March 26, 2008

Homework #5 is now due on Wednesday, April 2, the day of the next quiz. I suggest that you not wait until the night before it is due to start the homework. We have already covered all the material necessary to complete the homework.

We will return to the material about the distribution of thunderstorms on Friday. Today’s lecture will be about lightning.

Lightning

n Lightning is a discharge of electricity. It is actually a flow of electrical current through the air. Lightning most often occurs with thunderstorms. Although rare, lightning has also been observed with snowstorms, dust storms, and in the gas cloud of an erupting volcano.

n The lightning stroke heats the air through which it travels (possibly to 30,000° C, which is 5 times hotter than the surface of the sun). The heated air in the lightning channel expands violently, initiating a shock wave which is heard as thunder.

n Lightning is dangerous. In the United States alone, an average of 75-100 people are killed each year by lightning. Probably about 10 times that many people are struck and injured by lightning each year.

n Cloud-to-ground lightning strikes pose the largest threat to humans, however, this only accounts for about 20% of all lightning. The remaining 80% of lightning takes place within the clouds.

n We will use an in-class handout to go over through the physical mechanisms for lightning formation. Please keep in mind that there is still much that is not understood about lightning. I only present an overview of some of the more accepted theories. The handout also covers some issues involved in lightning safety.

n More on thunder

o Thunder and lightning always occur together … that is, there is never lightning without thunder and there is never thunder without lightning. An observer on the ground may not always be able to detect both the thunder and the lightning from a lightning strike.

o Draw a simple picture for the production of thunder.

o If you are very close to a lightning strike (within 100 yards or so), you may hear a crackling sound, followed by a single loud bang. If further away from the lightning strike, you will usually hear a rumbling sound.

o Distance to a lightning strike.

§ Near the surface of the Earth, on average sound travels about 1 mile in 5 seconds, i.e., speed of sound = 1 mile / 5 seconds

§ Light travels so much faster than sound that compared to sound the propagation of light is nearly instantaneous

§ Therefore, the distance to a lightning strike can be estimated by measuring (or counting) the time between seeing the lightning and hearing the thunder.

· distance to lightning = (speed of sound) x (time b/t seeing and hearing)

· example … suppose you see lightning, then count to 20 seconds before hearing thunder. How far away was the lightning strike?

o Distance = (1 mile / 5 seconds) x (20 seconds) = 4 miles

n 30/30 Rule for lightning safety (suggested by lightning safety experts)

o First 30 … if you see a lightning strike and count to less than 30 seconds before hearing the thunder, you should remain in the most lightning safe place as possible

§ Inside a substantial building (best) or inside a metal-topped car or truck

o Second 30 … you should wait 30 minutes after hearing the last thunder before venturing back outside. This is often the one people ignore. Keep in mind that lightning strikes can still occur well after the rain has stopped.

n Lightning detection network

o Cloud-to-ground that has already occurred can be located using an instrument called a lightning direction-finder, which works by detecting the radio waves produced by lightning. A network of these magnetic devices has been set up all around the United States as part of the National Lightning Detection Network. The location of each cloud to ground strike is pinpointed using triangulation.

§ Draw a simple diagram for this.

o This information is often displayed on maps showing the time and location of all detected cloud to ground lightning strikes (see link on the lecture summary page). This is valuable in showing the general motion of lightning producing storms and the density of lightning strikes, but it can not be used to predict when and where a newly developing storm will first produce lightning.

n Lightning prediction and warning

o The liklihood that cloud to ground lighting is about to occur is estimated by measuring the electric field build up in the air. The Electric Field is a measure of the strength of the charge build up. When a certain threshold is reached, lightning is about to hit somewhere. Electric field measuring equipment is used at many outdoor events (e.g., golf tournaments, state fairs, etc.) to allow authorities to give warning that people should take cover. Note that while the electric field instrumentation can provide information that tells us lightning is about to strike somewhere in the area, there is no way to predict exactly where the lightning will hit.

§ Again draw a simple diagram for this.