free download PNG images :Thunder

Thunder is the sound of lightning. Depending on the distance and nature of the lightning, it can range from sharp, loud cracks to long, low rumbles (brown). The sudden rise of pressure and temperature caused by lightning will cause the rapid expansion of air in and around the lightning path. In turn, this expansion of the air produces sonic shock waves, often referred to as "Thunderclap" or "thunderclap.".

The cause of thunder has always been the theme of speculation and scientific exploration. The early idea was that it was made by gods, but ancient Greek philosophers attributed it to natural causes, such as the wind blowing clouds (Aristotle) and the movement of air in the clouds (Democritus). [6] The Roman philosopher Lucretius believed that this was due to the sound of hailstones colliding in the clouds.

By the middle of the 19th century, the accepted theory was that lightning created a vacuum. The collapse of the vacuum produced thunder.

In the 20th century, it was agreed that because of the sudden thermal expansion of the plasma in the lightning channel, the thunder must start from the shock wave in the air. Through the measurement of spectral analysis, the temperature in the lightning channel will change when there is 50 μ s, rising sharply from the initial temperature of 20000 K to 30000 K, then gradually decreasing to 10000 K. The average is about 20400 K (20100 ° C; 36300 ° f). This heating causes a rapid outward expansion, striking the cooler air around it at a faster rate than sound can travel. The resulting outward motion pulse is a shock wave, similar in principle to a shock wave formed by an explosion, or at the front end of a supersonic aircraft.

The experimental results of simulated lightning are basically consistent with the model, although the exact physical mechanism of the process is still controversial. Other reasons are also proposed, depending on the electric effect of large current on the plasma in lightning.