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PUBLISHER’S LETTER www.siliconchip.com.au Publisher & Editor-in-Chief Leo Simpson, B.Bus., FAICD Production Manager Greg Swain, B.Sc.(Hons.) Technical Staff John Clarke, B.E.(Elec.) Peter Smith Ross Tester Jim Rowe, B.A., B.Sc, VK2ZLO Rick Walters Reader Services Ann Jenkinson Advertising Enquiries Leo Simpson Phone (02) 9979 5644 Fax (02) 9979 6503 Regular Contributors Brendan Akhurst Rodney Champness, VK3UG Julian Edgar, Dip.T.(Sec.), B.Ed Mike Sheriff, B.Sc, VK2YFK Philip Watson, MIREE, VK2ZPW Bob Young SILICON CHIP is published 12 times a year by Silicon Chip Publications Pty Ltd. ACN 003 205 490. ABN 49 003 205 490 All material copyright ©. No part of this publication may be reproduced without the written consent of the publisher. Printing: Hannanprint, Noble Park, Victoria. Distribution: Network Distribution Company. Subscription rates: $69.50 per year in Australia. For overseas rates, see the subscription page in this issue. Editorial & advertising offices: Unit 8, 101 Darley St, Mona Vale, NSW 2103. Postal address: PO Box 139, Collaroy Beach, NSW 2097. Phone (02) 9979 5644. Fax (02) 9979 6503. E-mail: silchip<at>siliconchip.com.au ISSN 1030-2662 Thunderstorms – nature’s monster light show! As we go to press, New South Wales is getting a fresh bout of drought-breaking rains over the state. In fact, it has been bucketing down over wide areas. No-one is complaining though; after such a severe drought, city dwellers are happy to endure the rain, in the hope that country districts are getting their fair share. But I enjoy it for another reason - I love thunder­storms. We haven’t really had a lot of thunderstorms in Sydney lately, having missed out on the usual summer storms because of the drought. So why do I like thunderstorms? Well, perhaps I had better qualify that. I don’t actually like being out in them, getting wet. I am not keen on that at all. I am also concerned about damage to electrical and electronic equipment during storms, so that is another negative. If a big storm is coming close, I go around the house and disconnect just about everything that is practical. The reason I love thunderstorms is the great spectacle - nature’s monster sound and light show. I like to sit in a dark­ened room with the curtains open, watching the progress of storm cells as they move up the coast. And while lightning strikes to ground can be very spectacular, the real fascination is in the constant and ever-changing internal lighting of storm clouds - so-called “sheet lighting” or cloud-to-cloud discharges. In fact, even when storm cells are a very long distance away, so far that no thunder can be heard, the constantly flickering light in the clouds can be marvellous. Just why is the electrical charge within the cloud bank changing so constantly? One reason is that each lightning strike causes the local charge distribution to be radically altered and it then has to equalise within the rest of the cloud. Another is that the storm cell is dynamic, with massive up-draughts and down-draughts, as more moist air is sucked in. I like to think of the charge distribution within a large cloud as akin to that on the ultor electrode on the back of your TV’s CRT (or the moving plate in an electrostatic loudspeaker). This large sheet electrode is a poor conductor and each local discharge (ie, lightning strike) causes all the charge distribu­ tion to readjust (the sheet lightning). Nor does this happen instantaneously and it can take several seconds for the disloca­tion caused by one lightning strike to ripple all around the cloud mass which can be huge – perhaps 50km or more across in a big storm system. All this happens constantly and so we have a wonderful random light display. And of course, each lightning strike that we see to ground is accompanied by an unseen equivalent discharge up into the stratosphere - the so-called “sprites” observed by astronauts. Sprites are a pinky, red colour, just what you would expect from an electrical discharge in a near-vacuum. With all that going on and the enormous energy involved in the dumping of perhaps millions or even billions of tonnes of water onto the land, how can you possibly watch some trivial show on TV during a big storm? Turn it off and watch nature’s vast and wonderful spectacle! Leo Simpson * Recommended and maximum price only. 2  Silicon Chip www.siliconchip.com.au