Monday Nov. 26, 2012

A couple of songs from Dire Straits before class this afternoon ("Down to the Waterline" and "Lady Writer")

The 1S1P Assignment #3 reports on Fog, Foucault's Pendulum, Atmospheric Stability, and Regional Winds have all been graded and were returned today.  Reports on the last topic (Rainbows, Mirages, and the Green Flash) were collected today.

You can always turn in late 1S1P reports.  I'll decide what to do with them later.  There's a good chance that you might only receive half credit on late reports.

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).

Air is normally an insulator, but 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 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 highlighted segments would be photographed).
 



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 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 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 covered 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 (10 min.) 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 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 are difficult to see (that's why nobody knew they were there for so long).  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.