Thunder vs. Lightning

Light and Sound Speed

Fig. 1: Lightening bolt striking the Earth's surface. (Source: Wikimedia Commons)

During a thunderstorm, you see a bright flash of lightning. Multiple seconds later, you hear the loud rumble of thunder. How does this happen if thunder and lightning come from the same place and occur at the same time? The answer involves the speed at which sound and light travel.

Light is the electromagnetic radiation within a certain portion of the electromagnetic spectrum. Lightning causes light in the form of plasma, which is created by the electrostatic discharge that take place between the electrically charged regions of two or more clouds, or the electrically charged regions of a cloud and the ground. [1] Fig. 1 shows an example of lightning striking the Earth.

The term "light" mostly refers to visible light, which is responsible for the sense of sight in the human eye. Wavelengths around 400-700 nano-meters make up visible light. Light travels at 186,282 meters per second (670,616,629 mph), so in theory, nothing moves faster than the speed of light. [2] The speed of sound is commonly stated as the speed of sound waves through dry air, which is about 343.2 meters per second (768 mph) in air at a temperature near 20°C. This is significantly slower than the speed of light.

This indicates that the reason why you would see lightning first in a thunderstorm is because the visible light waves formed by the plasma in the lightning travels to your eyes much faster than the rumble or crack of thunder reaches your ears. Therefore, when you hear rumbling thunder, the lightning bolt was far away because of the effect of dispersion, which is the action or process of distributing things over a wide area. When you hear a very loud crack or boom, this means that the lightning bolt was near your location. [1]

Fig. 2: Fireworks from the Lake of Annecy Festival in France. (Source: Wikimedia Commons)

Using this relationship between seeing lightning and hearing thunder, you can determine a rough estimate of how far a thunderstorm is from your current location. After seeing a lightning bolt in the distance, count how many seconds it takes to hear the thunder. The number of seconds it took to hear the thunder is approximately how many miles away the lightning bolt was from your position.

Fun Facts

There are several other circumstances where the relationship between light and sound are distinguishable. For example: You're sitting at the top of a huge baseball coliseum, and you see the baseball bat make contact with the baseball. A couple seconds later, you hear the faint "cling" from the contact. The tremendously fast visible light allows you to see the contact of the baseball bat and the baseball, but the much slower sound waves take longer to travel to your ears. Another circumstance where this relationship is observable would appear while watching fireworks. You see the fireworks explode in the sky, ad hear the boom of the eruption a couple seconds later. This again is because of the slow speed of sound waves of the fireworks reaching your ear drums slower than the colors. [2] (See Fig. 2 for an example of fireworks.) These little relationships happen around you constantly. Now that you know the reason why this happens, it will be easier to identify it when it happens!

© Tai Thomas. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.

References

[1] W. Pfeffer, Thunder and Lightning,/i> (Scholastic Reference, 2002).