What causes the rumbles after a lightning strike? Here's the phenomenon of thunder, explained.
Photo courtesy ISTOCKPHOTO/CLINT SPENCER
Much is written about lightning — why it occurs, its potential for danger and how to avoid it. Yet we rarely hear or read about an electrical storm's audio accompaniment: thunder. Because thunder comes from lightning strikes, however, we'll first cover a few facts about those "bolts from the blue."
To begin with, note that lightning bolts have no curves along their paths. All sections of a zigzagging bolt are straight and they connect at sharp angles. During thunderstorms, lightning bolts may occur inside a cloud, between a cloud and the earth's surface, between two clouds, or even between a cloud and the surrounding air. Within storm clouds, moisture, warm and cold temperatures and fast moving air all combine to create a strong electric charge. Once that charge builds to several million volts (enough to cause the electrical breakdown of air), those megavolts force a tremendous electric current to flow to or from a region with an opposing charge.
The path of this current can be many miles long, but it is no wider than a garden hose. The flow of current instantly heats surrounding air to an extremely high temperature. The gaseous expansion caused by that incredible heat then collapses, creating cylindrical shock waves along the entire bolt path. These waves are transmitted initially as one clap of thunder.
So, now we know what causes thunder. But here's the mystery: The shock waves that cause thunder last only a fraction of a second, so why does the booming carry on for up to several seconds? Echoes off ground surfaces, buildings and nearby mountains are one reason, but the main factor behind thunder's longevity is the extreme length of the lightning bolt.
Lightning races through the sky at more than 60,000 miles per second, but sound travels through air at only one-fifth of a mile per second. So, if a long vertical flash is one mile away, the first thunder arrives in five seconds. The sounds continues to arrive from higher and higher up the bolt, and may last several seconds or more.
In similar fashion, the speed of sound is also partly responsible for the rumbling we hear, due to refraction — the bending of sound waves. Although its change in speed is not great, sound travels faster in warm air than in cold, increasing in speed by about one foot per second with each degree Fahrenheit rise in temperature. These small changes are important because air temperature varies significantly across the atmosphere, and especially during storms.
Atmospheric temperature change can also be abrupt, as when incoming warm air slides over existing cold air. When this happens and a sound wave in cold air approaches the warm air at an angle, those portions of wave front that enter the warm air first will speed up, thereby rotating the entire front a few degrees and changing the wave's travel direction. Conversely, sound waves entering cold air from warm air at an angle will slow the wave and bend it in opposite manner. Bear in mind, even a gradual change in temperature can cause sound waves to bend. In some cases, rising sound waves can keep bending and, depending on rise-angle and other conditions, head back to Earth.
When sound waves radiate from a lightning bolt, vertical changes in air temperature cause extensive bending. Horizontal temperature changes, due mainly to air pockets and columns of rising or falling air, also bend sound waves. All this simultaneous refraction results in myriad crisscrossing sound waves.
Sound waves are also influenced by destructive interference. When similar waves are superimposed, their vibrations oppose or support each other, which results in a haphazard mixture of cancelled and reinforced sounds. Therefore, when lightning is a few miles away, we hear throbbing variations of sound — the rumble of thunder.