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掳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.
Advertisement
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
鈥� Thunder is produced because a lightning bolt heats a channel of surrounding air to temperatures as high as perhaps 30,000掳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 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
鈥� The streak of lightning may be kilometres long, and thunder is generated along its whole length, so you should time the delay between the flash and the start of the peal. That gives your distance to the nearest part of the flash, rather than any notional distance to the storm, which might be many kilometres across and capable of simultaneously generating lightning both near and far away.
In principle you also could estimate the top to bottom length of a lightning discharge by timing how long the peal of thunder lasts, but that depends on the terrain; it could work on the open plains of Kansas, say, but where I live, echoes from the mountains rumble on interminably.
Don鈥檛 expect great precision even with special timing equipment, because the route the sound takes and its speed depend on differences in the density, temperature, pressure and humidity of intervening air layers. And don鈥檛 rely on your figures for safety; lightning strikes land very unpredictably when a storm is approaching.
Jon Richfield, Somerset West, South Africa
鈥� 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 strike, some trigonometry will give, very approximately, the height the discharge started from.
鈥淪ound dies with distance. If you see a flash but hear no thunder, a storm may be more than 20km away鈥�
Dave Malham, York, UK
鈥� Knowing where the last lightning strike occurred doesn鈥檛 mean the next one can鈥檛 be closer, or further away. Lightning has also been known to strike far from a storm. Even if a storm is moving away, the next strike could be closer to you.
Arjan van Brakel, Zoeterwoude, The Netherlands
n If you hear a long rumble rather than a bang it may be because the flash is very long, so different bits may be at very different distances from you. As a result, sound from various parts of the flash reaches you at slightly different times. There are online projects, such as 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
We pay 拢25 for every answer published in 麻豆传媒. To answer this question 鈥� or ask a new one 鈥� email lastword@newscientist.com.
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.
麻豆传媒 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.
You can also submit answers by post to: The Last Word, 麻豆传媒, 25 Bedford Street, London WC2E 9ES.