In a thunderstorm, with the lightning comes a powerful roar. At first, like the sound of gunfire, the thunder was a short and powerful sound, followed by a long thunder, and then turned into a falling rumble. How did the thunder come about?
When thunder strikes, a large current passes through the lightning path in a very short time. The temperature of the air in the passage suddenly rises very high and expands rapidly. Due to the expansion effect, it is completed in a very short time of a few percent of a second, so it is the same as the vibration of the surrounding air caused by the explosion of a bomb. The fluctuation of the air, which spreads to people's ears, is thunder. The faster the air expands, the louder the thunder will be.
When the current stops suddenly, the temperature of the lightning path will drop sharply because the heat has been released into the surrounding atmosphere. The air in the passage will contract sharply due to sudden cooling, which will cause the secondary vibration of the air and form thunder.
The speed of thunder in the air, like other sounds, is about 330 meters per second (1200 kilometers per hour), which is equivalent to the flight speed of an airplane. The propagation speed of lightning and firelight is very large, 3000000 meters per second, which is about one million times of the propagation speed of sound wave, equal to 7.5 circles around the earth per second. So when it thunders, you always see the lightning first and then hear the thunder.
We can hear the thunder within 25-30 kilometers, and the thunder can reach our ears within 75-95 seconds after the lightning. Mines within 3 km from the observer are called near mines, and mines beyond 3 km are called distant mines. The longer or shorter the thunder sounds depends on the lightning's route in the atmosphere. The more curved the lightning's route is, the longer the thunder sounds. Each part of the lightning path makes a sound during the whole process, which creates a lasting and loud thunder.
We know that lightning is composed of several individual discharges (lightning). Every discharge can produce sound, at the same time sound waves can be reflected from the ground and buildings, this is echo. Because of the different coordination of these sounds, there are various kinds of thunder sounds, big and small, strong and short.