Tuesday Nov. 27, 2007

Here is an optional assignment that you can download, print, and turn in at the beginning of class on Tuesday.  You'll find the answers to the questions in the notes below and in the text (Ch. 10 pps 277-282).

Lightning is the topic today.  Lighning is usually produced by thunderstorms.

A typical summer thunderstorm in Tucson.  Remember that even in the summer a large part of the middle 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 the ice crystal process of precipitation formation operatures and is also where electrical charge is created.

Collisions between precipitation particles produces the electrical charge needed for lightning.  When temperatures are below -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 charging is 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) 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 of all lightning flashes strike the ground.  These are called cloud-to-ground discharges.

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 (a dangerous situation to be in).  St. Elmo's fire is a faint electrical discharge that sometimes develops at the tops of elevated objects during thundestorms.

Most cloud to ground discharges begin with a negatively charged downward moving stepped leader.  It 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.  An upward discharge is initiated when the stepped leader nears the ground.  A powerful return stroke travels back up the channel (and out into all the branches) once the upward discharge and the stepped leader meet.  These three steps are shown in additional detail below.

A sequence of stepped leader steps,

Several positively charged upward discharges begin to travel upward from the ground. One of these will eventually intercept the stepped leader. 
This is what determines what will be struck by the lightning.  Lightning doesn't really know what it will strike until it gets close to 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.

Stepped leader - upward connecting discharge - return stroke animation

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 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 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.

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.  These discharges are initiated by an upward leader.  This is followed by a more normal downward leader and an upward return stroke.

Scientists are able 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 was shown in class.

Near the end of the tape you will some cases where the lightning didn't follow the wire all the way to the ground (this is one reason why you need to be very careful doing experiments of this type).  When the lightning strikes the sandy soil (instead of striking instruments on the ground) it sometimes will leave behind a fulgurite.

This is a drawing of a science fair project.  If 10 to 20 Amps will cook a hotdog,
imagine what the 10,000 to 30,000 Amps in a lightning return stroke can do.  We wrapped up class with some lightning safety information.

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 people in Florida that were triggering lightning were inside a metal trailer and were perfectly safe).  The rubber tires really don't play any role at all.

You shouldn't use a corded phone and electrical appliances during a lightning storm because lightning currents can follow wires into your home.  Cordless phones and cell phones are safe.

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.

The latest lightning safety recommendation is the 30/30 Rule.  You 'll see that explained below.

The 30/30 rule
30/30 Rule graphicAny lightning safety plan should incorporate the 30/30 Rule. The 30/30 Rule states that people should seek shelter if the "Flash-To-Bang" delay (length of time in seconds between a lightning flash and its subsequent thunder), is 30 seconds or less, and that they remain under cover until 30 minutes after the final clap of thunder.

A 30 second lead time is necessary prior to a storm's arrival because of the possibility of distant strikes. A 30 minute wait after the last thunder is heard is necessary because the trailing storm clouds still carry a lingering charge. This charge can and does occasionally produce lightning on the back edge of a storm, several minutes after the rain has ended.

Studies have shown most people struck by lightning are struck not at the height of a thunderstorm, but before and after the storm has peaked. This shows many people are unaware of how far lightning can strike from its parent thunderstorm. DO NOT wait for the rain to start before seeking shelter, and do not leave shelter just because the rain has ended.

(from Jetstream An Online School for Weather )