WMOD00311

• Title: Harold D. Orville Symposium - Forty Years of Modeling Clouds and Weather Modification
• Date: 4/26/2003
• Weather Modification - Doc Type: Report

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THUNDERSTORM ELECTRICAL MODELING - THE ORVILLE LEGACY

John H. Helsdon
Institute of Atmospheric Sciences
South Dakota School of Mines and Technology
Rapid City, South Dakota 57701

1. INTRODUCTION

The study of thunderstorm electrification
dates back to the time of Benjamin Franklin and
his now-famous kite experiment (conceived in
1750 without the use of a kite and carried out in
June 1752), wherein he demonstrated that
thunderstorms, and lightning in particular, were
electrical in nature. Credit should also be given to
Frenchman, T. F. d'Alibard, who carried out
Franklin's originally-conceived experiment in
France in May of 1752. Subsequent research in
the field was spotty, mainly involving refinements
and duplications of Franklin's and d'Alibard's work.
In the 1850s and 60s LQrd Kelvin tyV.
Thomson) established that the electrical state of
the atmosphere could be represented as an
electric field and introduced the electric potential
and lines of force to help explain atmospheric
electrical phenomena. He also established the
first (and still operating) benchmark station in Kew,
England for measuring the earth's electric field.
The next development came in 1898 when J.
J. Thomson formulated the theory of ions. The
fact that the atmosphere was not an insulator, but
had a finite conductivity due to the presence of
ions helped explain several previous observations
including the relationship between the universal
diurnal variation of the observed electric field and
the frequency of thunderstorms. This also led to
the development of theories concerning the
electrification of thunderstorms. While many
theories or variations on theories currently exist,
only a few of the more prominent ones will be
mentioned here. For more details the reader is
referred to summary articles, such as those by
Mason (1972), Latham (1981), Williams (1985),
Beard and Ochs (1986), and Saunders (1993).
The first useful theory of charge separation
proposed was that of Elster and Geitel (1913). In
our present system of classification of
electrification theories (convective, inductive,
noninductive) theirs was an inductive theory
wherein two electrically neutral, colliding drops are
polarized in the ambient electric field. Upon
collision, the contacting hemispheres having
opposite induced charge, exchange charge (if the
drops do not coalesce), leading to gravitational

separation of opposite charges giving rise to the
electric field. The primary question regarding this
theory is, how readily do drops colliding in an
electric field separate rather than coalesce?
The next major contribution was that due to
. Wilson (1929) who proposed another inductive-
type mechanism. Wilson proposed that polarized
falling raindrops would preferentially capture ions
of one sign while smaller cloud droplets would
capture ions of the opposite sign followed by
gravitational separation (the so-called Wilson
effect). The sign of the charge attached to the
hydrometeors is a function of the ion speed in the
electric field relative to the fall speed of the drops.
The various combinations for attachment were
worked out by Whipple and Chalmers (1944).
Another contribution in the realm of inductive
charging was that of Muller-Hillebrand (1954).
Here, rather than water/water interactions, the
author considered collisions between ice crystals
and graupel particles in an electric field. While the
probability of such collisions resulting in separation
events is much greater than for water/water
interactions, questions have been raised because
of a relaxation time limitation on charge migration
between interacting ice surfaces.
The so-called convective hypothesis was
proposed independently by Grenet (1947) and
Vonnegut (1953, 1955). In this theory the polarity
of the thundercloud charge structure is determined
by the initial electric field and space charge
distribution at the time of cloud development.
According to Vonnegut and Moore (1958), net
positive ionic space charge is carried aloft in the
developing cloud updraft creating a positively
charged cloud through ion attachment to cloud
droplets (the Wilson effect). As the positive' cloud
penetrates levels of higher conductivity, a current
of negative ions from aloft leads to the formation of
a negative charge layer at cloud top, which is
carried to lower levels by cloud-edge downdrafts
where it is entrained into the cloud leading to the
formation of the classic positive-over-negative
dipole. The final stage is the creation of an
electric field at the surface strong enough to cause
positive corona discharge leading to a feedback
process.
The final major addition to charge separation

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