WHAT MAKES THUNDER?

Hokitika Guardian, 1 September 1923, Page 3

WHAT MAKES THUNDER?

(Bv a Meteorological Expert in Daily Mail"). One of the mysteries of such thunderstorms as have occurred in Great Britain during the last forty-eight hours is the relative smallness of the damage done in proportion to the terrific character of the forces operating. The impression produced yesterday morning, for instance, by some of the discharges was that of the firing of

a very heavy gun—l2in. or loin. There was the same concussion in the atmosphere ; the same crackling, rushing noise, and the same kind of report. Yet actually the power exerted by n single flasli is enormously greater than that of any human gun yet made. .Measurements recently carried out by Air ('. E. Wilson at Cambridge, not without considerable risk to himself, show that the power exerted in a flash is usually about 2,000,00011. tons, which would suffice to raise a weight of 2,000 tons I,oooft in file air. The power of (be British loin naval gun is 82,300 ft lons, or less flinn piietwcntictli of that developed hv the lightning discharge. The modern view of the thundercloud is that it is an .electreial generator, by which electrical currents are maintained between the earth (itself a powerful electric machine generating or carrying stupendous ebarges) and the conducting layer of the upper atmosphere. The upper atmosphere appears to be markedly affected when it thunderstorm is in progress below it. .Cathode rays, positive rays and Xrnys seem to he freely generated. The curious continual flickering of steely blue to green anil lilac light, which accompanies very had thunderstorms, may be due to these rays in the upper atmosphere, which is also the region where auroras show their marvellous colours.

The safest place in a thunderstorm is inside a house, or building. In London, lightning eonducinrs are not usually fitted except to very high structures, because of the large degree of protection given by high chimneys, spires, anil the mass of wires, nil ol winch contribute to iniercept the electrical force and send it harmlessly to the ground. But it is always wise in a heavy thunderstorm to get under shelter; the statistics proving that in a year only one person in a million is struck in this eoiiutrv do not apply lo those out of doors. Trees—il tony are mil isolated tree- —are salcr than the open, hut an isolated tree is the most dangerous shelter of all. The distance of a storm or of the centre of discharge can be calculated quickie bv counting the seconds that pass between ‘the Hash and the report of thunder. If live seconds elapse, the distance of the discharge is a mile; if the report is almost instantaneously heard, with only a second’s’ interval or less, the disturbance is overhead. On the whole, London is tolerably free from tlmmlerstorms. On the records of 1..0 years it lias :m average of 11 storms a year, as against (i Tor Edinburgh, 27 for Paris, and 120 for Batavia, in the Dutch East Indies. There is a well-marked curve showing the times when thunderstorms arc most prevalent, anil in London this curve reaches its peak between noon anil -1 in the afternoon.