Tuesday Nov. 23, 2010
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Three songs from Iron and Wine before class today ("Belated Promise Ring", "Boy with a Coin", and "Jezebel").

The Scientific Paper reports have been graded and were returned today.  Revised reports (if you decide to do one) will be due on or before Tue., Dec. 7 (the last day of classes).  Please return your original report with your revised report.

Keep an eye on the list of people that have earned 45 1S1P pts.  The list will be updated as my TA and I finish grading groups of reports over the break.

The entire Quiz #4 Study Guide (pt. 1 and pt. 2) is now available online.  The last quiz of the semester is Thursday Dec. 2.

We started today by looking at some of the conditions that can lead to severe thunderstorm formation.  Severe last longer, grow bigger, and become stronger than ordinary air mass thunderstorms.


Severe storms are more likely to form when there is vertical wind shear.  Wind shear (pt 1) is changing wind direction and/or wind speed with distance.  In this case, 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 and eventually chokes off the updraft which 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.  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 (low pressure also keeps winds spinning in a tornado). 

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.

Note (pt. 6) that a tilted updraft 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 forms a little bit below the rest of the base of the thunderstorm.  The figure above tries to explain why that is true.  Clouds normally 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.  In the figure we are assume that moving into 900 mb surroundings causes enough expansion and cooling for a cloud to 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.  The 900 mb pressure level and cloud formation are found a little bit closer to the ground. 



Here's a pretty nice photograph of a wall cloud and what is probably a relatively weak tornado (from the University Corporation for Atmospheric Research)

Thunderstorms with rotating updrafts often have a distinctive radar signature called a hook echo.
We haven't discussed weather radar in this class.  In some ways a radar image of a thunderstorm is like an X-ray photograph of a human body.  The Xrays pass through the flesh but are partially absorbed by bone.



Xrays pass through flesh and reveal the skeletonal structure inside a body.



The radio signals emitted by radar pass through the cloud itself but are reflected by the much larger precipitation particles.  The intensity of the reflected signal (the echo) is color coded.  Red means an intense reflected signal and lots of large precipitation particles.  The edge of the cloud isn't normally seen on the radar signal.

Here are some actual radar images with prominent hook echoes.





This is the radar image of a thunderstorm that produced a very strong tornado that hit Oklahoma City in May 1999 ( http://www.spc.noaa.gov/faq/tornado/radscel.htm ).  The hook echo is visible near the lower left hand corner of the picture.  Winds in the tornado may have exceeded 300 MPH.

This seemed like a good place to briefly discuss supercell thunderstorms.


Here is a relatively simple drawing showing some of the key features on a supercell thunderstorm.  In a supercell the rotating updraft (shown in red above) is strong enough to penetrate into the stratosphere.  This produces the overshooting top or dome feature above.  A wall cloud and a tornado are shown at the bottom of the mesocyclone.  In an ordinary thunderstorm the updraft is unable to penetrate into the very stable air in the stratosphere and the upward moving air just flattens out and forms an anvil.  The flanking line is a line of new cells trying to form alongside the supercell thunderstorm.


 Here is a second slightly more complicated drawing of a supercell thunderstorm.  A typical air mass thunderstorm (purple) has been drawn in for comparison.

A short segment of video was shown at this point.  The video first showed some good quality video of a close tornado.    This was followed by photographs of a distant supercell thunderstorm and photographs of the bases of nearby supercell thunderstorms.  Here you could see the spectacular wall cloud that often forms at the base of these storms.  Finally a computer simluation showed some of the complex motions that form inside supercell thunderstorms, particularly the tilted rotating updraft.

Next it was time for some introductory information on tornadoes.



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.


In the spring, cold dry air can move all the way from Canada and collide 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 numbering in the figure above may differ slightly from what we did in class)

1.  About 2/3rds of tornadoes are F0 or F1 tornadoes (see below) 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. 

Here's an easy to remember version of the Fujita Scale used to rate tornado strength and damage potential.  It is very hard to actually measure the speed of the rotating winds in a tornado.  Researchers usually survey the damage caused by the tornado to come up with a Fujita Scale rating.  The Fujita Scale has recently been revised.  The newer scale is called the Enhanced Fujita Scale to differentiate it from the original scale introduced in 1971.

2.  A very strong inwardly directed pressure gradient force is needed to keep winds spinning in a circular path.  The PGF is much stronger than the Coriolis Force (CF) and the CF can be neglected.  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.

3.  Tornadoes can spin clockwise or counterclockwise, though counterclockwise rotation is more common. 

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.  We will look at an exception on Friday.

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.

Most tornadoes last only a few minutes and leave a path a few miles long on the ground.  One of the exceptions is shown below.



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.



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.  During this two day period the front moved from the NW part toward the SE part of the figure (see sketch below not shown in class).  Note that all the tornado paths have a SE toward NE orientation.





These two tables above are on p. 161 in the photocopied ClassNotes.  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 was about where another short video with images of several different tornadoes was shown.  Descriptions of the tornadoes are given in the table below (the numbers in the left most column were used on the video to identify each tornado)
54a
 F3
 Grand Island, NE
 Mar. 13, 1990
 tornado cloud is pretty thick and vertical
61f
 F3
 McConnell AFB KS
 Apr. 26, 1991
 this is about as close to a tornado as you're ever likely to get.  Try to judge the diameter of the tornado cloud.  What direction are the tornado winds spinning?
52
 F5
 Hesston KS
 Mar. 13, 1990
 Watch closely, you may see a tree or two uprooted by the tornado winds
51
 F3
 North Platte NE
 Jun. 25, 1989
 Trees uprooted and buildings lifted by the tornado winds
65
 F1
 Brainard MN
 Jul. 5, 1991
 It's a good thing this was only an F1 tornado
57
 F2
 Darlington IN
 Jun. 1, 1990
 Tornado cloud without much dust
62b
 F2
 Kansas Turnpike
 Apr. 26, 1991
 It's sometimes hard to run away from a tornado.  Watch closely you'll see a van blown off the road and rolled by the tornado.  The driver of the van was killed!
47
 F2
 Minneapolis MN
 Jul. 18, 1986
 Tornado cloud appears and disappears.




Tornadoes begin in and descend from a thunderstorm.  You would usually see a funnel cloud dropping from the base of the thunderstorm.  Spinning winds will probably be present between the cloud and ground before the tornado cloud becomes visible.  The spinning winds can stir up dust at ground level.  The spinning winds might also be strong enough at this point to produce some minor damage. 

In Stage 2, moist air moves horizontally toward the low pressure in the core of the tornado.  This sideways moving air will expand and cool just as rising air does (see figure below).  Once the air cools enough (to the dew point temperature) a cloud will form. 

Tornadoes can go from Stage 2 to Stage 3 (this is what the strongest tornadoes do) or directly from stage 2 to stage 5.  Note a strong tornado is usually vertical and thick as shown in Stage 3.  "Wedge tornadoes" actually appear wider than they are tall.

The thunderstorm and the top of the tornado will move faster than the surface winds and the bottom of the tornado.  This will tilt and stretch the tornado.  The rope like appearance in Stage 5 is usually a sign of a weakening (though still a dangerous) tornado.


The tornado cloud forms when moist air moves into lower pressure in the core of the tornado.  The air expands and cools to the dew point and a cloud forms.  This is just like the cloud that forms when air rises (and moves into lower pressure and expands). 


Time for one more tornado video in class today.  This showed a tornado in Laverne Oklahoma and illustrated well the first three stages in the tornado life cycle (jespecially the first "dust swirl stage").

We'll finish up the section on tornadoes at the start of class next Tuesday and spend the remainder of the period on lightning.