Why does thunder follow lightning?

          During the rainy season we often see lightning in the sky followed by thunder. Do you know what this lightning is and how thunder follows it?

          In ancient times whenever man saw lightning in the sky and heard thunder he used to believe that gods were angry and punishing him for some sin. Benjamin Franklin was the first person who, in 1872, scientifically explained the occurrence of lightning. In fact, whenever the sky gets overcast with clouds, the small particles of water present in them get charged due to air friction. In the process, some clouds become positively-charged, while some others negatively. When a positively-charged cloud approaches a negatively-charged one, there develops a potential difference of millions of volts between them. Because of this high voltage, there is a sudden electric discharge through the air between the two clouds and a streak of light is seen. This is called ‘lightning’. The electric discharge through the air produces a large amount of heat due to which the atmospheric air suddenly expands. With this sudden expansion, the innumerable molecules of the air collide with one another and produce sound. This is called ‘thunder’. In other words, the thunder is the acoustic shock waves, which may be a sudden clap depending upon the lightning path. Although lightning and thunder are produced simultaneously, yet we see the flash of lightning first. It is so because the speed of light is very high i.e., 300,000 kms per second. On the other hand speed of sound is only 332 metres per second. Thus, because of high velocity, light immediately reaches our eyes, but the sound takes some time to reach our ears.

          Whenever a charged cloud passes by some tall tree or high building, by induction, it produces the opposite charge on that tree or building. When the amount of charge so produced is very high, a giant electric spark travels between the cloud and the ground. It is then said that lightning has struck such a tree or building.

          To protect high buildings from such mishaps, pointed rods of copper or some other metal are fixed on the top of buildings which run to the bottom and are buried deep in the earth. These are called ‘lightning conductors’. Whenever some charged cloud passes by such a building and produces opposite charge on it, the charge goes to earth through the metal rod and does not damage the building. This how buildings are protected from the lightning.