Tuesday Apr. 24, 2012

I heard a song I liked, "A Dream Within a Dream" by The Glitch Mob, at the School of Dance Student Spotlight performance last Thursday night.  That was the 2nd song I played before class this morning.  The first was "We Can Make the World Stop" by the same group.

All of the Foucault Pendulum and Regional Winds 1S1P reports have been graded and were returned in class today along with the Optional Assignment that some people turned in early last Thursday.

The last of the 1S1P reports (Story and the Enhanced Fujita Scale) were collected today.  I'll make an initial pass through the papers and will update the list of students that have earned 45 pts.  The last Optional Assignment was collected today.  You'll find answers to the questions here.

We have a little material on tornadoes to finish up before spending the rest of the class period on lightning.  Tornado season this spring has already been particularly destructive and deadly.  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).

Positive polarity cloud to ground lightning (Pt. 3) accounts for a few percent of lightning discharges.  Upward lightning is the rarest form of lightning (Pt. 4).  We'll look at both of these unusual types of lightning later in the lecture.


Most cloud to ground discharges begin with a negatively-charged downward-moving stepped leader (the figure above is on p. 166 in the ClassNotes).  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 periodically 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.
 



Here's an actual slow motion movie of a stepped leader.  The video camera collected 7207 images per second ( a normal video camera takes 30 images per second).  The images were then replayed at a slower rate.  A phenomenon that takes a fews tens of milliseconds to occur is spread it out over a longer period of time so that you can see it.

As the leader channel approaches the ground strong electrical attraction develops between negative charge in the leader channel and positive charge on the surface of the ground.  Several  positively charged sparks develop and move upward toward the stepped leader.  One of these will intercept the stepped leader and close the connection between negative charge in the cloud and positive charge on the ground.






Here's a sketch of one of the best photographs ever taken of an upward connecting discharge.


You can see the actual photograph on the photographers homepage.  There were at least 3 upward discharges initiated by the approach of the stepped leader (1, 2, and 3 in the sketch).  Streamer 1 connected to the bottom of the stepped leader.  It isn't clear where the exact junction point was.  The downward branching at Point 4 indicates that was part of the descending stepped leader.  A very faint upward discharge can be seen at Point 3.

Lightning rods (invented by Benjamin Franklin) make use of the upward connecting discharge.



Houses with and without lightning rods are shown above.  When lightning strikes the house without a lightning rod the powerful return stroke travels into the house destroying the TV and possibly starting the house on fire. 
With a lightning rod, an upward discharge will intercept the stepped leader and safely carry the lightning current through a thick wire around the house and into the ground.  They do work and have changed little since their initial development in the 1700s.  Most of the newer buildings on campus are protected with lightning rods.

The connection between the stepped leader and the upward discharge creates a "short circuit" between the charge in the cloud and the charge in the ground. 


A powerful current travels back up the channel from the ground toward the cloud.  This is the 1st return stroke.  Large currents (typically 30,000 amps in this 1st return stroke) heat the air to around 30,000K (5 times hotter than the surface of the sun) which causes the air to explode.  When you hear thunder, you are hearing the sound produced by this explosion.

The figure below shows what we've learned so far in simplified form
 






Many cloud-to-ground flashes end at this point.  In about 50% of cloud to ground discharges, the stepped leader-upward discharge-return stroke sequence repeats itself (multiple times) with a few subtle differences.


A downward dart leader travels from the cloud to the ground. The dart leader doesn't step but travels smoothly and follows the channel created by the stepped leader (avoiding the branches).  It is followed by a slightly less powerful subsequent return stroke that travels back up the channel to the cloud.  This second stroke might be followed by a third, a fourth, and so on.



A normal still photograph would capture the separate return strokes superimposed on each other.  If you bumped or moved the camera during the photograph the separate return strokes would be spread out on the image.

Here's a stepped leader-upward connecting discharge-return stroke animation.





The image above shows a multiple stroke flash consisting of 4 separate return strokes. There is enough time between separate return strokes (around 1/10 th second) that your eye can separate the individual flashes of light.



When lightning appears to flicker you are seeing the separate return strokes in a multiple stroke flash.  The whole flash usually lasts 0.5 to 1 second.

Here are some unusual types of lightning.


