This seemed like a good place to briefly discuss supercell thunderstorms (see p. 163 in the ClassNotes)

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 a short ways 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 (similar to convergence between prexisting winds and thunderstorm downdraft winds that can lead to new storm development alongside a dissipating air mass thunderstorm).

Here is a second slightly more complicated and realistic drawing of a supercell thunderstorm.  A typical air mass thunderstorm (purple) has been drawn in so that you can appreciated how much larger supercell thunderstorms can be.

A short segment of video was shown at this point.  It showed 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.  I haven't been able to find the video online.

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Thunderstorms with rotating updrafts and supercell thunderstorms often have a distinctive radar signature called a hook echo.

We haven't discussed weather radar in this class yet.  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.

The radio signals emitted by radar pass through the cloud itself but are reflected by the much larger precipitation particles. The radar keeps track of how long it takes for the emitted signal to travel out to the cloud, be reflected, and return to the radar antena.  The radar can use this to determine the distance to the storm.  It also knows the direction to the storm and can locate the storm on a map.  The intensity of the reflected signal (the echo) is often 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 is an actual radar image with a prominent hook echo.  The hook is evidence of large scale rotation inside a thunderstorm and means the thunderstorm is capable of, and may already be, producing tornadoes.

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.  You can read more about this tornado here.  And here is some storm chase video of the tornado.

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The figure below (p. 162 in the ClassNotes) illustrates the life cycle of a tornado.  Have a close look at the next tornado you see on video and see if you can determine whether it is in one of the early or late stages of its development.



A tornado cloud forms is mostly the same way that ordinary clouds do.  In an ordinary cloud (left figure above) rising air moves into lower pressure surroundings and expands.  Expansion cools the air.  If the air expands and cools enough (to the dew point) a cloud forms.  In a tornado air moves horizontally into lower pressure at the core of the tornado.  The air expands and cools just like rising air does.  If the air cools enough a true cloud apears.

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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 and assign a Fujita Scale rating.  The original scale, introduced in 1971 by Tetsuya (Ted) Fujita.  A simplified, easy to remember version is shown below.  A very basic and grossly oversimplified damage scale is included.  This is simple enough that I can remember it and can use it to estimate tornado intensity when I see damage on the television news.

The original scale has been revised because the estimated wind speeds were probably too high.  The newer scale is called the Enhanced Fujita Scale and became operational in 2007.  Here's a simplfied version of the EF scale

Now EF2, EF3 and EF4 levels have winds between 100 and 200 MPH and only EF5 tornadoes have winds over 200 MPH.  More accurate versions of both scales are compared below


 
The original Fujita Scale actually goes up to F12.  An F12 tornado would have winds of about 740 MPH, the speed of sound.  Roughly 3/4 of all tornadoes are EF0 or EF1 tornadoes and have winds that are less than 100 MPH.  EF4 and EF5 tornadoes are rare but cause the majority of tornado deaths. 

The EF scale considers 28 different "damage indicators," that is, types of structures or vegetation that could be damaged by a tornado.  Examples include:

Damage Indicator	Description
2	1 or 2 family residential home
3	Mobile home (single wide)
10	Strip mall
13	Automobile showroom
22	Service station canopy
26	Free standing light pole
27	Tree (softwood)

Then for each indicator is a standardized list of "degrees of damage" that an investigator can look at to estimate the intensity of the tornado.  For a 1 or 2 family home for example

degree of damage	description	approximate wind speed (MPH)
1	visible damage	65
2	loss of roof covering material	80
3	broken glass in doors & windows	95
4	lifting of roof deck, loss of more than 20% of roof material, collapse of chimney, garage doors collapse inward, destruction of porch roof or carport	100
5	house slides off foundation	120
6	large sections of roof removed, most walls still standing	120
7	exterior walls collapse (top story)	130
8	most interior walls collapse (top story)	150
9	most walls in bottom floor collapse except small interior rooms	150
10	total destruction of entire building	170

You'll find the entire set of damage indicators and lists of degrees of damage here.
Here's some recent video, not shown in class, of damage being caused by a tornado as it happened (caught on surveillance video).  The tornado struck West Liberty, Kentucky on March 2 this year.

The photos below show examples of damage caused by EF2, EF4, and EF5 tornadoes.

EF2 Damage roof is gone, but all walls still standing	EF4 Damage only the strong reinforced concrete basement walls are left standing.  It doesn't look like there would have been anywhere in this building that would have provided protection from a tornado this strong.	EF5 Damage complete destruction of the structure

  Here are some additional, older, photographs of typical damage associated with all the levels on the Fujita Scale.  None of these photographs was shown in class.

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And finally, something that was initially a puzzle.


The sketch above shows a tornado located SW of a neighborhood.  As the tornado sweeps through the neighborhood, the suction vortex will rotate around the core of the tornado.



The homes marked in red would be damaged severely.  The others would receive less damage.  Remember that there are multiple suction vortices in the tornado, but the tornado diameter is probably larger than shown here.

At this point we watched the last of the tornado video tapes.  It showed a tornado that occurred in Pampa, Texas (here are a couple of videos that I found on YouTube: video 1, video 2, they're missing the commentary that was on the video shown in class).  Near the end of the segment, video photography showed several vehicles (pick up trucks and a van) that had been lifted 100 feet or so off the ground and were being thrown around at 80 or 90 MPH by the tornado winds.  Winds speeds of about 250 MPH were estimated from the video photography (though the wind speeds were measured above the ground and might not have extended all the way to the ground).