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Thunder Megaphone

Source: Personal Experience

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Thunder is an impressive sound, but under some conditions it is utterly extraordinary.

We've all heard thunder, and we all know what causes it. Many of us have heard two distinct kinds of thunder, but perhaps we never really noticed or thought about it. Recently, I heard a third kind of thunder.

"Ordinary" thunder - a thoroughly extraordinary sound, but the kind of thunder we hear most often - happens when lightning occurs at some distance from the observer. The initial sound of the lightning bolt echoes off surrounding objects and air masses. Because it is echoed so many times, the thunder stretches out into many, many seconds, even though the initial sound might have lasted a second or two at most. Moreover, because the initial sound echoes off soft things with indistinct surfaces - clouds, thermoclines, and weather fronts - and because many echoes reach the ears of the observer at different times, the original sound is greatly distorted. Almost all high frequency components are filtered out, and the observer hears mostly a low-pitched rumble that rises and falls in intensity several times over a period of seconds.

When lightning happens to strike very close to the observer, within a few hundred feet, the sound is entirely different. The observer might not hear echoes of the thunder at all, but only the pure initial sound. It is a single, sharp, intense "POW!" It may be followed by a much quieter, but still loud, whistling or hissing sound that appears to be moving away from the observer. This aftereffect is actually the sonic boom of the lightning bolt arriving at the observer's location after the supersonic lightning itself. The observer hears first the sonic booms made last, followed by the earlier sounds catching up later, and so on, which creates the illusion of a sound source moving away from the observer.

This same phenomenon can be observed when a supersonic airplane passes nearby, but very few of us ever get to experience this. If you ever do get buzzed by a supersonic airplane, as I have been, pay careful attention. After the initial sonic boom, you will hear the airplane departing in both directions. That is, if you can still hear anything at all. If ever you see a plane coming toward you but you can't hear it, cover your ears until you hear the sonic boom.

But back to that third kind of lightning.

I had spent that day hiking up and down Mount Jackson, then had myself a nice little campsite dinner of canned beef stew. It looked like thunderstorms moving into Crawford Notch, so I tidied up my campsite before settling into my camp chair for a cup of merlot and a nice cigar for "dessert." Everything was in the car except the tent, the sleeping bag, the lantern, the camp chair, "dessert," and me. I figured that even if the rain started in an instant, as thunderstorms often do, I could have the lantern extinguished and under the picnic table, the chair folded and thrown into the car, and me and my dessert in the tent without getting too wet.

Halfway through dessert, Mother Nature put my figuring to the test.

It was a success. I had to let my cigar go out so I wouldn't choke on my "dessert," but such are the risks one takes when one goes camping.

Darkness had fallen by the time the second thunderstorm rolled up from the south. I occupied myself by counting the time interval between lightning and thunder to track the movements of the storms. Fifteen seconds before the thunder rolled up from somewhere west of Mount Bemis, and I knew the storm was just under three miles southwest of me. Seven seconds between the flash and the rumble beyond Frankenstein Cliff, and I knew the storm was passing nearly a mile and a half to my west.

And then it happened!

A flash. I counted eleven seconds. And I heard a sound unlike any thunder I had ever heard before.

The cacophony included at least half a dozen rapid repetitions of the kind of "POW!" of a nearby lightning strike. But at the same time, there was the rumbling and rolling and roaring of "ordinary" thunder, but much, much louder than usual.

Before I could figure out what that sound was, there was another flash somewhere to the north. Again I counted eleven seconds, and again I heard that utterly incredible crackling and powing and rumbling and roaring.

This time, I figured it out.

It was a lightning strike right within the upper reaches of Crawford Notch just a couple of miles north of me. It was a lightning strike right within a gigantic stone megaphone formed by Webster Cliff on the east, Mount Field and Mount Willey on the west, and the old glacial cirque of Mount Willard for a backstop on the north.

And this 1,500 foot deep, three-mile-long granite megaphone was pointed right at Dry River Campground.

Yes, the beautiful U-shaped glacial valley of Crawford Notch is a nearly perfect megaphone, albeit open on top. The bare stone faces of Mount Willard and Webster Cliff echoed the initial "POW!" of the thunder almost undistorted. The western slope of the notch is a bit more heavily wooded, but there's enough bare ledge and rockslide there to provide a pretty good echo. The open top of the notch was covered by the underbelly of the thunderstorm itself, which provided enough of a soft echoic surface to create the usual rumbling of thunder in addition to the clean "POW!" echoes off the rock faces.

But all of this sound was extraordinarily loud because of the megaphone that focused it all right on me and my campsite.

After I got this all figured out, there was a third lightning flash in the north. Yes, eleven second later, there was that glorious, unearthly sound again.

Half an hour later, the third thunderstorm of the night passed just over a mile east of me. A flash of lightning. Six seconds later, there was an anemic roll and rumble from somewhere just beyond the top of Montalban Ridge. Even though this lightning strike was only half as far away, the sound was nowhere near as loud, nor as spectacular.

Now I began to wonder why I had never heard this kind of thunder before. I have probably visited Crawford Notch on at least two hundred day trips and some forty camping trips, and I had experienced thunderstorms there at least a dozen times. Why had I never heard the Thunder Megaphone before?

My best guess is that I probably have heard it before, but never noticed it. Most of the times I've camped there, it was with a crowd of friends and family. Much goes on when a thunderstorm rolls in. Split-second decisions have to be made regarding whether to finish cooking whatever we were cooking in the rain, or put it aside and try to finish it after the rain stops. Ponchos have to be broken out and put on, while at the same time, various disorderly what-nots need to get stashed into cars and tents before they get soaked. There is a bit of yelling and shouting to be done, just to make sure the efforts are coordinated properly. And paradoxically among the mayhem, kids and dogs need to have their fears calmed. Meanwhile, tarps over the tents and picnic tables are flapping in the gales, making a poor imitation of thunder themselves.

In all my 25 years camping in Crawford Notch, this may have been the first time I experienced a thunderstorm while I was camping there alone. There was no tarp over the tent, as I figure the tent can withstand the rain just fine - that's what it was designed for, you know. There wasn't much miscellany lying around the campsite to begin with, and I had anticipated the thunderstorm well enough to get everything that I didn't want to get wet into the car long before the rain started.

So, when the lightning and thunder came, I had nothing to do but observe.

What a treat!

I half hope we get a thunderstorm the next time we go camping in the mouth of the Thunder Megaphone.

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