1. Lightning
Pretty dangerous
How?
Why?
Where?

2. Homework in More lightning Thunder
Today
Homework in
More lightning
Thunder

3. The (CG) Lightning Stroke
The lightning stroke begins when the electric fields exceed breakdown voltage.
Initially streams of electrons surge from the cloud base toward the ground in steps of 50 to 100 m.
Start and stop steps as the stepped leader progresses toward ground.

4. Stepped Leader The stepped leader is: Very Faint
Essentially invisible to the human eye
Produces an ionized channel that will allow for the flow of charge during the remainder of the lightning stroke.

5. © 1969 Martin Uman -- From: Uman, Lightning
Stepped Leader
Stepped leader moving in ~50m steps.
© 1969 Martin Uman -- From: Uman, Lightning

6. The (CG) Lightning Stroke
When the stepped leader gets near the ground (~100 m or so):
Positive charge moves from the ground up toward the stepped leader -- these are called streamers.
The streamers may come from almost any pointed object on the ground:
Trees Antennas Grass
Flagpoles Telephone Poles People
Really Tall Towers Etc.

7. Streamers
A streamer rising from a part of the tree. The return stroke was created when the stepped leader met with a streamer from another part of the tree.
© 1998 Oxford University Press -- From: MacGorman and Rust, The Electrical Nature of Storms

8. Return Stroke
One of the streamers will meet the stepped leader -- not necessarily the one from the tallest object!
When they meet a pulse of energy flows up toward the cloud (along the ionized path) and toward the ground.
This luminous pulse of electrical energy is called the return stroke.
Occurs very fast -- we see it as a flash!

9. Dart Leader
Often a second series of stepped flow of electrons moves from the cloud toward the ground.
Since the ionized channel already exists, the stepped flow is much faster.
This stepped flow is called the dart leader.

10. Return Stroke
When the dart leader connects with a streamer from the ground (usually along the same path) another return stroke moves toward the cloud and ground.
May get several sets of dart leader/return stroke pairs.
Appears as if the lightning “flashes.”

11. Thunder Lightning is very hot (~30 000C).
Violently expanding air causes an audible shock wave.
Nearby Lightning
Thunder sounds like a “crack” or a loud bang.
Distant Lightning
Thunder sounds like a rumble. We are hearing the sound from different parts of the lightning channel.

12. Thunder How far is that lightning???? No thunder?
Light travels much faster than sound.
Count the time between when you saw the flash and heard the thunder.
5 seconds per mile
3 seconds per km
No thunder?
Could be too far away
Sound could be “bent” by the atmosphere away from you.

13. Thunder The man hears thunder from the bottom of the lightning
channel in 6 seconds. He
will continue to hear the
thunder until sound from the
top of the channel (2.8 km
away) in 8.4 seconds.
The short sound of thunder
indicates a nearby strike.
2.8 km
2 km
2 km

14. Thunder The man hears thunder from the bottom of the lightning
channel in 8.4 seconds. He
will continue to hear the
thunder until sound from the
top of the channel (4.8 km
away) in 14.4 seconds.
The long sound of thunder
indicates a long strike,
probably in-cloud.
2 km
2.8 km
2 km

15. Lightning Rods
Lightning rods act as locations where streamers can be launched toward descending stepped leaders.
It is better for the lightning rods to be struck, where the electrical energy can be safely transferred to ground, than the house.
The “cone of safety” is about 45o under the lightning rod.

16. Lightning Rods & Fulgurite
Figure 15.26
Figure 15.25
Metal rods that are grounded by wires provide a low resistance path for lightning into the earth, which is a poor conductor.
The fusion of sand particles into root like tubes, called fulgurite, may result.

17. For Atmospheric Science majors: Weather Briefing Very important information