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.

Image comes from Wikimedia Commons, and is used under a  Creative Commons Attribution-ShareAlike 3.0 Unported license.

Why is it colder in winter and warmer in summer?

If you walked up to people on the street and asked them why it’s colder in the winter, what do you think most of them would say? I bet they’d say that it’s colder because the sun is farther from the earth in the winter. I’ve never done an actual survey, but I’ve asked the question in my classes several times over the years, and that’s the most common answer I get.

In summer in the northern hemisphere, the north pole points 23 degrees towards the sun, while in winter it points 23 degrees away from the sun..

Actually, the distance to the sun has practically nothing to do with the change of seasons. The earth revolves around the sun in a very nearly circular orbit, which keeps it pretty much the same distance from the sun at all times. Yes, earth’s orbit is just a tiny bit elliptical, so the distance from the sun does vary by something like 3% plus or minus, but that’s not nearly enough to produce the dramatic changes in temperature between winter and summer. In fact, winter in the northern hemisphere actually happens when the earth is that little bit closer to the sun in its orbit, which is the opposite of what so many people think. This relationship does change very slowly over time, however, so in another 10,000 years we will indeed be just a little bit farther from the sun during the northern hemisphere winter. Something to look forward to.

What does cause the change in seasons? Even if you haven’t thought much about this, you’ve probably noticed that the sun is lower in the sky in the winter, which also makes the days shorter. This is caused by the tilt of the earth’s axis relative to its orbit around the sun. In the northern hemisphere summer, the north pole is tilted 23 degrees towards the sun, which causes the sun to be higher in the sky from our perspective here on earth’s surface. In the northern hemisphere winter, the north pole is titled 23 degrees away from the sun, so the sun appears lower in the sky.

On December 21 at noon, the sun is directly overhead at the Tropic of Capricorn (23 degrees south of the equator). As you go further north or south from that latitude, the sun’s rays strike the earth’s surface at a shallower angle and thus the the sun appears lower in the sky.

The light coming to us from the sun is a stream of photons. When those photons are absorbed by the material of the earth, their energy is transformed into thermal energy, which heats up the earth’s surface and atmosphere. Each photon contributes a bit of energy, so the earth heats up more when the density of photons striking its surface is greater.

When the sun is directly overhead, rays of sunlight strike the earth’s surface head-on, which means that the photons in a given patch of sunlight are spread over the smallest possible area of the earth’s surface. But the sun is only ever directly overhead if you’re in the tropics, and even then only at noon on two days of the year. The rest of the time and everywhere else, sunlight is always hitting the earth’s surface at an angle. The further that angle is from the perpendicular (i.e., the further the sun is down towards the horizon), the more the photons in sunlight are spread over a greater area of earth’s surface.

You see an example of this spreading out of the photons in a patch of sunlight in the lengthening of shadows towards the end of the day. A shadow represents the patch of sunlight that an object is intercepting. As the shadow lengthens, the area of earth’s surface that that patch of sunlight would have been spread across if the object hadn’t gotten in the way becomes larger and larger.

Of course, this illustration is only really accurate if the object is spherical—like a beach ball, for example—because only a sphere intercepts the same area of sunlight no matter what angle the sun is coming from. So if you have the time, you could set a beach ball on the ground in the late afternoon and watch the shadow gradually get larger as the sun gets closer to setting. But don’t do it on a windy day, because the beach ball might blow away. Or you could just imagine it.

Anyway, when the sun is lower in the sky in winter, the photons in sunlight are spread more thinly over a greater area of the earth’s surface, so there is a lower density of photons being absorbed by the material of the earth and its atmosphere. Fewer photons being absorbed means less thermal energy, so that part of the earth is colder. In short, the change of seasons is caused not by a change in the distance to the sun but by a change in the angle of the sun’s rays as they strike the earth’s surface.

© Joel Benington, 2012.

All images come from Wikimedia Commons, and are used under a  Creative Commons Attribution-ShareAlike 3.0 Unported license.