Added on 29/12/2011Article

Lightning Strikes and Trees


By Chris Skellern



Introduction


A Lightning strike is a gigantic arc of electricity jumping between clouds or jumping between clouds and ground. The arc occurs once the potential difference (PD), measured in volts, between two points becomes so great that the insulation between the two points breaks down. Air is generally a good insulator against electricity while metals and water are not. However, air can become ionised due to a very high PD and actually start to conduct the electricity. During a storm a potential difference of over 100 million volts can exist between the charged cloud and ground. Once a strike occurs, a current of over 20,000 amperes can discharge causing instantaneous heating of the surrounding area. They can produce temperatures up to 50,000 º F. Thunder is the result of the air around the strike becoming superheated.

Trees act as a convenient conductor to ground, especially when wet due to rainfall. The sapwood under the trees bark also provides a favourable conducting route to ground. When the sap under the trees bark is exposed to the intense heat produced by the current surge, it turns instantly to steam which then literally 'blows' a vertical strip of bark of the tree. This would happen instantaneously and with great force. And this is the cause of the tree damage, high pressure steam needing to escape to a low pressure area. If a pocket of sap deeper within the tree structure is turned into steam then the damage will be more explosive with branches or stems splitting open.

[Please note: It has been pointed out to me that the same damage may actually be caused by the shock wave from around the strike rather than the sap becoming a gas].



The above photographs were taken early in the New Forest. The tree is a 75-80ft Douglas Fir (Pseudotsuga menziesii). The strike appears quite recent as the exposed wood is still fairly fresh. The above picture is a close up of the damage showing resin formation (still sticky in places).

The following shots were taken by Jonathon Price close to the M4 who thinks the tree was struck on the 3/8/04.



Lightning Conductors


Important trees can be fitted with lightning conductors to help protect them against the damage caused by lightning. Such a device would not prevent them from being struck, but will just provide a more attractive route for the lightning surge to take, if struck.

A typical lighting conducting system consists of an air terminal (the metal 'spike' at the top of the tree), the conducting pathway (the metal wire or strip to take the surge down the tree) and the earthing system (a staking system to connect the cabling to ground).

Current best practice for the installation of lightning conductors (in the UK) is set out in BS6651:1999. However, this document only briefly addresses the subject of trees and fails to take account of many arboricultural constraints. As an example, the direct effects on the tree by the installation of the conducting hardware is not considered, with the result that trees are likely to experience some degree of long-term harm. In the case of the conducting wire, as the tree grows, the conductor and its fixing screws will become encapsulated within the bark. If a lightning surge were now to occur the damage to the tree bark would be extensive. Such an installation would need to be constantly inspected and then removed/replaced when encapsulation begins.

Jerry Ross B.Sc F.Arbor.A and Ben Fuest MD have spent much time investigating the whole subject of lightning protection and the various systems used in the UK and USA. Ben has developed a usable system complete with a specially designed (patented) fixing bolt which addresses the problems of adjustment. Its called the Arborbolt™ and is available for sale to other arboriculturists who have the technical knowledge and experience in lightning conductor installation.

The Arborbolt™ is specially designed to allow the conductor to move with the tree bark growth but at the same time to keep the conductor in contact with the bark surface (see following picture).



The design is clever as it allows extension cylinders to be added once bark growth approaches the top of the current cylinder. Thus, the screw fixing only need installing once. The use of stainless steel prevents corrosion.

Lightning conductors should only be fitted by those with the appropriate technical knowledge. An incorrectly designed or fitted system can be useless to the tree and dangerous to nearby homes. Each individual tree must have a system specifically tailored to suit it. It must be electronically tested to ensure its resistance is low enough to be of use. Most arboriculturists will not have the necessary knowledge (or insurance) to attempt such an installation so should consider contacting an experienced team for guidance.

Many thanks to Jerry & Ben for the supply of the Arborbolt image and related information.

Arborbolts are available from Ben Fuest MD. The Allt Farm, Llansantffraed, Talybont on Usk, Brecon, Powys LD3 7YF. Tel 01874 676 489 Email: benfuest@ukonline.co.uk