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THE LAST WORD Counted out I remember being told that when you see a flash of lightning and start counting until you hear the thunderclap, the number you have counted to will amount to the number of miles distant the storm is. Is this true? If not, is there any way of calculating the distance based on the observed time difference between lightning and thunder?
n You can estimate the distance lightning is from you by counting seconds until you hear thunder, but it equates to more than 1 second per mile. Sound travels at approximately 343 metres per second in air at 20°C. That means it covers just over 1 kilometre in 3 seconds, and a bit more than 1 mile in 5 seconds. So counting the seconds between the flash and thunder gives about 1 kilometre for every 3 seconds and 1 mile for every 5 seconds. Because sound dies away with distance, if you see a flash, but hear no thunder, the storm may be more than 20 kilometres away. Storm Dunlop East Wittering, West Sussex, UK
thunder are produced almost simultaneously. However, in a cloud-to-ground lightning strike, one can expect the ground end of the bolt to be closer to an observer than the cloud end, so the thunder generated there will be heard first. Therefore, a distance calculation using the start of the thunderclap will indicate how far away the lightning is striking the ground. Richard Swifte Darmstadt, Germany
figures for safety; lightning strikes land very unpredictably when a storm is approaching. Jon Richfield Somerset West, South Africa n You can tell the distance to the nearest point of a strike by counting in seconds from the flash to the first rumble of thunder. If you keep counting until the end and note both distances, this will give you the nearest and furthest points. For a cloud-to-ground
n Thunder is produced because a lightning bolt heats a channel of surrounding air to temperatures as high as perhaps 30,000°C, causing rapid expansion. The subsequent cooling and collapse of this channel of air produces a sonic shock wave: thunder. The whole process takes place in a fraction of a second, which means the lightning and
n The streak of lightning may be “Sound dies with distance. If you see a flash but hear kilometres long, and thunder is no thunder, a storm may generated along its whole length, be more than 20km away” so you should time the delay between the flash and the start of strike, some trigonometry will the peal. That gives your distance give, very approximately, the to the nearest part of the flash, height the discharge started from. rather than any notional distance Dave Malham to the storm, which might be York, UK many kilometres across and capable of simultaneously n Knowing where the last generating lightning both near lightning strike occurred doesn’t and far away. mean the next one can’t be closer, In principle you also could estimate the top to bottom length or further away. Lightning has also been known to strike far from of a lightning discharge by timing how long the peal of thunder lasts, a storm. Even if a storm is moving away, the next strike could be but that depends on the terrain; closer to you. it could work on the open plains Arjan van Brakel of Kansas, say, but where I live, Zoeterwoude, The Netherlands echoes from the mountains rumble on interminably. n If you hear a long rumble rather Don’t expect great precision than a bang it may be because the even with special timing flash is very long, so different bits equipment, because the route the may be at very different distances sound takes and its speed depend from you. As a result, sound from on differences in the density, various parts of the flash reaches temperature, pressure and you at slightly different times. humidity of intervening air There are online projects, such as layers. And don’t rely on your
We pay £25 for every answer published in New Scientist. To answer a question or ask a new one please email [email protected]. Questions should be scientific enquiries about everyday phenomena, and both questions and answers should be concise. We reserve the right to edit items for clarity and style. Please include a postal address, daytime telephone number and email address. You can also send questions and
answers to The Last Word, New Scientist, 25 Bedford Street, London, WC2E 9ES. New Scientist Ltd retains total editorial control over the published content and reserves all rights to reuse question and answer material that has been submitted by readers in any medium or in any format and at any time in the future. All unanswered questions and previous questions and answers are at newscientist.com/lastword/
Mind. Full The perfect present stuffed with ideas and discoveries
lightningmaps.org, that plot strikes in real time, along with the expanding sound wave. If the lightning was nearby, you can view the strike on screen as the thunder rolls over you. Andrew Wells Portland, Dorset, UK
This week’s questions FAB SPROUTS
Brussels sprouts sold loose taste fabulous, whether steamed, microwaved or roasted. Now, more and more shops sell them pre-prepared in sealed bags, which presumably contain some inert gas, such as carbon dioxide or nitrogen. However you cook them, these sprouts taste off. Is the gas affecting their flavour, or is this because the shelf life is extended and we are being sold produce that isn’t fresh? Richard Kubiak Usk, Monmouthshire, UK FLEECE POLICE
I spend a lot of time outdoors and wearing fleece jackets seemed a good use of recycled plastic. I now learn that every time I wash them, I pollute the oceans with microscopic fibres. If they go to landfill, they will presumably stay there for hundreds of years and may leach toxins into the environment. Should I continue to wear them? What is the best way of disposing of them? Elizabeth Carter Penselwood, Somerset, UK
FREE BOOK
WORTH
£25
GREAT GIFTS from £69 newscientist.com/12051 or call 0330 333 9470 quoting 12051
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THE LAST WORD Counted out I remember being told that when you see a flash of lightning and start counting until you hear the thunderclap, the number you have counted to will amount to the number of miles distant the storm is. Is this true? If not, is there any way of calculating the distance based on the observed time difference between lightning and thunder?
