Tue., Apr. 25 notes

Tue. Apr. 25, 2006

We (the TAs and I) have made some real progress on the grading of the 1S1P reports. The most recently graded papers were returned in class today. All of the reports turned in last week have also been graded and were returned in class. Note that any report revisions must be turned in by next Tuesday (the last day of classes in this T Th section)

A copy of the Quiz #4 Study Guide was handed out in class.

Rather than introducing today's topic Lightning with some grim statistics (see above) the course instructor read from a recently completed literary work.

This is review. Thunderstorms easily climb above 20,000 feet altitude, so even on the hottest day in Tucson, these clouds will contain ice crystals. The mixture of supercooled water droplets and ice crystals in the middles of clouds was important in the ice crystal process of precipitation formation. It is also important in generating the electrical charge needed for lightning.

Collisions between precipitation particles produce electrical charge. When graupel and ice crystals collide at temperatures colder than -15 C the ice crystal is positively charged and is carried toward the top of the cloud by the updraft. The graupel ends up with a negative charge and creates a layer of negative charge in the middle of the cloud. At temperatures warmer than -15 C (but still colder than freezing) the polarity of the charging is reversed. Small pockets of positive charge are found below the main layer of negative charge. Note also the positive charge that accumulates in the ground under the thunderstorm. The electrical attraction between objects on the ground can create some interesting phenomena. We saw a slide of a woman standing on a mountain ridge under a thunderstorm. Her hair was standing on end (a dangerous situation). A faint blue green glowing discharge is sometimes visible on the tops of tall objects under a thunderstorm. This is known as St. Elmo's Fire.

Intracloud lightning discharges between the main positive and negative charged regions in the cloud is the most common type of lightning. We are more concerned with and know more about cloud to ground lightning because that is what kills people and causes billions of dollars of damage to structures on the ground.

Cloud to ground lightning normally begins with a downward moving negatively charged stepped leader. It takes a 50 yard long jump every 50 microseconds or so. It emits a short pulse of light everytime it takes a jump. When the stepped leader nears the ground a positively charged upward connecting discharge progagates upward to meet the stepped leader. When they connect, the cloud and ground are effectively short circuited and a very powerful spark, the return stroke travels back up the channel to the cloud.

A "flip book" movie of a downward progagating stepped leader, the upward connecting discharge, and the upward return stroke was "showed" in class.

About half of all cloud-to-ground lightning flashes consist of a stepped leader and a return stroke, that's it. In the other 50% of the cases the process repeats itself.

Instead of a stepped leader, a dart leader travels down the channel created by the stepped leader and first return stroke. The dart leader doesn't step and doesn't go out into the branches. An upward discharge intercepts the dart leader when it nears the ground and a subsequent return stroke travels back up the channel into the cloud.

Often this downward dart leader - upward return stroke combination is repeated a 3rd and often a 4th time. The separate return strokes are 0.05 (1/20th) to 0.1 (1/10th) of a second apart which is long enough that your eyes see them as separate flashes of light. When you see a lightning flash flicker, you are seeing the separate return strokes.

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. The process begins with an upward leader (#1) which is then followed by a downward leader (#2) and then an upward return stroke (#3).

French scientists were the first to perfect a technique of triggering 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 Florida was shown in class.

When the lightning strikes the sandy soil (instead of striking instruments on the ground) it sometimes will leave behind a fulgurite (note spelling in the figure below is incorrect). The end of the video showed Florida archaeology students digging up some fulgurites.

Lightning kills around 100 people a year in the US, it is worth learning some lightning safety rules.

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. You should also stay out of the ocean or a swimming pool during a thunderstorm.

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 Florida scientists were very careful to be sure that no wires of any kind came inside the trailer from outside.

You shouldn't use a corded phone and electrical appliances during a lightning storm because lightning currents can follow wires (and even the plumbing) into your home.

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.

If there is less than 30 seconds delay between the light and the thunder then the lightning is close enough to present a risk to you. You should wait 30 minutes after the last lightning before assuming that a storm has dissipated or moved out of your area.