~ National Lightning Safety Institute ~

Section 5.4.2

A contemporary understanding of lightning's "leader - streamer" profile is available to us through research from Schonland (1927,1933), Trichel (1938), Chalmers (1957), Loeb (1965), Dawson (1970), Winn/Moore (1971), Phelps (1974), Llewellyn (1976), Vorgucic (1978), Moore/Brook/Krider (1981), Uman (1987), Stahmann (1993), Berger (1994), Fisher/Schnetzer (1994), and other contributors. Uman, for example, describes the process as:

1. Leader departs cloud in steps of 1 us duration.
2. Length of leader is in tens of meters.
3. Pause time between steps is 20-50 us.
4. A fully developed leaders can lower 10 coulombs or more of charge.
5. Downward speed of propagation is about 2 X 105 ms-1.
6. Average leader current is about 100-1000A.
7. Leader step peak pulse currents are at least 1 kA.
8. Leader pauses produce other downward branches.
9. Potential difference between the leader and the earth is in excess of 107 V.
10. Breakdown occurs in excess of 107 and streamers move to the leader.

The present paper will focus on streamer launch models and their associated current intensities. The generally accepted electrogeometric behavior model holds that step leaders influence the electrification processes of ground-based objects. The approximate stages of this activity are: 1) ionization, 2) corona, or St. Elmo's Fire, and 3) arcing or launching of upward streamers.

Moore, Llewellyn and others have proposed that the behavior of blunt vs. pointed rods can be explained by current intensities induced by leader electric field.

From Moore on outdoor experiments in New Mexico (1):
"With lightning at great distances only the sharpened rod emitted point discharge currents but for the larger field changes with nearer lightning strokes the blunt rod emitted transient burst of ions also. The behavior of the two rods shows a significant difference as the field changes become larger yet. The sharp rod at all times readily the charges required by the electric field whereas the blunt rod usually emitted little charge until the field became very intense. With the approach of the negative leader, it then typically emitted large burst of positive charge... our results indicate that the sharpened rod usually acted to protect itself by emitting ions whenever the electric field exceeded the breakdown threshold. The blunt rod, on the other hand, emitted ions with great difficulty...the fields around the blunt rod often large values that when breakdown did occur at the blunt rod a positive streamer could propagate for appreciable distances away from the blunt rod. It thus appears to us that there is a significant difference in the response of a sharp rod from that of a blunt rod during the development of the lightning discharge."

Moore measured the pointed rod (0.1 mm dia.) Efield breakdown as being 1.1 X 107 V/m, while the blunt rod (1 cm dia.) Efield breakdown was some 3.7 X 106 V/m.

From Llewellyn on electrostatic fields & corona around tall towers (2):
[consider]..."two towers 30 m high with a radius curvature [of one with sharp point of] 1/10 mm and [the other with blunt point of] 10 cm. Lines of equal value were drawn for the exposure factors in an area around the top of the towers, again using double logarithmic scales to show detail near and far. The enhancement at the tip is of the order of 10,000 for the sharp point but only 100 for the blunt point. This means that only fields of the order of 100/V/m are required for the sharp point to be in corona, which is in agreement with our experimental results from a sharp point giving up to 1/4 uA current in fair weather fields. For the blunt point, however, storm conditions of 10,000 V/m are required before corona is given off...The sharp point goes into corona in low fields and just immediately around the tip, the blunt point goes into corona only in high fields, but out to greater distances from the tower."

Field trials in the summers of 1994-98 at Langmuir Labs (New Mexico Institute of Mining and Technology, Socorro NM) have provided electric current measurements flowing from various shaped lightning rods. These were correlated to changes in electric fields caused by lightning discharges. Results of this work confirm earlier theoretical models about sharp vs blunt rod behavior. (3) Sharp rods are poorer receptors for lightning than blunt rods.

References:

1. Moore, CB, Study of Sharp and Blunt Lightning Rods in Strong Electric Fields, published in Review of Lightning Protection Technology for Tall Structures, ONR/NASA/FAA/USAF, Nov. 6, 1975.

2. Llewellyn, SK, Theoretical Investigation of Electric Fields and Corona Around Tower Structures, published in Review of Lightning Ptorection Technology for Tall Structures, ONR/NASA/FAA/USAF, Nov. 6, 1975.

3. W. Rison, et al, Proceddings 24th ICLP, Birmingham UK, Sept 14, 1998.

Note: See ongoing NLSI Lightning Rod Research