n You can estimate the distance lightning is from you by counting seconds until you hear thunder, but it equates to more than 1 second per mile. Sound travels at approximately 343 metres per second in air at 20°C. That means it covers just over 1 kilometre in 3 seconds, and a bit more than 1 mile in 5 seconds. So counting the seconds between the flash and thunder gives about 1 kilometre for every 3 seconds and 1 mile for every 5 seconds. Because sound dies away with distance, if you see a flash, but hear no thunder, the storm may be more than 20 kilometres away. Storm Dunlop East Wittering, West Sussex, UK
thunder are produced almost simultaneously. However, in a cloud-to-ground lightning strike, one can expect the ground end of the bolt to be closer to an observer than the cloud end, so the thunder generated there will be heard first. Therefore, a distance calculation using the start of the thunderclap will indicate how far away the lightning is striking the ground. Richard Swifte Darmstadt, Germany
figures for safety; lightning strikes land very unpredictably when a storm is approaching. Jon Richfield Somerset West, South Africa n You can tell the distance to the nearest point of a strike by counting in seconds from the flash to the first rumble of thunder. If you keep counting until the end and note both distances, this will give you the nearest and furthest points. For a cloud-to-ground
n Thunder is produced because a lightning bolt heats a channel of surrounding air to temperatures as high as perhaps 30,000°C, causing rapid expansion. The subsequent cooling and collapse of this channel of air produces a sonic shock wave: thunder. The whole process takes place in a fraction of a second, which means the lightning and
n The streak of lightning may be “Sound dies with distance. If you see a flash but hear kilometres long, and thunder is no thunder, a storm may generated along its whole length, be more than 20km away” so you should time the delay between the flash and the start of strike, some trigonometry will the peal. That gives your distance give, very approximately, the to the nearest part of the flash, height the discharge started from. rather than any notional distance Dave Malham to the storm, which might be York, UK many kilometres across and capable of simultaneously n Knowing where the last generating lightning both near lightning strike occurred doesn’t and far away. mean the next one can’t be closer, In principle you also could estimate the top to bottom length or further away. Lightning has also been known to strike far from of a lightning discharge by timing how long the peal of thunder lasts, a storm. Even if a storm is moving away, the next strike could be but that depends on the terrain; closer to you. it could work on the open plains Arjan van Brakel of Kansas, say, but where I live, Zoeterwoude, The Netherlands echoes from the mountains rumble on interminably. n If you hear a long rumble rather Don’t expect great precision than a bang it may be because the even with special timing flash is very long, so different bits equipment, because the route the may be at very different distances sound takes and its speed depend from you. As a result, sound from on differences in the density, various parts of the flash reaches temperature, pressure and you at slightly different times. humidity of intervening air There are online projects, such as layers. And don’t rely on your
We pay £25 for every answer published in New Scientist. To answer a question or ask a new one please email [email protected]. Questions should be scientific enquiries about everyday phenomena, and both questions and answers should be concise. We reserve the right to edit items for clarity and style. Please include a postal address, daytime telephone number and email address. You can also send questions and
answers to The Last Word, New Scientist, 25 Bedford Street, London, WC2E 9ES. New Scientist Ltd retains total editorial control over the published content and reserves all rights to reuse question and answer material that has been submitted by readers in any medium or in any format and at any time in the future. All unanswered questions and previous questions and answers are at newscientist.com/lastword/
Mind. Full The perfect present stuffed with ideas and discoveries
lightningmaps.org, that plot strikes in real time, along with the expanding sound wave. If the lightning was nearby, you can view the strike on screen as the thunder rolls over you. Andrew Wells Portland, Dorset, UK
This week’s questions FAB SPROUTS
Brussels sprouts sold loose taste fabulous, whether steamed, microwaved or roasted. Now, more and more shops sell them pre-prepared in sealed bags, which presumably contain some inert gas, such as carbon dioxide or nitrogen. However you cook them, these sprouts taste off. Is the gas affecting their flavour, or is this because the shelf life is extended and we are being sold produce that isn’t fresh? Richard Kubiak Usk, Monmouthshire, UK FLEECE POLICE
I spend a lot of time outdoors and wearing fleece jackets seemed a good use of recycled plastic. I now learn that every time I wash them, I pollute the oceans with microscopic fibres. If they go to landfill, they will presumably stay there for hundreds of years and may leach toxins into the environment. Should I continue to wear them? What is the best way of disposing of them? Elizabeth Carter Penselwood, Somerset, UK
FREE BOOK
WORTH
£25
GREAT GIFTS from £69 newscientist.com/12051 or call 0330 333 9470 quoting 12051
2018_Xmas_LastWordAd.indd 1
09/11/2018 17:25