WHO, WHAT, WHY?
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The blast at the Hertfordshire oil depot could reportedly be heard 200 miles away in the Netherlands and Belgium. Can sound really travel that far?
Smoke from the fire chokes the sky
A series of blasts signalled the explosion at the Buncefield oil depot, near Hemel Hempstead, early on Sunday.
The depot, storing oil, petrol as well as kerosene, supplies airports across the region, including Heathrow.
Local residents spoke of hearing a "humungous blast" and their houses shaking, but there were also reports of the explosions being audible in the Netherlands and Belgium. Could the noise really have travelled that far?
Sound can travel as long as there is something for it to travel through. This could be a liquid such as water, a solid such as a metal or, as in this case, a gas such as air.
It travels as a wave, which is produced when something vibrates. If a drum is hit for example, it vibrates. This causes sound waves to travel away from the drum towards you.
The waves travel by forcing the particles of air surrounding the drum to vibrate back and forth, colliding with each other in the same direction as the wave is travelling. Such a wave is called a longitudinal wave as the vibrations move in the same direction as the wave moves.
How far sound travels depends on different variants. Tim Wilton, who works for Vibrock, an environmental consultancy, says it is unusual for it to travel 200 miles - but possible.
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He says a strong "temperature inversion" on Sunday will have been one of the reasons the sounds of the blast travelled so far. This happens when there is a layer of warmer air at altitude rather than air cooling as it goes up. This bends the audible energy back down to earth and the sound can be heard miles from the source.
This strong temperature inversion, combined with the huge amount of energy at the source of the blast and the easterly direction of the wind could have resulted in people in other countries hearing the sound of the explosion.
"To hear it 200 miles away is unusual," says Mr Wilton. "But the blasts were unusual because you don't usually get that amount of energy at the source. On Sunday there were classic conditions for sound to travel further than usual."
The blasts are said to be the largest of their kind in the UK and Europe during peacetime.
During World War II there were frequent tales of Londoners hearing bombings in France and residents in Dover hearing gun fire from across the Channel.
"It's quite possible as it's not unusual at all to hear things 30 or 40 miles away," says Mr Wilton.
Add your comments on this story.
During the first world war the sound from the guns on the Somme could be clearly heard in parts of the UK on the arc of a circle. The guns could not be hearrd within this arc. This phenomenum lead to the discovery of The Heaviside Layer which reflected the sound back to earth. The original research was carried out in the department of physics at the University of Birminhgham.
Graham Williams, UK
I dont think that it could be heard in Holland because I live in Harpenden six miles away and was loud but not that loud
greg clarke, england
With such a loud explosion, will there be a legacy of deafness and/or tinnitus throughout Hemel Hempstead and its environs?
John Thomas, UK
I live within 5 miles of the Hemel depot. Myself, my partner and our 2 young children were all tucked up in bed at the time of the explosion. We didn't hear a thing!! The first we knew of it was when concerned relatives called. Sound may travel, but deep sleepers don't notice!!
When the volcano in krakatoa exploded in 1883 it was desribed as the explosion that shook the earth ,both politically and literally . Reports of the explosion were heard around the globe and water levels were effected as far north as London .
john macpherson, uk
The historian Chateaubriand claimed he heard the cannon of the Battle of Waterloo in 1815 from the city of Ghent. That must be a good 125 kms.
In 1946 I was 7 years of age and the BBC announced on the radio news that the bomb dump in Heligoland was to be blown up. Just after 12.00Hrs I recall hearing a very faint rumble.
DAVID TASSELL, U.K.
In fog and mist, sound is conducted much further than usual. On watch during still foggy night in the North Atlantic, I've clearly heard warning fog horn of other ships, long before they appeared on the radar.In fact, their clarity was deceptive, and the radar was always consulted since they could sound much closer than they really were. Old country ladies in Jamaica frequently practice the tradition of "yoahing." On Bob Marley's Misty Mornings, my neighbour frequently yoah her friend almost four miles away to tell her she's leaving for the bus stop, and to co-ordinate their departure for the market in town. They have a special code and recognise each other's voice, such is the sound clarity, despite its travel of more than three miles, and bouncing around two steep hills. Packs of dogs also use the Jamaican mist to communicate over miles, most annoyingly. We can clearly hear Friday-night music from sound systems - again despite intervening hills - as much as almost eight miles away on occasion. I know this is in a valley and the DJ - because I delivered and set up the system. I won't mention the three or four nearer systems!
