Thunder

Thunder is the powerful sound produced by a lightning strike. It can range from a sudden, loud crack to a long, low rumble. This phenomenon occurs because lightning creates an intense and rapid change in the atmosphere. The sound is often called a thunderclap or a peal of thunder. Scientists who study these sounds specifically are practicing a field known as brontology. While many find the noise interesting, some people experience an irrational fear of it called brontophobia.

To understand the mechanism, we must look at the extreme heat within a lightning channel. During its brief existence of about 50 microseconds, the temperature inside the bolt changes rapidly. Spectral analysis shows the temperature rises from 20,000 K to about 30,000 K. It then drops gradually to about 10,000 K, leaving an average temperature of 25,000 K. This sudden thermal expansion causes the air to expand outward very quickly. The air expands faster than the normal speed of sound, creating a sonic shock wave. This pulse is similar to the shock wave from an explosion or a supersonic aircraft.

Thunder can be categorized into several distinct types based on its characteristics. Claps are loud sounds that last between 0.2 and 2 seconds and have higher pitches. Peals are sounds that change in both their pitch and their loudness. Rolls consist of irregular mixtures of different pitches and loudness levels. Rumbles are less loud and have a low pitch, often lasting more than 30 seconds. Additionally, inversion thunder can occur during a temperature inversion. This happens when warm, moist air passes above a cold front, making the air near the ground cooler than the air above. In these conditions, sound energy cannot disperse vertically and stays concentrated near the ground.

Humans have speculated about the cause of thunder for many centuries. Early civilizations often believed that deities were responsible for the noise. Some folklore even suggested that gnomes were playing games to create the sound. Ancient Greek philosophers offered natural explanations, such as wind striking clouds or air moving within clouds. The Roman philosopher Lucretius suggested that the sound came from hail colliding inside clouds. By the mid-19th century, the accepted theory was that lightning created a vacuum. People believed the sound was caused by the collapse of that vacuum. It was not until the 20th century that scientists agreed on the shock wave model.

Because thunder is so powerful, it can have significant physical consequences. Near the source, the sound pressure level is usually between 165 and 180 dB. In some extreme cases, the sound can even exceed 200 dB. These high levels of pressure are sufficient to cause property damage. They can also cause physical injury to people standing nearby. This includes internal contusions or the rupturing of eardrums. Such injuries can lead to permanent hearing impairment, though temporary deafness is also possible.

A very bright flash of lightning followed by an almost immediate crack is called a thundercrack. This indicates that the lightning strike was very close to the observer. You can estimate the distance of a strike by using the speed of light and sound. Light travels much faster than sound, which moves at about 343 meters per second in dry air. When you see a flash, start counting the seconds until you hear the sound. To find the distance in miles, divide the number of seconds by five. To find the distance in kilometers, divide the seconds by three.

Thunder is also shaped by the specific way lightning interacts with the Earth. Cloud-to-ground lightning typically involves two or more return strokes moving from the ground to the cloud. These later return strokes actually possess greater acoustic energy than the very first stroke. The sequence of sounds you hear also changes based on the part of the bolt. Close-in lightning may first sound like clicking or cloth tearing. This is followed by a sound like a cannon shot or a loud snap. Finally, the sound settles into a continuous rumbling as the distant parts of the return stroke reach you.