WHEN LIGHTNING FLASHES

Nelson Evening Mail, Volume LVI, 9 April 1924, Page 8

WHEN LIGHTNING FLASHES

MECHANISM OF A THUNDER

STORM

In his .inaugural address to the Institution of Electrical Engineers, Dr Alexander Russell gave the following description of the mechanism of a thunder stonn: — Owing to the small potential differences produced by the ordinary earthair electric current, and consequently the minute amount of electrostatic energy stored in the field, wc are justified in assuming that the most important function of this current is in assisting to start the storm. A thunder storm usually travels over' considerable distances, and keeps on general mg prodigious quantities _of electrical energy. It functions like and electrostatic machine driven by a very powerful motor. We have to consider what produces the energy which the lightning flash converts into heat, light, sound and radio waves. Aviators and balloonists report that the air in the centre of a thunder cloud is practically always moving upwards with considerable velocity. In the upper layers of the troposphere, owing to ionisation, there are sometimes many free electrons. If the humidity of'the upward current of air be high enough the vapour condenses round the electrons, and so the air is full of minute globules of water. Some of these coalesce together, forming larger drops. If the vertical velocity of the air be greater than 8m per secern! (17 miles per hour) the largo drops, as they are blown upwards, often break into smaller .drops. Experiment shows that the charge of the smaller drops is generally negative, and on the larger drops positive. As the larger drops are often kept, oscillating in the higher and very cold strata of the atmosphere, owing to fluctuations in the velocity of the air currents, they keep melting and freezing again, producing ordinary hailstones • As the gusts vary we have the large drops and hailstones positively charged descending. Electrical eueigy, due partly to the work done by 1 gravity on the descending drops, is stored in the atmosphere. On the upper part of the cloud we have a negatively charged layer and on the lower part a positively charged layer. The falling drops keep increasing the potential difference until it gets so high that disruptive charge ensues. We have imagined the cloud to he kqxdfjr. Several kinds of electric discharge pan therefore ensue. We may have brush discharges between the upper layer and the better conducting layers higher up. Some kinds of sheet lightning could be caused in this way. On a summer evening it is not very unusual to see sheet lightning in broad flashes coming from a cloud and I ending in the upper air. An ordinary I flash also has sometimes been observed to end in clear air. If another cloud j drifts up we may have a flash between j the upper layer and this cloud. We ' may have flash discharges between the lower layer and the earth or another j cloud. Possibly also we may have a ! flash-over between the two poles of the thunder ploud rfhen they are not too far apart.