RANDOM NOTES

Dominion, Volume 32, Issue 68, 13 December 1938, Page 10

RANDOM NOTES

Sidelights On Current

Events

(By Kickshaws).

We note that a German doctor claims he can smell colour, and all we hope is that the fellow never meets a rainbow.

It looks as if Eden is telling the people of America that the place he comes from is no paradise.

It is stated that campers are attracted by the more elaborate type of caravan. People just have to put up with the discomforts of the home until the camping season starts.

“Our part of the country has just been visited by a violent thunder and lightning storm, with the result that conversation has turned to reasons for the thunder and lightning. Could you please tell me, in your column, the following?” says “Emma”“(1) Does every flash of lightning have to make contact with the earth? (2) Coul'd you tell us briefly what causes the thunder and lightning? I am sure you will earn the gratitude of many of your readers if you will answer these.”

In view of the fact that Wellington has been honoured with two thunderstorms within a week, perhaps it is timely to explain a few details about this sort of visitation. Every flash of lightning does not have to make contact with the earth. There are three types of lightning discharge. The most usual is from cloud to cloud. The next is from cloud to earth. The third is from earth to cloud. The last is the rarest and most 'dangerous. The second is the more usual. The first provides ns with what is often called summer lightning. The thunderstorm we had about a week ago was of the second type. Although the cloud to cloud discharge is usually harmless, this need not necessarily be the case. In mountainous country it is uncomfortable to form part of the conducting medium of a cloud to cloud discharge. Moreover, heavy lightning discharges, whatever be their type, produce induced effects in their vicinity which may be nearly as deadly as the main discharge.

The mechanism of a thunderstorm is still only partially understood. The problem is to discover a method whereby electrical voltages of 200,000,000 or even more may be built up. Man has only been able to produce artificially a voltage of 2,000,000. We know that! the earth has a negative charge of electricity. We know that the air conducts electricity very feebly until the main arc has been struck, when the conductivity becomes better. Even jf the earth were generating electricity in its interior, as has been suspected, it would not explain the immense voltages that are built up between cloud and cloud or cloud and earth. It is believed that the huge electrical voltages built up are produced from a combination of a rapid upward wind and falling rain. A drqg of water falling through an upward Wind breaks up into smaller drops. The water is positively charged, and the surrounding air negatively charged. Experiment proves this to be the case. In order to produce a thunderstorm, however, the upward wind must first converge and then diverge. At the centre the wind is more rapid than at the top or bottom. This assumption has never been proved, partly because nobody has been there to see.

Provided one has atmospheric conditions as depicted in the preceding paragraph the scene is set for a firstclass thunderstorm. Raindrops fall from a high, cold region. They- increase in size as they fall because they are cold: water vapour condenses on them like it does on a cold pane. Eventually they grow so large they break up. When they reach the narrow centre of the rising current of air they have broken so small that the wind carries them up again. Each drop carries up its positive charge of electricity. When the drops break the positive charges increase, as two drops will have two positive charges instead of the one that the larger drop had. This means that the voltage is building up all the time. The rising drops grow again and start to fall when they reach a weight too heavy for the rising wind to lift. These drops fall and break up once more. This process is continually repeated until tremendous voltages are produced. Eventually tlje positive charges build up such a huge voltage that a flash results, either to another cloud or to earth. The thunderstorm is merely the noise of the smack that the air gets when the flash disrupts it. That, then, is the theory of a thunderstorm.

Improvements in the technique of taking high-speed measurements has enabled experts to obtain numerous facts about the mechanism of a flash of lightning that was not known before A camera with a film that moves at the rate of over a mile a minute, has enabled experts to take 10 separate pictures of a single flash of lightning. The pictures show that a flash of lightning does not strike with a single bolt. The charge that flashes from cloud to earth returns after a few millionths of a second. Alternations continue until electrical equilibrium is restored. Moreover, before the flash takes place, a curious dart of light precedes the flash. The speed of this dart or leader is about 18ft. every millionth of a second. The main flash travels at the rate of 150 feet in a millionth of a second. After the flash the air remains illuminated for about one two-thousandth of a second. A lightning flash takes, in all. about one seven-thousandth of a second to complete itself, depending on the length of the flash, which travels at about a sixth of the speed of light.

The amount of electrical energy in a flash of lightning varies tremendously. It may be as little as that contained in a flash-lamp cell or as much as man could supply at a cost of £9OO. The damage done is more or loss in proportion. The man who sees the flash and hears the thunder is not the man to be struck. Thunder travels at about 1000 feet a second. The light from the flash of lightning travels at 186.000 miles n second. A person struck might see the flash, but be would not lie.tr the thunder until he had been struck. Nevertheless, when caught in a thunderstorm, it is unwise to shelter under a lone tree. It is dangerous to shelter under even a small tree if it is beside water. It is not so dangerous to shelter under a low tree among taller trees, and it is comparatively safe to shelter in a clearing in bush. Oak, poplar and elms are struck most frequently. If caught in the open it is safer to lie down than to stand up. Sica in from the body affords an attraction to lightning. ail'd for that reason it is better to put on a coat. It is most unwise to shelter near a wire fence. A flash that strikes a mile away may electrify the whole fence. Moreover, it is not wise to remain under power lines er relej bone lines during a storm. Nevertheless. despite the fact that there are 2060 thunderstorms in the world every minute, only n few hundred people are killed each year out of 2,060,000,000.