Friday Apr. 20, 2012

Something I heard at the School of Dance Student Spotlight performance last night, "A Dream Within a Dream" by The Glitch Mob before class this afternoon.

I made an initial pass through all the 1S1P reports turned in on Wednesday and simply gave points to students that were very close to 45 pts and had turned in a reasonable report.  You can check this list to see if you have made it yet or not.  I should be able to grade at least the Foucault Pendulum reports and hopefully also the Regional Winds reports by Monday. 

The last Optional Assignment of the semester was collected today.  I'll grade those this weekend and return them next Monday.  I have already graded the assignments that were turned in early.  I don't remember anyone answering all the questions correctly.  For me that's a good assignment as it points out areas where you need to do some additional study.

And one other thing.  Something new today after I had an opportunity to sit in on one of the other Atmo 170A1 sections earlier this week.  An indefinite ban on laptops in class.  And something I'm calling it a "Some Signs of Life" card.  I'm not sure what it will look like and haven't decided what it will be good for but I've handing them out to people (I've got a list of names at this point).  If you were in class and didn't get your name on my list or weren't in class, I would consider reading through these online notes to be a Sign of Life.  So bring me some evidence that you're studying and understanding this material.  If your evidence is convincing I'll either give you a card or add your name to my list.


A quick review of tilted rotating updrafts (mesocyclones) and wall clouds, features found on severe thunderstorms.  You can see a wall cloud at about the 1:46 minute mark on the video of the Laverne Oklahoma tornado.

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 little 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 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.

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.



Xrays pass through tissue but get absorbed by bone.  They reveal the skeletonal structure inside a body.  In some respects radar is similar.




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 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.

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.


Tornado season this spring has already been particularly destructive and deadly.  Next we'll look at some of the kinds of damage tornadoes can do and we'll introduce the Fujita Scale used to rate tornado strength or intensity.

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, has recently 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.  The chart below compares the two scales.



 
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 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.

I like to keep a very basic damage scale in mind so that I can estimate tornado intensity when I see video on the television news.



The photos above 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.



Several levels of damage (EF1 to about EF3) are visible in the photograph above.  It was puzzling initially how some homes could be nearly destroyed while a home nearby or in between was left with only light damage.  One possible explanation is shown below.




Some big strong tornadoes may have smaller more intense "suction vortices" that spin around the center of the tornado (they would be hard to see because of all the dust in the tornado cloud.  Tornado researchers have actually seen the pattern shown at right  scratched into the ground by the multiple vortices in a strong tornado.



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 (the wind speeds were measured above the ground and might not have extended all the way to the ground).

Lightning kills just under 100 people every year in the United States (more than tornadoes or hurricanes but less than flooding, summer heat and winter cold) and is the cause of about 30% of all power outages.  In the western United States, lightning starts about half of all forest fires.  Lightning caused fires are a particular problem at the beginning of the thunderstorm season in Arizona.  At this time the air underneath thunderstorms is still relatively dry.  Rain falling from a thunderstorm will often evaporate before reaching the ground.  Lightning then strikes dry ground, starts a fire, and there isn't any rain to put out or at least slow the spread of the fire.  This is so called dry lightning.

Lighning is most commonly produced by thunderstorms (it has also be observed in dust storms and volcanic eruptions such as the 2010 eruption of Eyjafjallajokull in Iceland). 



A typical summer thunderstorm in Tucson is shown in the figure above (p. 165 in the photocopied ClassNotes).   Even on the hottest day in Tucson in the summer a large part of the middle of the cloud is found at below freezing temperatures and contains a mixture of super cooled water droplets and ice crystals.  This is where precipitation forms and is also where electrical charge is created.  Doesn't it seem a little unusual that electricity can be created in such cold and wet conditions?


Collisions between precipitation particles produce the electrical charge needed for lightning.  When temperatures are colder than -15 C (above the dotted line in the figure above), graupel becomes negatively charged after colliding with a snow crystal.  The snow crystal is positively charged and is carried up toward the top of the cloud by the updraft winds.  At temperature warmer than -15 (but still below freezing), the polarities are reversed.  A large volume of positive charge builds up in the top of the thunderstorm.  A layer of negative charge accumulates in the middle of the cloud.  Some smaller volumes of positive charge are found below the layer of negative charge.  Positive charge also builds up in the ground under the thunderstorm (it is drawn there by the large layer of negative charge in the cloud).

When the electrical attractive forces between these charge centers gets high enough lightning occurs.  Most lightning (2/3 rds) stays inside the cloud and travels between the main positive charge center near the top of the cloud and a large layer of negative charge in the middle of the cloud; this is intracloud lightning (Pt. 1).  About 1/3 rd of all lightning flashes strike the ground.  These are called cloud-to-ground discharges (actually negative cloud-to- ground lightning).  We'll spend most of the class learning about this particular type of lightning (Pt. 2)

A couple of interesting things can happen at the ground when the electrical forces get high enough.  Attraction between positive charge in the ground and the layer of negative charge in the cloud can become strong enough that a person's hair will literally stand on end (see two photos below).  This is incidentally a very dangerous situation to be in as lightning might be about to strike. 




St. Elmo's Fire (corona discharge) is a faint electrical discharge that sometimes develops at the tops of elevated objects during thunderstorms. The link will take you to a site that shows corona discharge.  Have a look at the first 3 pictures, they probably resemble St. Elmo's fire.  The remaining pictures are probably different phenomena.  St. Elmo's fire was first observed coming from the tall masts of sailing ships at sea (St. Elmo is the patron saint of sailors).

Pts. 3 and 4 in the thunderstorm sketch are a couple of relatively rare forms of lightning.  We'll discuss them in more detail in class on Monday.