Monday, Apr. 15, 2019

Cafe Accordion Orchestra "L'Indifference" (5:23), Tuesday at Mona's documentary (first 3:30), The Hot Club of San Francisco "Don't Panic" (4:41), Gipsy Klezmer Band "Djelem Djelem" (5:35)

Many of the figures that follow are found on page 161, page 162, page 163a, page 163b, page 164a, and page 164b in the ClassNotes.

The 1S1P/EC assignment about atmospheric stability was collected today.  The revised Expt. #3 reports were also due today.

We have the last part of the tornado material to finish up, mainly tornado damage and the Enhanced Fujita Scale

Here are simplified, easy to remember, versions of both scales.




Here is a comparison of the actual scales



There's also a much more detailed set of guidelines for determining the EF scale rating from a survey of tornado.  Different objects and structures react differently when subjected to tornado (or microburst) strength winds.

The EF scale has 28 "damage indicators" that can be examined to determine tornado intensity.  You can think of these as being different types of structures or objects that could be damaged by lightning..  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).    It shows a house roof being lifted intact off a house.  If you listen to the news commentators, there was someone in a pickup truck in the street that survived the tornado.  The tornado struck West Liberty, Kentucky, on March 2, 2012.

Here are photographs of some actual tornado damage and the EF Scale rating that was assigned to each
EF2 Damage
roof is gone, but all walls still standing
 EF4 Damage
only the strong reinforced concrete basement walls (part of the wall was below ground) 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




At this point we watched the last of the tornado video tapes.  It showed a tornado that occurred in Pampa, Texas.  Here is a pretty similar video that I found online.  It's 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 (the large dark objects seen between about 5:40 and 6:10 on the video).  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).

Here's probably the best photo comparison of the different levels of tornado damage that I've been able to find.  I don't think I can embed the images in the lecture notes without worrying about a copyright violation.

Multiple vortex tornadoes

 And finally, something that was initially something of a puzzle to tornado researchers.









One of the better examples that I've seen of very different levels of damage in close proximity.  This is damage from an EF4 tornado that hit Northwood ND on Aug. 26, 2007.  (National Weather Service photo, source click on the Track Segments and Photos link)


Several levels of damage (EF1 to about EF3) are visible in the photograph above.   Here's another example with level F0 through F5 damage all found in a few block area.

 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.






Sketch of multiple vortices in a large tornado and the damage pattern they could leave on the ground
 An actual aerial survey of tornado damage.  This was an EF 4 tornado that hit Washington Illinois on Nov. 17, 2013 (here is YouTube video of the tornado and the damage left behind).  Photo by Zbigniew Bzdak for the Chicago Tribune (source)
 

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 damage pattern shown above 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.  Just one suction vortex was used here, there are usually several.  But the tornado diameter is also probably larger than shown here.

This ends our coverage of tornadoes.

We had some time left to begin the next topic - Lightning

Data collected in the past 30 years indicate that lightning kills about 50 people every year in the United States (floods kill about 80 people per year, tornadoes about 70 people per year and hurricanes about 50 people per year).  Lightning 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 (virga).  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.  Strong downdraft winds from the thunderstorm can "fan the fire" and help the fire grow and spread.

1.  What produces the electrical charge needed for lightning?
The short answer is collisions between precipitation particles in the cloud (collisions between graupel and snow crystals)



For reasons that are still not completely understood initially uncharged particles become charged during the collision.  The charge production takes place in the middle, mixed phase region, of the cloud.  Mostly the ice crystals become positively charged and are carried up to the top part of the cloud.  The negatively charged graupel particles form a layer of negative charge in the middle part of the cloud.  The are also smaller pockets of positive charge found below the layer of negative charge.  The distribution of charge in a thunderstorm is shown in the figure below.
2.  The 4 different types of lightning
Note the distribution of positive and negative charge in the cloud (and in the ground under the thunderstorm)



We'll be concerned with the lightning produced by thunderstorms.  There are 4 main types of lightning.  Intracloud lightning is the most common type (2/3 to 3/4 of all lightning discharges, sometimes more).  Negative cloud-to-ground is the next most common type (~1/4 to 1/3).  Positive cloud-to-ground lightning accounts for a few percent of cloud-to-ground lightning.  Upward lightning is pretty rare and "needs some help" such as a mountain or tall building in order to occur.   Photographs of a negative cloud-to-ground flash and an upward lightning discharge are shown below. 













Cloud to ground lightning with downward branching (source of this photo)
 An upward lightning discharge initiated by the Eiffel Tower in Paris.  At the top of the photograph you can see that the branching points upward.  Photographed by Hakim Atek, source of this photo


Lightning has also been observed in dust storms and volcanic eruptions such as in these otherworldly pictures of the 2010 eruption of Eyjafjallajokull in Iceland.  And these more recent pictures from the Calbuco volcano in Chile.
 
A couple of interesting things can happen at the ground under a thunderstorm.  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; I wouldn't wait around for my picture to be taken.  I recently stumbled upon an article that described the circumstances under which the photographs below were taken. 


Michael McQuilken is shown at far right next to his brother Sean.  Their sister Mary is shown in the left photo.  All three were on top of Moro Rock in Sequoia National Park in California.  Sean was struck by lightning but survived.  Another man in the area was struck and killed by lightning.  An elevated exposed location like this is a very dangerous place to be during a thunderstorm.


St. Elmo's Fire (corona discharge) is a faint electrical discharge that sometimes develops at the tops of elevated objects during thunderstorms.  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).  Sailors in those days were often very superstitious and I suspect they found St. Elmo's fire pretty terrifying.

3. Cloud to ground lightning - the stepped leader, upward discharge, and 1st return stroke
A cloud to ground lightning flash is actually a sequence of several separate events.

Most cloud to ground discharges begin with a negatively-charged downward-moving stepped leader.  A developing channel makes its way down toward the cloud in 50 m jumps that occur every 50 millionths of a second or so.  Every jump produces a short flash of light (think of a strobe light dropped from an airplane that flashes on and off as it falls toward the ground).  The sketch below shows what you'd see if you were able to photograph the stepped leader on moving film.  Every 50 microseconds or so you'd get a new picture of a slightly longer channel displaced slightly on the film (the flash of light would come from the highlighted segments would be captured on film).
 
Here's an actual slow motion movie (video not film) of a stepped leader (its the second video on the page).  The video camera used here was able to collect 7207 images per second ( a normal video camera takes 30 images per second).  The images were then replayed at a slower rate.  1/8 of a second of lightning is stretched out to about 30 seconds on the video.

We were just about out of time at this point.  I've moved the rest of the material on lightning to the Wednesday, Apr. 17 notes.