The perception of thunder as a long, drawn-out rumbling sound is primarily due to the way sound waves propagate through the atmosphere and interact with various factors such as distance, reflections, and absorption.
When lightning occurs during a storm, it rapidly heats the surrounding air to extremely high temperatures, causing it to expand rapidly. This rapid expansion creates a shock wave that travels outward from the lightning channel. This initial shock wave produces the characteristic cracking or snapping sound, often referred to as the "thunderclap."
However, what we perceive as thunder is not just the direct sound of the initial shock wave. As the shock wave travels through the atmosphere, it encounters variations in air density, temperature, and other atmospheric conditions. These variations cause the sound waves to undergo diffraction, reflection, and refraction, which can alter their path and result in a complex sound propagation pattern.
When thunder travels over long distances, the higher-frequency components of the sound tend to get attenuated more quickly due to air resistance. The lower-frequency components, on the other hand, can propagate over longer distances with less attenuation. This phenomenon leads to a gradual decay of the higher-frequency components of the sound and emphasizes the lower-frequency components, which are perceived as a prolonged rumbling or rolling sound.
Furthermore, when thunder encounters obstacles such as hills, buildings, or other objects, it can bounce off these surfaces and create echoes. These echoes can reinforce certain frequencies or introduce delays in the sound arrival, further contributing to the rumbling effect.
Overall, the complex interaction of sound waves with the atmosphere, along with the attenuation of higher frequencies and the occurrence of echoes, creates the perception of thunder as a long, drawn-out rumbling sound, even though the initial cause is the sharp crack of the lightning bolt.