Tuesday Nov. 24, 2015

Had a hard time setting on some music selections for class today.  Here's what I ended up with: Gaelic Storm "Before the Night is Over", Los Lobos "One Time, One Night", Israel Kamakawiwo'ole "Over the Rainbow/What a Wonderful World", Lake Street Dive "Use Me Up"

The 1S1P/Optional Assignment on Thermal Circulations and the 3-cell Model were collected today (late submissions via email will probably be accepted up until about midday Wednesday).  The last 1S1P assignment on El Nino is due next Tuesday, Dec. 1.

The Quiz #4 Study Guide is now online in nearly its final form (I'll add points total for each of the sections once the quiz has been written).

Lightning

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).  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 help the fire grow and spread.

We'll be concerned with the lightning produced by thunderstorms but it has also be observed in dust storms and volcanic eruptions such as in these other worldly pictures of the 2010 eruption of Eyjafjallajokull in Iceland.  And the pictures from the Calbuco volcano in Chile.




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, static electricity, can be created in the wet interior of a thunderstorm?

1.  What produces the electrical charge needed for lightning?
2.  Different types of lightning



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, because it is smaller and lighter, 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).

Air is normally an insulator, but when the electrical attractive forces between the volumes of charge in the cloud gets gets high enough lightning occurs.  Most lightning (2/3 rds, maybe even 3/4) stays inside the cloud and travels between the main positive charge center near the top of the cloud and the 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).  It's what kills people and starts forest fires.  A photograph of a negative cloud to ground flash is shown below at left.

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).  The photo below at right shows an upward lightning discharge that was initiated by the Eiffel Tower in Paris.   We'll look at both of these unusual types of lightning later in the class.
Cloud to ground lightning with downward branching (source of this photo)
 An upward lightning discharge initiated by the Eiffel Tower in Paris.  The branching, visible at the top of the photograph, is upward.  Photographed by Hakim Atek, source of this photo



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.


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).  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 (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 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.

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 (the actual image is copyrighted so I can't stick it in the ClassNotes).



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, perhaps at the point where the channel bends indicated with the red arrow.  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.  Here's another more recent photograph (click on Galleries on the bar near the top of the page, then click on Lightning Gallery 1).  We'll learn later in the class that a lightning flash often consists of several strikes to the ground that occur in less than 1 second.  You can clearly see separate ground strikes in this photo.

4. Lightning rods
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 at left 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 launched off the top of the lightning rod intercepts the stepped leader and safely carries the lightning current through a thick wire around the house and into the ground.  Lightning rods do work and they have changed little since their initial development in the 1700s.  Most of the newer buildings on campus are protected with lightning rods.  If you look carefully at the roof of Old Main, which was recently remodeled, you'll see 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 is 6000 K) which causes the air to explode.  When you hear thunder, you are hearing the sound produced by this explosion.

The figure below summarizes what we've covered so far in simplified form


Does lightning travel upward or downward?  The answer is it does both.  It starts with a downward leader than is followed by an upward moving return stroke.

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.  That's covered below.

5. Multiple strokes flashes - dart leaders and subsequent return strokes


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.  The subsequent return stroke channel usually doesn't have branches.



Here's a stepped leader-upward connecting discharge-return stroke animation (you'll see the stepped leader, upward discharges, and the first return stroke.  Two additional subsequent strokes are shown without the dart leader).


The sketch above and the photo below show a multiple stroke flash consisting of 4 separate return strokes. There is enough time between separate return strokes (around 1/20th to 1/10th of a second) that your eye can separate the individual flashes of light.  Separate return strokes cause the flickering you sometimes see when looking at lightning.







6. Positive 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.

7. Upward lightning



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 not followed 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.

8. Rocket triggered lightning
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.
Photograph of a triggered lightning discharge taken from a few 100 meters away.  The straight part of the channel is where the discharge followed the wire.  The lightning channel becomes much more jagged when traveling through air above the wire.  (source of this photo)
 A closeup of triggered lightning striking the launch tower.  The green portion of the image at left is produced by heating and vaporization of the wire used to trigger the discharge.   The brighter whiter strokes of lightning are seen at right.  They have been spread across the picture by wind.  Photo credit: Doug Jordan and Martin Uman International Center for Lightning Research and Testing


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. 

Here's a link to the video that was showed in class.

The abbreviation NLDN that you'll see at the start of the video stands for National Lightning Detection Network.  The headquarters of this company is located here in Tucson.

In the first 1:30 of the video you'll see natural lightning occurring in the Tucson area during the summer (both intracloud and cloud to ground discharges).  Look for the flickering that means multiple return strokes in a flash.

Between 1:30 and about 2:00 you'll see lightning activity photographed at the Grand Canyon.  Lightning at the Grand Canyon preferentially strikes the edges of the canyon, a location to avoid if you're there during a thunderstorm.

The first 2 minutes of the video wasn't shown in class.

Next, between about 2:00 and 2:40 photographs of lightning striking large wind turbines in Kansas.  A lightning strike to one of the turbine blades can cause damage that is very expensive to repair.  At 2:16 and again at about 2:24 you'll see very bright lightning flashes that momentarily overexpose the video.  These are probably positive cloud to ground discharges.  And look carefully at the discharge that occurs between about 2:28 and 2:31 on the video.  Notice the upward pointing branching.  This was an upward discharge initiated by one of the wind turbines.

The remainder of the video shows rocket triggered lightning.  These experiments were done at the International Center for Lightning Research and Testing (ICLRT) run by the University of Florida near Gainesville, FL.

A student in a previous class asked a question, after the video, about what causes the green color that is sometimes seen in photographs of triggered lightning.  The answer that is probably from vaporization of the copper wire that is carried upward by the rocket.  If you're someone that enjoys watching lightning storms you may remember having seen a green glow when lightning strikes the ground.  This is often produced by an exploding transformer on an electric power pole.  The copper wire in the transformer is vaporized by the lightning.

The vaporization of different chemical compounds is what gives fireworks their distinctive colors.  This link lists some of the chemical compounds and the colors they produce.

9. Fulgurites









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 photographs of fulgurites above were found at the University of Florida lightning triggering site. 


10. Lightning safety
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.  When you hear of someone being struck by lightning and living to tell about it, it was often a nearby rather than a direct strike. 

An automobile with a metal roof and body provides good protection from lightning.  Many people think this is because the tires insulate the car from the ground.  But the real reason cars are safe is that 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 in the video that were triggering lightning with rockets 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 are connected to the plumbing and go up to the roof of the house which puts them in a perfect location to be struck by lightning.

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
(the lightning is within 6 miles).

People should remain under cover until 30 minutes after the final clap of thunder.  The powerful positive strokes often occur at the ends of thunderstorms.