# why do we hear thunder after we see lightning

Dr. Catherine Doyle received her Ph. D. in Physics from Dublin City University and has worked as a postdoctoral researcher in the Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN), Trinity College Dublin. Weвre all familiar with flashes of lightning and rolls of thunder during a thunderstorm, and the fact that we see the lightning before we hear the thunder. ВLightning and thunder are caused by a build-up of static charge in a cloud discharging to another cloud or to the ground. ВBut if the lightning and thunder are both caused by the same event, why do we observe a delay between them? Itвs because of the different properties of light and sound. Both are forms of energy, and both are waves, but they are different types of wave. Waves transfer energy from one place to another, through oscillations or vibrations. Light is a transverse wave в this means that the oscillations are perpendicular to the direction in which the wave is travelling. ВOne way to picture this is to think about a skipping rope. Flicking one end of the rope sends a ripple down to the other end. The rope moves up and down as the ripple passes along from one end of the rope to the other. ВВ Sound is a longitudinal wave, meaning that the oscillations are parallel to the direction in which the wave is travelling. Think of a slinky в moving one end back and forth causes the coils to bunch up and stretch out as the vibration passes down the slinky to the other end. В The coils move in the same direction as the vibration. Transverse wave motion (top); longitudinal wave motion (bottom). Image credit: www. frankswebspace. org. uk, via Google. Light is an electromagnetic wave, which means that it does not need a medium through which to travel.

This is why light from the Sun reaches Earth across the vacuum of space. Sound is a mechanical wave, however, which means it can only travel through a medium. Sound is transferred via the vibrations of air molecules в if two objects collide, for example, this causes a vibration in nearby air molecules which then travels along through the air until it reaches your ear and causes the ear drum to vibrate, which your brain perceives as sound. В If there is no medium, there is nothing to carry the vibration and so the energy canвt be transferred. This is why there is no sound in space. All electromagnetic waves travel at the same speed в around 300,000,000 metres per second. This is incredibly fast. In fact, nothing in the universe can travel faster. ВSo if there is a lightning strike within a few kilometres of where you are, the light will reach you in fractions of a second. ВSound travels much more slowly. The speed of sound in air is about 340 metres per second в almost a million times slower than the speed of light. So the sound of thunder from that lightning strike a few kilometres away will take a few seconds to reach you. В The further away the storm is, the further the sound has to travel, and the longer the delay between seeing the lightning and hearing the thunder. ВYou can estimate how far away the storm is by multiplying the time delay by the speed of sound in air, e. g. a 10 second observed delay between lightning and thunder means the storm is roughly 3. 4 km away.
Fersiwn. Thunder is the sound that accompanies during a thunderstorm.

Sounds simple enough, but why does lightning even make a sound? Any sound you hear is made up of vibrations. The vibrations travel as a sound wave through the air, until they reach your ear. is a huge discharge of electricity, and this electricity shoots through the air, causing vibrations to be formed in two ways: 1. The electricity passes through the air and causes air particles to vibrate. The vibrations are heard as sound. 2. The lightning is also very hot and heats up the air around it. Hot air expands, and in this case the air expands very quickly, pushing apart the air particles with force and creating more vibrations. This is what we hear and call thunder the rumbling of thunder is simply caused by the vibration or sound of the air affected by lightning. If you re nearby to a lightning strike, you may have heard thunder as a really loud crack, almost like the sound of a whip being cracked. But, most of the time we hear thunder as a loud, long rumble. In fact, the crack sound is the direct sound of the lightning near us, reaching our ears. The more common rumbling effect happens when thunder echoes off objects all around us. This happens a lot in towns and cities, where there are lots of buildings for the noise to bounce off. However even in flat areas of land, with no trees or other objects, there is quite often a rumble as the thunder simply bounces off the ground on its way to our ears. All this echoing transforms the original crack sound into a longer rumble! Why is thunder not at the same time as the lightning? We see the lightning before we hear the thunder because light travels faster than sound. PThe light from the lightning travels to our eyes much quicker than the sound from the lightning. so we hear it later than we see it.

There is an old myth that counting seconds between a lightning flash and the accompanying thunder gives you the distance of how far away the storm is, in miles. However, from a mathematical point of view we know this isn t true, as the speed of sound is roughly 330 metres per second. So it takes roughly 3 seconds for the thunder to travel one kilometre, and therefore about 5 seconds for thunder to travel a mile. So, a more scientific rule would be, count the number of seconds between the lightning flash and the thunder noise, and then divide that number by five, and that is how many miles away the thunderstorm is. Extreme sounds like this are incredibly fascinating, especially to scientists like us who investigate how such things happen. Extreme sounds don t just have to be loud, they can also be powerful enough to shatter a wine glass, or can even be used as a weapon in the animal kingdom. We ve spent so much time finding out new ways that sound shapes and transforms our lives, that here at science made simple Pwe even wrote a whole science show about, so we can tell as many people about them as we can. Why not try it yourself? Try out a light and sound experiment yourself. Get another person to stand along way away from you but so you can still see them. Clap your hands: get the other person to raise their left hand when they see you clap and their right when they hear you clap. You might notice a very small gap, similar to that of lightning and thunder; except that lightning is even further away so the sound and light gap is usually bigger!