TREES AND LIGHTNING.

Sun (Christchurch), Volume I, Issue 160, 12 August 1914, Page 11

TREES AND LIGHTNING.

SOMETHING OF THE RISKS.

■ls it, as- is usually supposed,. moire dangerous to'take refuge under trees in a thunderstorm than to remain in an open field ? As a matter of fact, statistics rather .favour the tree aud .so does what' dim reasoning science has. so far

been able to bring to bear (says a , writer in an English exchange). In | Hungary, during 1901 to 190:5, of the j total number of- ascertained deaths by j lightning, 15 per cent, occurred .under trees arttl 57 per cent, wholly in the open. In the United States in 1900 only 10 per cent, of the deaths where the precise conditions were ascertained occurred under trees, as against 52 per cent, in the open. . If the risk under trees ..exceeds the risk in the open, at least live or six times as many people, must have, remained in the open as sought shelter under trees. .The deaths in closely built industrial areas, though, always less than in rural districts, are considerable, yet we hear very little about them. The tree gets all the notoriety. Certainly, an isolated tree in an exposed position is, On a commonsense view, more likely to be struck than the much less tall human being in a similar position, but even this is assuming for lightningy a conformity with regular electrical laws which scientific observation does not positively support. Something would appear to.: depend upon the species of tree. Records taken for a good .many years in Lippe give the following order of the liability of different- trees to lightning stroke (the figures being corrected to the assumption that!.the total number of each species : -of tree in the area Avas the same)r-Oak 57,• fir 39, pine s,.beech 1. Observation in Styria makes oak, poplar, and pear trees, the most liable to be struck, and , beech, again, exceptionally safe.- The apparent differences -may,and probably, are, to same..externt influenced! by the height of the-; tree, its .proximity to wafer, and the nature of the. soil.! .In Germany the liability of different -soils to be the recipients of lightning has. been put 1 at—chalk l;,j clay 7, sand 9j and-loam 1 • i • TREES IN OPEN FIELD. With' regard to the reasoning of science on the question of tree versus open lielTl, most experts refrain from dogmatism. A few main pohits ' may be given'. To begin with, we need not go' beyond the early Victorian text-book Conception of electricity as being of two' kinds', positive and negative, and of charges of opposite kinds trying to come 1 together. " Usually, biit" not always, the upper atmosphere is posi-: tively charged in relation, to the lower atmosplfere and the earth itself, and consequently there is a neutralising current from aloft to earth. This current is very slight in any given area of observation because the resistance of dry air is ; very great. The special conditions necessary to produce a lightning flash simply the conditions sary to increase greatly the intensity of the charge, or its voltfige," in some given area aloft. ' ' ; v Two 1 influences will bring this about. First, 1 sunshine increases the! positive electricity ; in the upper air;-. But the increase is, we may suppose, fairly uniform over an enormous area;. ! Every cloud partakes of if. Then comes the | second, influence. . Each of j ; the tiny ! (lrop's"'bf 'Water "which liiake the" cloud carries its own little charge 1 of' electricity/' all charges being of the, same intensity 'or voltage. The intensity depends dn'the actual amount Of the electricity and on the extent of Mi-face to : I which it adheres. Increase the amount of .electricity or decrease the -.surface,; ami,up,g#es the intensity, pressure, or! " voltjige" of the charge, , and,: conse(pieiitiy,, .its effort' toi. escape. The. cloud becomes a' rain cloud by the process, of' coalescence between the originally tiny drops. Efiit a. big..drop lias . l<?ss than j twice the surface of the two small ones which compose it, so the pressure of the jcharge goes up. % v One or two things may happen first: i The cloud 'may come down as rain, | bringing its electricity harmlessly with ! it;' or the electricity may come down | violently as a lightning flash, followed | by the cloud itself as a thunder shower, j A flash happening in this way has been I given the title of an "A" flash, or in j Germany, JKaJtum (not causing lire). J Its action <&n: be anticipated very often by a lightning conductor, the conductor, showing by a "brush" discharge of electrk'ity that its point or points are being made a passage for the neutralising current between cloud and earth; and if the flash does follow, in spite of this relief, it will probably get to earth through the same channel. A "B" FLASH. A "B" .flash, known to German scientists as Zundenden, or "causing fire," is much more serious and elusive. A preliminary lightning flash between one cloud ami another cloud, which is common and in itself quite harmless, [results in leaving some cloud of the | group with a much more . intense, unj neutralised charge than it. before; and this charge waits for nothing. It goes to earfh wit.li such suddenness and violence that it may pass quite near to a well-designed lightning conductor and shatter , a brick , chimney. If may go .straight, to earth, while there are trees and people, dotted all around. It mayavoid striking the flat land even, and go straight- to the bottom of a quarry or a sandpit, as liappeued recently. It may depart, as photographic observation has shoAvn, from a fairly straight course .and .dart, aside to a falling raindrop, then back to another rain drop, in apparent contradiction of the Jaw that electricity, takes the line of least resistance. .Similarly, it may leave its easiest course to strike an insulated metal object, apparently in ignorance of the fact, that the metal dors- not present a path to earth. THE SE VER AL CI I AN (' IOS. Sum up the several chances. If you are in the open you m.iy be struck, because you .lessen the length of the current's path. If you are uiider a tree, but not touching the trunk or too 1 near to it, the tree, Being taller than you, may take what- you otherwise would get. and so save you. If you are in a clump of trees ton'get, as it were, the prot.eejtion of hi I of them, and only the lesser j danger of your own particular tree b'e- ! i.ncr struck,' assuming that there is a | danger in being under or ('lpse to a j struck tree'. In this assumption we come to what is perhaps .the' crux of the question. Some flashes.give off dangerous side-strokes. Are such flashes always "B" flashes? It is considered almost certain that they are, and so one ,j might, be under a tree struck by an :i"A." flash and take no harm. Thatj brings us to ask. What is the usual proportion ,of A" to "B'' flashesf We do not know. If the "B" flashes were very rafe, iv : e might go under a louely : tree out of the rain and feel.fairly safe, if the. "B" flashes preponderate, Ave may still go into a clump of trees and

feel that our chance of harm is 110 worse than in the open. But to stand close under an isolated tree is, in spite of statistics, more than one has the courage to recommend.