back into the strong core of the updraft and get carried back up toward the top of the cloud.




A wall cloud can form a little bit below the rest of the base of the thunderstorm.   Clouds form when air rises, expands, and cools as shown above at left.  The rising air expands because it is moving into lower pressure surroundings at higher altitude.  Only when the air has risen high enough, moved into low enough pressure, expanded and cooled enough will a cloud form.

At right the air doesn't have to rise to as high an altitude to experience the same amount of expansion and cooling.  This is because it is moving into the core of the rotating updraft where the pressure is a little lower than normal for this altitude.  Cloud formation is a little bit closer to the ground.




Here's a picture of a portion of the bottom of a thunderstorm with a wall cloud and, what appears to be, a relatively weak tornado (narrow diameter and almost transparent). Photo from the University Corporation for Atmospheric Research

Now on to tornadoes.
The United States has roughly 1000 tornadoes in an average year.  That is more than any other country in the world .

A year's worth of tornado activity plotted on a world map.  Note the name at bottom left: T.T. Fujita, "Mr. Tornado."  The scale used to rate tornado strength and intensity is named after him. 

Part of the reason why the central US has some many tornadoes is just a consequence of geography. 


Without any mountains in the way, cold dry air can move in the spring all the way from Canada to the Gulf Coast.  There it collides with warm moist air from the Gulf of Mexico to form strong cold fronts and thunderstorms.  There are some other meteorological conditions that come into play that make these storms capable of producing tornadoes.


This map (found on p. 161 in the ClassNotes) shows the average frequency of tornado occurrence in the US.  Tornadoes have been observed in every state (including Alaska), but they are most frequent in the Central Plains, a region referred to as "Tornado Alley" (highlighted in red, orange, and yellow above). 



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

1.  About 2/3rds (maybe 3/4) of tornadoes are F0 or F1 tornadoes (this is referring to the Fujita Scale, which we'll learn more about on Thursday) 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.  The same pressure drop can be found in strong hurricanes but it takes place over a much larger distance.  The PGF isn't as strong and the CF does play a role.

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 where winds often blow from the SW.   Most tornadoes have diameters of 10s to a few 100s of yards but tornadoes with diameters over a mile have been observed.  Tornado diameter can also be much larger near the base of the thunderstorm than it is near the ground.

9, 10.  Tornadoes are most frequent in the Spring.  The strongest tornadoes also occur at that time of year.  You don't need to remember the specific months.  Tornadoes are most common in the late afternoon when the atmosphere is most unstable.


A little more infomation from p. 161 that wasn't mentioned in class.  At the present time about 75 people are killed every year in the United States by tornadoes.  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 than tornadoes.  Hurricanes kill fewer people on average than tornadoes.  The increase in the number of tornadoes observed per year is probably more due to there being more people in locations that are able to observe and report a tornado rather than a true increase in tornado activity. 

I hurried through the following figure at the end of class.



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 still today the deadliest single tornado ever in the United States (you'll find a compilation of tornado records here).  The Joplin Missouri tornado (May 22, 2011) 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 and all of the tornado paths are oriented from SW to NE.

The April 25-28, 2011 outbreak is now apparently the largest tornado outbreak in US history (358 tornadoes, 346 people killed)


Tornado activity and tornado outbreaks in November, like we had last weekend, is unusual.  The outbreak this past Sunday is one of the largest November outbreaks ever.

Here's the answer to a question embedded in today's notes.

The first 3 pictures shows the different stages in the lifetime of an air mass thunderstorm.  There's a tilted updraft in the 4th picture which is a characteristic of a severe thunderstorm.