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Flying circus of physics

Upward streamers in electric storms

Monday, December 01, 2014

 

Upward streamers in electric storms
Jearl Walker  www.flyingcircusofphysics.com
December 2014  The woman in this image certainly appeared to be happy, perhaps largely because of the amusing way her hair was standing up from her head. But she was in great danger for that very reason.

A highly electrified cloud was over her and the lookout platform there in Sequoia National Park, California. The huge electric field in the air above her drove some of the conduction electrons out of her body and into the lookout platform, leaving her positively charged. Thus, each strand of hair was positively charged, and the strands repelled one another. Many of them stood up in order to move away from the other strands.

This might look like fun, but she was in danger of being killed by two processes. One, with the electric field that high, lightning could hit the platform, which would electrocute her. Indeed, minutes after she left the platform, lightning did strike the platform, killing the person that was still on it. The danger of being near a lightning strike, or being hit directly by lightning, is well known. The second danger she faced on that platform is little known and recognized only fairly recently. The electric field could have been strong enough to suddenly ionize the air molecules in a narrow channel above her head in what is called an upward streamer. Once freed from the molecules, the electrons would then be driven down along the channel, through her body, and into the platform. The ionized channel might then connect with ionizing channels that snake down from the storm clouds to complete a lightning strike. But even if it did not and remained isolated, the discharge through the woman could have been fatal. Here is a photograph in which you can see both a completed lightning strike and two separate upward streamers.

Upward streamers appear to have been involved in a 2000 tragedy in South Africa. Under a large tent, 23 girls and 2 adults were asleep when lightning apparently struck one of the tent poles, leaving 4 girls dead and 19 injured. However, the burns marks and other evidence suggested that many of the victims were not hurt by the direct strike or by the strike branching out into adjacent objects. Rather they were hurt by upward streamers that developed in the intense electric field that brought the lightning to the tent pole.

Upward streamers may be responsible for injuries at some outdoor sporting events. In this video link you see several football players knocked down during their game. There is a flash of light off camera to the left from a lightning strike to the playing field. The players could have been hurt by the resulting current through the ground, aptly call ground current. But they could have also been hurt by upward streamers from their heads. Notice how many of them clutched their heads, strongly suggesting upward streamers discharged into their head.

https://www.youtube.com/watch?v=MNJRPOltifI

In my textbook (Fundamentals of Physics in the American version and Principles of Physics in the international version) I show how we can estimate the amount of positive charge on the woman on the Sequoia lookout platform when an upward streamer is on the verge of forming. Let’s simplify her shape to be a narrow cylinder of height L = 1.8 m and radius R = 0.10 m. Let’s also simplify the charge distribution as being uniformly distributed along the cylinder. If the upward streamer is on the verge of forming, then the electric field has a critical magnitude of about E = 2.4 X 106 volts per meter, the so-called breakdown voltage of air. Because the cylinder is narrow compared to its length, we can use the formula for the electric field E outside a line of uniform charge: E = 2kd/r, where k is a constant of value 9.0 X 109 N m2/C2, d is the linear charge density (charge per length along the line of charge), and r is the radial distance out to where we measure the field.

For our model of the woman, the linear charge density is the total charge Q on her divided by her height L and we are concerned with the electric field at her surface, thus at distance r = R. Substituting all this into the expression for E, setting the field to the critical value, and solving for Q, we have

Q = RLE / 2k = 2.4 X 10-5 coulombs,

which is equivalent to 24 microcoulombs. If an upward streamer had formed, at least this amount of negative charge from the ionized air molecules would have flowed through the woman’s body in a few microseconds, producing a current of several amperes, more than enough to injure or kill.

My point here is simple (simple enough to be on a tee-shirt): If your hair stands up, don’t pose for a photograph --- run for shelter.

References
Dots · through ··· indicate level of difficulty
Journal reference style: author, journal, volume, pages (date)
· Krider, E. P., and C. G. Ladd, “Upward streamers in lightning discharges to mountainous terrain,” Weather, 30, 77-81 (1975)
· Krider, E. P., and R. H. Wetmore, “Upward streamers produced by a lightning strike to radio transmission towers,” Journal of Geophysical Research, 92, No. D8, 9859-9862 (20 August 1987)
· Smith, T., “On lightning: hair standing on end may be warning of an impending strike,” British Medical Journal, 303, No. 6817, 1563 (21-28 December 1991)
· Anderson, R. B., “Does a fifth mechanism exist to explain lightning injuries?” IEEE Engineering in Medicine and Biology, 20, No. 1, 105-113 (January/February 2001). This paper studies the South African tragedy.
· Cooper, M. A., “A fifth mechanism of lightning injury,” Academic Emergency Medicine, 9, No. 2, 172-174 (February 2002)
··· Morrow, R., and T. R. Blackburn, “The stepped nature of lightning, and the upward connecting streamer,” Journal of Physics D: Applied Physics, 35, L69-L73 (2002)


Are you looking for the stories from last month? They are in the archives:

Loop-the-loop with cars, bikes, skateboards, and simply running, 1.270
Pub  trick --- spoon into mug, 1.271
Transparent when wet, 6.86
Hula-hoop, 1.84

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