What Causes Thunders?

Speed of Sound

Thunder and lightning are phenomena mundane to the people of tropics. In fact statistics state that 70% of the time lightning is observed in the tropics. Thunder is always associated with lightning and in fact follows a lightning. But why does thunder always follow lightning, what is the relationship between these two phenomena? How does such a large amount of sound is generated and what is the source of its energy? Can we calculate the distance to the point of origin of that sound without any sophisticated device?

Lightning is a discharge of large amounts of current that moves across two regions that have different electric potential formed due to the piling up of charge. This movement of large amounts of charge heats the surrounding air to very high temperatures. Kinetic theory of gases states that when the temperature is high the gas particles move at higher speeds. Fast moving gas particles impart high pressure. And hence there is a formation of high pressure region in the atmosphere that propagates like a wave, called as shock wave. Sound travels in the air as compressions and rarefractions, compressions being the high-pressure regions and low-pressure rarefractions.

When a sound generating system like speaker vibrates back and forth, air adjacent to the speaker gets pressurized and depressurized. A combination of one pressurized section and depressurized section makes one sound wave. These pressurized and depressurized sections propagate and as result of this process sound waves traverse in the medium like gas. And thus the shockwave is nothing but a sound wave with very high intensity. Now you know that lightning causes thunder, the relation between both is clear.

As we have learnt during our Physics tuition classes, speed of sound is around 330 meters per second and the speed of light is around 3 x 108 meters per second. Consequently, light travels very fast almost instantaneously (for the distances of the order of tens of kilometers). Let’s say you see the lightning and hear thunder after ‘t’ seconds. Since the light from the lightning takes virtually no time to reach earth, ‘t’ seconds is the time taken by the sound to reach the point at which you are present. From the fact that light travels at a speed of 330 meters per second, the distance between point of origin of lightning and you is (330 x t) meters. Since the point of origin of lightning and thunder are same, the distance to point of origin of thunder is (350 x t) meters.