Lightning

Where Will It Strike?

By Doug Criner

Annually in the U.S., lightning kills several hundred people.  Also, it causes untold millions of dollars of losses due to structural damage, fire, electrical power outages, and damage to sensitive electronic equipment.

Surprisingly, the physical causes of lightning remain somewhat elusive.  Within a thunderstorm, there is a partially understood tempest of wind, water, and ice–all in the presence of gravitational and temperature gradients.  These elements give rise to electrically charged regions of thunderclouds.

The following are typical lightning data reported in the literature:

Where it Strikes

Even as the causes of lightning are poorly understood, there are gaps in the scientific knowledge about lightning's characteristics (amperage, rates of rise, and frequency content).  Also, there is the age-old question of what terrestrial features "attract" lightning.

A lightning strike begins with a stepped leader, a low-power stroke that progressively propagates downward toward earth in discrete jumps, each about 50 m in length.  The stepped leader traverses a path without regard to local terrestrial features until it reaches within about 10–100 m of earth or the top of an elevated structure.  At that point, a much more powerful return stroke emanates from ground, meeting the stepped leader.  The electrical circuit is then "complete," and several repetitive strokes flow along the established path.

So, lightning's path is unpredictable or "mindless" until it reaches within 10–100 m of earth.  Only then do the geometry and electrical characteristics of the earth and structures affect the lightning's striking point.  This explains why lightning often strikes a point that seems less inviting than another nearby target.

The stepped leader takes a relatively long time to reach ground (about 20 msec).  This sequence may explain this engineer's perception of a faint, audible "click" an instant before a nearby thunderclap.

Because lightning is essentially a high-frequency signal, it is not a simple "Ohms Law" problem to predict its behavior.  For example, a sharp bend in a ground wire may present enough high-frequency impedance to divert the current to another, less obvious path.

Lightning Protection

A system of grounded lightning rods (the technical term is "air terminals") can shield critical facilities.  Protection is afforded by purposely inducing the return stroke to initiate from the lightning rod, thus diverting the current directly to ground.  National Fire Protection Association codes provide useful guidance on the design of such protective systems.

Properly grounded, modern steel-framed buildings have good inherent protection against lightning.  However, during construction, hazardous conditions can occur while grounding is incomplete.  Special risks, such as explosive materials, tanks, and chimneys, require special treatment.

Wood structures are particularly vulnerable to lightning damage.  Of course, there is the hazard of fire.  Also, wood struck by lightning heats up instantly, causing moisture in the wood to vaporize rapidly–which can lead to explosive forces that are very destructive.

Electric power systems are particularly susceptible to lightning.  Wiring offers a convenient path throughout a building for lightning to travel and do its devilment.  A measure of protection can be attained by installing lightning arrestors on electrical systems and by installing surge suppressors for sensitive electronic equipment.

Modern arrestors and suppressors include metal oxide varistors, arc-discharge devices, and various semiconductor devices.  Selection of type and electrical rating is always a compromise.  Since the simulation of a lightning stroke is rather impractical, testing is performed by Nature itself.  Compounding the design problem, manufacturers of computers and other electronic equipment seldom specify the maximum voltage spikes that their equipment can withstand.

Interestingly, some observers attribute more computer crashes to thunder than to lightning.  A loud thunderclap can shake a building and cause voltage transients due to loose wiring connections.

Suggested Reading List

1.  Lightning, Martin A. Uman, Dover Publications, Inc., New York, 1984.

2.  "Protecting Industrial Plants Against Lightning's Dangers," Marvin M. Frydenlund, Consulting/Specifying Engineer, September 1990, pp. 60-68.

3."Thunder," Arthur A. Few, Scientific American, pp. 80-90.  (My copy of this article does not reveal the date of issue.)

4.  "The Electrification of Thunderstorms," Earle R. Williams, Scientific American, November 1988, pp. 88-99.

5.  "Lightning Protection Code," National Fire Protection Association, NFPA 78.

6.  Tree Maintenance, P.P. Pirone, 6th edition, 1988, pp. 146-147.  (Protection of trees against lightning and treatment of tree damage due to lightning.)

® Doug Criner, June 2002