We've been looking at strikes that originate in the negative charge center is a thunderstorm (discharge at left in figure above).  Occasionally a lightning stroke will travel from the positive charge region in the top of the thunderstorm cloud to ground (shown at right in the figure above).  These types of strikes are more common at the ends of storms and in winter storms.  This is probably because the top part of the cloud gets pushed sideways away from the middle and bottom portions of the cloud.  Positive strokes are very powerful.  They sometimes produce an unusually loud and long lasting clap of thunder.



Here's an even rarer form of lightning.  Lightning sometimes starts at the ground and travels upward.  Upward lightning is generally only initiated by mountains and tall objects such as a skyscraper or a tower of some kind (the Empire State Building is struck many times every year but lightning and usually it's lightning that the building itself caused). 

Note the discharge is different in another way also.  These discharges are initiated by an upward leader.  This is followed by not by a return stroke, like you might expect, but by a more normal downward leader.  Once the 2nd leader reaches the ground, an upward return stroke travels back up the channel to the cloud.
The fact that lightning could begin with an upward discharge that begins at the ground led (French) scientists to develop a technique to trigger lightning by firing a small rocket up toward a thunderstorm.  The rocket is connected by a thin wire to the ground.  When the rocket gets 50 to 100 m above the ground an upward streamer will develop off of the top of the wire.  Once the streamer reaches the cloud it can initiate a "normal" series of downward dart leaders and upward subsequent return strokes.

Scientists are able to take closeup photographs and make measurements of lightning currents using triggered lightning.  Triggered lightning can also be used to test the operation of lightning protection devices.  A short video showing rocket triggered lightning experiments being conducted by University of Florida scientists in northern Florida was shown in class.  There are also some good videos of the experiments on YouTube.




When lightning strikes the ground it will often melt the soil (especially sandy soil) and leave behind a rootlike structure called a fulgurite.  A fulgurite is just a narrow (1/2 to 1 inch across) segment of melted sand (glass).  The video showed archaeology students excavating around the lightning triggering site after the summer's experiments.  They were able to uncover and reveal a very long (perhaps world record length) fulgurite. 

Lightning is a serious weather hazard.  Here are some lightning safety rules that you should keep in mind during thundery weather.


Stay away from tall isolated objects during a lightning storm.  You can be hurt or killed just by being close to a lightning strike even if you're not struck directly.  Lightning currents often travel outward along the surface of the ground (or in water) rather than going straight down into the ground.  Just being close to something struck by lightning puts you at risk.

An automobile with a metal roof and body provides good protection from lightning.  The lightning current will travel through the metal and around the passengers inside.  The rubber tires really don't play any role at all.  The people in Florida that were triggering lightning with rockets (shown on a video last week) were inside a metal trailer and were perfectly safe.  All of the connections made to equipment outside the trailer were done using fiber optics, there were no metal wires entering or leaving the trailer. 

You shouldn't use a corded phone or electrical appliances during a lightning storm because lightning currents can follow wires into your home.  Cordless phones and cell phones are safe.  It is also a good idea to stay away from plumbing as much as possible (don't take a shower during a lightning storm, for example).  Vent pipes that are connected to the plumbing go up to the roof of the house which puts them in a perfect location to be struck.



To estimate the distance to a lightning strike count the number of seconds between the flash of light and when you first hear the thunder.  Divide this by 5 to get the distance in miles. 


For example, a delay of 15 seconds between the flash of light and the sound of thunder would mean the discharge was 3 miles away.  Research studies have shown that about 95% of cloud to ground discharges strike the ground within 5 miles of a point directly below the center of the storm.  That's a 10 mile diameter circle and covers the area of a medium size city.

The latest lightning safety recommendation is the 30/30 Rule.

 

The 30/30 rule
People should seek shelter if the delay between a lightning flash and its  thunder is 30 seconds or less.

People should remain under cover until 30 minutes after the final clap of thunder.

The following information wasn't covered in class and won't be on this week's quiz
Some fairly new and unusual upper atmospheric phenomena are sometimes called lightning.  The figure below (source: Wikipedia) gives you an idea of where these so-called sprites, elves, and blue jets are found and sort of what they look like.  They're very faint and don't last very long so they difficult to see.  They don't really involve lighning channels and large currents like we have been discussing.  Rather these are phenomena most likely caused by the electromagnetic fields produced by lightning.





You find some good actual pictures of sprites (mainly) at this sky-fire.tv website.  A site maintained by the New Mexico Institute of Mining and Technology (NM Tech) also has some good photographs, video, and more information about these phenomena.