Wednesday Nov. 17, 2010
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A couple of songs from The Ragbirds ("Space" and "Roar, Claw, and Bite").  This is a group that a NATS 101 student said I might like and she was right.

The 1S1P "Record Setting Storm" reports have been graded and were returned in class today.

The Quiz #4 Study Guide pt. 1 is now available.

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 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.  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 can form 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. 

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 forms 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 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 (we'll learn about the Fujita scale used to rate tornado intensity 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 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.