Why does lightning cause thunder?
We humans have done a remarkably thorough job of domesticating our world. We live on earth, but we have remade the environment around us to suit how we would like earth to be. We’re still subject to clouds and sun, rain and snow, and the earth’s changes in temperature, but we’ve invented ways to avoid the worst of those. In the heat of summer and the cold of winter, we shuttle from climate-controlled buildings via climate-controlled cars to other climate-controlled buildings; and in winter we cover our bodies with insulated materials to keep us warm when we’re in the climate-uncontrolled outdoors (no one has yet invented practical air-conditioned clothing for the summer, but we can still hope).
Likewise, most of us spend most of our time in domesticated landscapes. We walk on floors, stairs, sidewalks, lawns, and parking lots. We sit on furniture and sleep on beds. And these trappings of our domesticated world are often quite similar all over the world.
The world we came to take for granted from our earliest childhood was dominated by this domesticated environment and domesticated landscape, sculpted by humans to make humans as comfortable as possible. Thanks to our many determined accommodations, our planet has largely faded into the background, as a passive place things happen rather than an active agent in our lives. Only occasionally does our part of the planet become unstable enough to feel dangerous. In some places the earth itself moves underfoot, and some mountains sometimes spew out rocks and dust and flow with molten lava. But these catastrophes are rare. The most commonly unstable part of our planet is the atmosphere, stirred into motion by differences in pressure and temperature, bringing winds, and water falling from the air in drops and frozen crystals.
How does a young child experience these changes? Many are pleasant: a cooling breeze or drenching summer rain, the thrilling first snowfall of the winter. Even the less comfortable ones are merely inconvenient, no more remarkable to a child than an upset stomach or a scraped knee. Only in storms do the motions of the atmosphere become so dramatic that a child could experience them as the actions of a violent threatening being. High winds can tip over tall trees like an invisible monster, and hurl objects through the air as if in anger.
But for human children, the most emotionally evocative of these atmospheric effects is the thunderstorm. It brings with it winds and rain, but also the unpredictable flash of lightning and the deep, awesome voice of the thunderclap. When children fear thunder, they may have in mind the damage that thunderstorms can do, but the emotional heart of their fear is I think the sense of thunder as an angry and powerful cosmic being—announcing itself ominously from a distance, then growling louder and more deeply as it comes closer. As adults, we may have overcome these childish fears, but for most of us there is still something thrilling in the onset of a thunderstorm.
As children, we did learn about lightning and thunder—and some of what we learned was even true. At some point, I was told that thunder is caused by two clouds colliding—an explanation not so different from what Aristotle proposed in his Meteorologica. I must have been old enough to know something about lightning being caused by positive and negative charges, like a spark, because I interpreted that parental lesson in terms of positively charged clouds coming in contact with negatively charged clouds, and I have to admit that I held onto that idea well into adulthood. Like most children, I also learned to figure out how far away the lightning struck by counting the seconds between lightning and thunder and dividing by five.
But how many of us actually ever wonder what it is about a lightning bolt that causes the sound we call thunder? Much scientific thought has been devoted to this question over the centuries, and in broad terms at least we do now have an answer. But before we get to that, let’s make sure we’re clear on what a sound is.
We hear sounds when our eardrum vibrates, and that happens when the air around us vibrates. Those vibrations are caused by pressure waves moving through the air at—yes—the speed of sound, which is almost 800 miles per hour. In an air pressure wave, the molecules in the atmosphere are crowded together at the peak of the wave and spread apart (at lower pressure) at the trough of the wave. So sound is caused by anything that generates enough force to push against the molecules of the atmosphere and press them together. When you whack two sticks together, the collision between them pushes against the nearby air, producing the cracking sound you hear. More force generates more concussion, which produces a louder sound.
Storm clouds produce lightning because they accumulate a major difference in electrical charge between themselves and the ground or other clouds. Once this difference in electrical charge becomes great enough to overcome the electrical resistance of the atmosphere, streams of electrons feel their way towards the ground, eventually connecting with a return stroke of opposite charge coming up from the ground. This connection produces a long narrow channel about an inch thick in which massive amounts of electrical current can pass between the cloud and the ground. As the current begins to flow, there is enough electrical resistance within the lightning channel to convert some of the current’s electrical energy into thermal energy, rapidly heating up the air around the channel.
When the temperature within the lightning channel rapidly increases, the air around the channel becomes intensely pressurized (because the pressure of a gas is proportional to its temperature divided by the volume it occupies, according to the combined gas law). Then a moment later, the air expands faster than the speed of sound, producing a shock wave just like in an explosion. As that shock wave spreads out, it produces sound pressure waves that can be sensed by our ears. You can read about the details of how the sound of thunder varies depending on the shape of the lightning bolt and the atmospheric conditions in the references and external links cited in the Wikipedia article on thunder.
My sense from the little reading I’ve done on this subject is that there are still open questions about what exactly is happening from moment to moment within the channel of a lightning bolt, and how some of the electrical energy is transformed into the sound pressure wave that produces thunder. But the above should at least provide enough of an answer to satisfy the next inquisitive child you run across.
© Joel Benington, 2012.