Friday, Apr. 20, 2018

Blue Man Group "The Complex Rock Tour" (18:30) or maybe an appearance on the NPR Tiny Desk Concert (13:15)



Quick recap of the series of events that comprise a negative cloud-to-ground lightning strike


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 (cloud-to-ground) 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




A very nice picture of an upward lightning initiated by Freedom Tower in New York City.  The photo was taken from the Empire State Building (source of this image).

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, it's usually lightning that the building itself initiated; some very early studies of lightning were carried out there). 

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 copper 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.

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 were 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.

Starting about 2:50 and for the remainder of the video you'll see some 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.

The green glow that you might have noticed in some of the triggered lightning video probably comes 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 similar green glow when lightning strikes the ground.  I suspect this is caused by a strike to a 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.