Storm in a Teacup: The Physics of Everyday Life(42)

Light waves hitting the water surface are either reflected back up into the sky or pass through and travel down into the depths. But sometimes, a tiny particle or even the water itself acts as an obstacle, sending the wave off in a new direction. This redirection may happen to the same light wave enough times that it eventually makes its way back out to the air. And on that long journey, the water has filtered the light. The light waves coming from the Sun are a mixture of lots of different wavelengths, all the colors of the rainbow. But the water can absorb light, and it absorbs some colors more than others. The first to go is the red light—a few yards of water is enough to get rid of most of that. And then the yellows and greens follow after a few tens of yards. But blue light is hardly absorbed at all—it can travel for huge distances. And so by the time the light is on its way out of the ocean, most of what’s left is blue. The reason tap water is colorless is that there isn’t enough of it to make a difference. Tap water does have a color, the same color as all the other water in the world. But that color is so faint that you need a huge amount of water all together to actually see the effect that the water is having on the waves going through it.# When you do see it, it’s spectacular, and bright blue crayon really is the right choice. But you’d never learn that from a tap.

So as waves travel, they can be absorbed by whatever they’re passing through. It’s a very slow process of attrition, sneaking away wave energy tiny bit by tiny bit. The amount that’s lost depends on what type of wave it is and also its wavelength. All this variability means there’s a huge richness in what waves are doing and what they can tell us. We can see and hear some of the contrasts in one of my favorite atmospheric phenomena: the thunderstorm.

A thunderstorm is a magnificent spectacle, a dramatic reminder that air is far more than an invisible filler for the sky. Our atmosphere is host to vast quantities of water and energy, and usually these hefty commodities are shunted around slowly and peacefully. The thundercloud, the mighty cumulonimbus, develops in order to rebalance the atmosphere when peaceful shunting is no longer enough. The system starts when buoyant, warm, moist air near the ground shoves upward into the cooler air above, taking huge amounts of energy with it. In the center of the vast cloud, hot, humid air rises rapidly, churning the atmosphere above it and liberating huge raindrops. Most dramatic of all, the churning causes electrical charges to be separated and redistributed to different parts of the clouds. The charges accumulate until nearby clouds or the Earth itself are stabbed by giant pulses of electrical current, carrying the excess electrical charge away. Each lightning bolt lasts for less than a millisecond, but the thunder echoes across the landscape for far longer. I love thunder and lightning, both for the theatrical spectacle and for the glimpse it gives us into the atmospheric engine. Thunderstorms produce such unlikely opposites: the sharp, shocking flash of lightning contrasting with the deep, drawn-out rumble of thunder. But both are beautiful examples of the versatility of waves.

The lightning bolt is temporary. The electrical connection is a superheated tube of atmosphere, stretching from the thundercloud to the Earth or perhaps to another cloud. It’s a corridor full of molecules that have been blown apart by the energy rushing past them. For a brief instant, the temperature in that tube may reach 90,000°F, and so it blazes blue-white. A giant pulse of light waves whooshes outward from the tube, filling the landscape, but they rush away at such an enormous speed that they’re gone in an instant. As the superheated tube carrying the electrical current heats up, it expands sideways, thumping into the air around it. This gigantic pressure pulse ripples outward through the air, following the light, but much more slowly. These are sound waves, and this is the thunder. We know that lightning bolts exist because they make both light and sound waves.

The most important thing about a wave is that it’s a way of letting energy move, but without also having to move air, water, or “stuff” of any kind. This means that waves can billow through our world very easily, disturbing things enough to be interesting and useful, but not so much that they’re shoving our world about and causing major disruption. A lightning strike liberates a lot of energy, and light and sound waves can carry some of that energy out into the rest of the world, sharing it out. Even though the air doesn’t go anywhere overall as the sound ripples past, huge amounts of energy are transferred onward. Light and sound are different types of wave, but the same basic principles apply to both. For example, both light and sound can be changed by the environment that they pass through. In the case of thunder, we can directly hear what’s happening to the waves.

My favorite place to be is about a mile from the lightning strike. Once the flash has signaled that the sound is on its way, I like to imagine that giant pressure ripple spreading out toward me. As I look out across the landscape, I can see right through the ripple, but it takes a few seconds to reach me with the first whipcrack of thunder. These sound waves are traveling at about 1,100 feet every second or 767 mph, which means they’re taking 4.7 seconds to cover a mile. That sharp crack is similar to the original sound made as the lightning bolt expanded right at the ground. But here’s what makes the sound of thunder so distinctive: What I hear just after the initial crack is the sound from slightly higher up the lightning bolt. It started as the same sound, but it took longer to reach me because it had to travel a sloping, and therefore longer, path. And then as the thunder rumbles on, I’m hearing the sound from higher and higher up that same lightning bolt. If it takes five seconds for the first crack to reach me, it’ll take two more seconds before the sound from one mile up hits me, and another four seconds before the sound from two miles up arrives. All these sound waves started off more or less the same, just in different places. And that means that as I listen, I can hear how the atmosphere is changing these waves. As time goes on, the only difference is that they’ve traveled farther. So the highest-pitched sounds, that first sharp crack, disappear very quickly as the high-frequency waves are absorbed by the atmosphere, but the lower-frequency waves rumble on. As time goes on, and the waves have traveled farther and farther, the overall pitch gets lower and lower, because the highest notes are consumed by the air, but the lowest notes just keep going. If you’re far enough away, the air takes it all and the sound never reaches you. But the lightning has a greater reach—these light waves are different and they don’t depend on the air for assistance as they travel. They don’t get absorbed by air as easily, but they can be altered in other ways as they whoosh through the world.