Lightning Rods: Recent Investigations
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
- Leader departs cloud in steps of 1 us duration.
- Length of leader is in tens of meters.
- Pause time between steps is 20-50 us.
- A fully developed leaders can lower 10 coulombs or more
- Downward speed of propagation is about 2 X 105 ms-1.
- Average leader current is about 100-1000A.
- Leader step peak pulse currents are at least 1 kA.
- Leader pauses produce other downward branches.
- Potential difference between the leader and the earth is
in excess of 107 V.
- Breakdown occurs in excess of 107 and streamers move to
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.
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.
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.
W. Rison, et al, Proceddings 24th ICLP, Birmingham UK, Sept 14, 1998.
Note: See ongoing NLSI
Lightning Rod Research