Martin Harle, UK
Surely 200 miles is nothing when you comapre it to reports of other sounds heard in history. For example the expolison of the volcano on the island of Krakatau is said to have been heard over 4600 km away. I also read that it was claimed the nosie rang round the world word for 9 days and made water slosh the other side of the world. If sound can travel that far surely 200 miles is nothing.
Gareth, Sheffield, UK
It is refreshing to see a presentation of the educational aspect of a current news item. Though it is still a shame is did not appear coincident with that news and replace the emotional and sensational content there.
Nigel Andrews, United Kingdom
I heard an ominous booming sound on Sunday morning just after 6am. I was in my house in Penarth and wondered what on earth it could be as I had never heard anything like it before. When I saw the news reports I realised that it must have been the blast from the explosions.
We are approximately 120 miles (as the crow flies)from Hemel Hempstead and we overlook the Bristol channel, so the sound would have travelled quite well over the water. My friends doubted that I would have been able to hear it, but I know what I heard.
Sue Richards, South Wales
I live in Harlow about 30 miles away and I heard nothing
David Ewles, UK
In September 1961 the soviet union constructed a thermonuclear bomb called tsar bomba (king of bombs) to date this is the largest man made explosion in history and the shockwave was detected travelling around the world an amazing three times!
bernard mc verry, ireland
To add to Mr Wilton's last comment on this:
If a given explosion is just heard at 40 miles, then to hear an explosion at 200 miles (5 fold increase in distance), requires a 25 (or 5 squared) fold increase in the sound wave's energy at the source. That's a big explosion! See http://en.wikipedia.org/wiki/Sound_intensity for more details...
Jamie Ballin, England
I know I'm a heavy sleeper but I didn't hear a thing on Sunday morning... I live about 30 miles east of the blast so how it could be heard 200 miles away is beyond me!
It occurred to me that the sound I heard in Guildford, 40 miles away, would have taken about 3 mins to reach me!
Tom Blandford, UK
We live at the end of St Albans that is furthest from Hemel Hempstead, and whilst our friends a mile or so across twon could hear all three + explosions, we only heard the first low 'grumble' and the shock wave... so how can they have heard the explosions on the other side of the channel, I wonder?!
Atmospheric conditions have the same effect on radio waves and its not unusual for VHF and UHF radio signals broadcast in the UK to picked up as far afield as Italy.
Its also very lucky that it was a clear day on Sunday as the shock wave from an explosion can be refelcted off a cloud base. If this had occured on Sunday - the damage would have been much more widespread.
I certainly heard the explosion over on the Suffolk coast, it woke me up. It wasn't quite right for thunder, at first I was ready to curse out Farmer Dave as I thought one of his bird scarers had misfired. Since there's nowt but sea from here to Holland I find it quite plausible that the sound could continue on that far.
Wasn't Krakatoa heard all around the world?
What if it was something different they heard at the same time, e.g. thunder or something? i didn't hear it in london, and people in scotland didn't hear it!
We heard this in Norwich, Norfolk. We were already awake and heard several noises just after 6am that sounded like an odd, short dull rumbling thunder clap. We haven't heard a sound like that before, ever.
Pete Irving, UK
How come when people could hear it in Holland, we did not hear it in West Sussex? I assume that soundwaves travel like waterwaves when a stone is thrown in water, i.e. in all directions?
Marjanne Worsley, England
We were definitely woken up by the sound of the explosion on Sunday morning, and we live in central London.
sally fee, uk
Ever hear (sorry about the inadvertant pun) of the Halifax Disaster of December 6 1917 ? The Mont Blanc (a Belgian ammunition ship) exploded in Halifax harbor -- the blast was hear at least 200 miles away, although I confess that I don't have the references at hand; I think some people on PEI heard the explosion, which was one of the largest ever until WWII's nuclear weapons and some blasts disposing of captured German ammunition.
Greg Williamson, USA
The truth of this story could be checked by examining the time at which the reports were made. Sound is much slower than light and therefore would take longer to reach the Netherlands and Belgium. If the reports are say, 10 minutes late, that could be accurate. However if the reports are earlier (closer to the actual time of the explosion) it may well be made up.
C Gillatt, england
On Sunday morning at appr. 7.17 in the moring I heard my glass door shaking.
First I thought that there was a gas explosion in my local neighbourhood but later I learned that the explosion occured in the UK.
So I confirm that the sound ( at leat the air-movment was present in my City.
I'm living at Kortrijk in Belgium about 60 km from the coast line.
Joseph Mattelaer, Belgium
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