Tuesday Nov. 02  Election Day
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A little music "Crystal Frontier" (http://www.youtube.com/watch?v=JF8V_EZB5vI) from a local group Calexico playing together with Mariachi Luz de Luna at the Barbican Theater in London in 2004.

I hope you're planning to attend the All Souls Procession this coming Sunday night.  Calexico will be playing at the Rialto Theatre

Most of the class period today was devoted to lightning.  Here's a little introductory information that wasn't covered in class.

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. 

Lighning is most commonly produced by thunderstorms (it has also be observed in dust storms and volcanic eruptions such as the one that occurred earlier this year in Iceland).  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.


A typical summer thunderstorm in Tucson is shown in the figure above.   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 a cold and wet environment?

Before going into how electrical charge is produced we took a short detour to look at how hail forms.  Large hailstones from in strong severe thunderstorms.  These storms will probably get more coverage in your course.  Severe thunderstorms can also produce tornadoes.


The figure above shows the formation of graupel.  The process (called riming or accretion) begins with a falling ice crystal.  The crystal collides with supercooled water droplets (cooled to below freezing but not able to freeze).  The droplets stick to the ice crystal and freeze.  If the process goes on long enough the ice crystal gets completely covered with frozen water droplets.  Graupel particles have a frosty milky white appearance and can grow up to 1/4 inch or more.  When they fall from a cloud they are often mistaken for hail.  Graupel is sometimes referred to as soft hail or snow pellets.  Graupel also usually serves as an embryo or nucleus for a hailstone.



This figure illustrates the formation of hail.  We start with a graupel particle (Pt. 1, colored green to represent rime ice).  The graupel falls or gets carried into a part of the cloud where it collides with a large number of supercooled water droplets which stick to the graupel but don't immediately freeze.  The graupel gets coated with a layer of water (blue) at Pt. 2.  The particle then moves into a colder part of the cloud and the water layer freezes producing a layer of clear ice (the clear ice, colored pink, has a distinctly different appearance from the milky white rime ice), Pt. 3.

Hail that falls to the ground in Tucson usually just has a graupel core and a single layer of clear ice.  In the severe thunderstorms in the Central Plains, the hailstone can pick up additional layers of rime ice and clear ice and hailstones can be composed of many alternating layers of rime and clear ice.

There was some severe weather in early October in the Pheonix and Flagstaff area.  Strong supercell thunderstorms produced large hail and also tornadoes (Flagstaff area).  Here are some photographs.


A pickup overturned either by tornado winds or strong thunderstorm downdraft winds.  Serious damage also the garage door.


Roof and siding damage produced by tornado winds (~100 MPH). 



Train cars tipped over by tornado winds.  It looks like the rail cars were carrying shipping containers stacked two containers tall.  This would present a large crossection to the winds.

The next two photographs show what appear to be wall clouds at the bases of the supercell thunderstorms.






This is a little better sketch of a severe thunderstorm or supercell thunderstorm.  Normal thunderstorms have a vertical updraft.  The updraft often tilts in severe thunderstorms and sometimes rotates.  A rotating updraft is referred to as a mesocyclone.  A rotating wall cloud will sometimes form at the bottom of the mesocyclone and extend below the rest of the thunderstorm base.  Up at the top of the sketch you can see how the updraft can keep a growing hailstone inside the thundercloud allowing it to grow bigger.

The next photographs are of large hailstones observed in the Pheonix area.







Note that hailstones often have a jagged not a smooth round shape.


My favorite photograph because you can clearly see the concentric layers of white rime ice and clear ice.

Here's the hail video shown in class (http://www.youtube.com/watch?v=-EPkDWW0N94)
and a longer video (not shown in class) showing hail damage to automobiles (http://www.youtube.com/watch?v=LUphYcACHgk)

OK back to lightning.

Collisions between precipitation particles produce the electrical charge needed for lightning.  When temperatures are colder than -15 C, 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.  Large positive and negative charge centers begin to build up inside the cloud.  When the electrical attrative 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.  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 spent most of the rest of the class learning about this particular type of lightning.



A couple of interesting things that 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 photo below).  This is incidentally a dangerous situation to be in as lightning might be about to strike.  St. Elmo's fire is a faint electrical discharge that sometimes develops at the tops of elevated objects during thundestorms.  It was first observed coming from the tall masts of sailing ships at sea (St. Elmo is the patron saint of sailors).



Most cloud to ground discharges begin with a negatively-charged downward-moving stepped leader.  A developoing 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. 



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.



Lightning rods take advantage of this principle.

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. 
A lightning rod is supposed to intercept the stepped leader and safely carry the lightning current around the house and into the ground.

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 return stroke.  Large currents (typically 30,000 amps in the first 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.


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.

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's a stepped leader-upward connecting discharge-return stroke animation.
We also watched a fast time resolved video of an actual stepped leader that is on YouTube (http://www.youtube.com/watch?v=2XwFF5idD_0)

Here are some unusual types of lightning.

Occasionally a lightning stroke will travel from the positive charge region in the top of the thunderstorm cloud to ground.  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.  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 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 lead 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 upward lightning will develop off of the top of the wire.

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 in northern Florida was shown in class.



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.  A couple of small pieces of fulgurite were passed around class.

Lightning is a serious weather hazard and kills just under 100 people every year in the United States.   We discussed 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.

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 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.

The latest lightning safety recommendation is 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.