Monday Apr. 26, 2010
click here to download today's notes in a more printer friendly format

Three songs ("Get Off of My Cloud", "19th Nervous Breakdown", and "Paint It, Black") if I remember correctly from the Rolling Stones before class today.

The In-class Optional Assignment from last Friday was returned in class today.  Answers to the assignment are now available online.

Here's a little review of some of what we covered last Friday.


The most common type of cloud-to-ground lightning is shown above.  The strike is initiated by a negatively charged, downward moving stepped leader.  This is followed by an upward propagating return stroke.  The figure above shows a single stroke flash.


This is a multiple stroke flash.  The first return stroke is preceded by a stepped leader.  The subsequent return strokes are preceded by downward moving negatively charged dart leaders that do not step.  The subsequent return stroke channels aren't branched.



This figure shows both a negative cloud-to-ground strike (on the left) and a positive stroke on the right.  The positive stroke is preceded by a downward moving positively charged leader that is followed by a very powerful return stroke.  Positive polarity flashes usually only have one return stroke.


This is a rare type of lightning that is initiated by an upward moving leader.  A downward leader and an upward return stroke then follow.  Lightning channel branches on a stroke like this would point upward.  Upward lightning is generally only initiated by mountains and tall objects such as a skyscraper or a tower of some kind.


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. 

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 material on sprites, blue jets, and elves won't be covered on the quiz.  Notes on this material will be added in the next day or so.

sprite pictures (sky-fire.tv) (also blue jets and elves)

sprite movie (Univ. AK, Geophys. Inst.)

sprite research (NM Tech)
sprite movie high speed video (NM Tech)
sprite movie high speed video (NM Tech)

blue jet movie (Univ. AK, Geophys. Inst.)