The sound of thunder

Last Sunday (April 14) we had a series of thunderstorms roll through the region, kind of unusual for upstate New York at this time of year.  We're not particularly stormy in general, but most of the thunder and lightning we do get comes in the heat of midsummer.  On Sunday, though, a warm front brought in turbulent, moist air, and we got some decent storms and rain for most of the day.

At 11:51 AM (EDT), though, something odd happened.  There was a deep, shuddering rumble that repeated three times within the span of about two or three minutes.  (The first was the strongest.)  I grew up in the Deep South, where thunder is a frequent occurrence, and to my ears this didn't feel or sound like thunder.  Immediately I thought of a mild earthquake -- primed, of course, by the April 6 quake, centered in New Jersey, which was felt over large regions of New York and the neighboring states.

The rumble we experienced preceded the arrival of the strongest of the storms; because of that, and the fact that it "sounded wrong," I was convinced that we'd experienced an earthquake.  That conviction intensified when reports began to pour in that the same noise had been heard at the same time -- in locations separated by fifty kilometers or more.  (Thunder ordinarily can only be heard about fifteen kilometers from the source.)  

My wife, on the other hand, was absolutely sure it was thunder, albeit rather powerful and deep-pitched.

Well, let it never be said that I won't admit it when I'm wrong.

I started to doubt myself when the Paleontological Research Institution in Ithaca (only ten miles from my home) reported on Monday morning that despite numerous people calling in to report noise and shaking, their seismometer had not recorded an earthquake.  That seemed pretty unequivocal -- and after all, there had been storms in the area, even though at the time we heard the rumble, the center of the front wouldn't arrive for over an hour.  But if it had been thunder, how had a single thunderclap (or three in rapid succession) been heard over such a great distance?

The answer turns out to be a temperature inversion.  Ordinarily, temperature decreases as you go up in altitude; but this effect competes with the fact that cool air is denser and tends to sink.  (This is why in winter, the greatest risk of frost damage to plants is in isolated valleys.)  So sometimes, a wedge of warm air gets forced up and over a blob of cooler air, meaning that for a while, the temperature rises as you go up in altitude.

This is exactly what happens in a warm front; the warm air, which carries more moisture, rises and forms clouds (and if there's enough moisture and a high enough temperature gradient, thunderclouds).  But this has another effect that is less well known -- at least, by me.

The difference in density of warm and cool air means that they have different indices of refraction -- a measure of how fast a wave can travel in the medium.  A common example of different indices of refraction is the bending of light at the boundary between air and water, which is why a pencil leaning in a glass of water looks kinked at the boundary.  At a shallow enough angle, the wave doesn't cross the boundary at all, but reflects off the surface layer; this causes the heat shimmer you see on hot road surfaces, as light bounces off the layer of hot air right above the asphalt.

Sound waves can also refract, although the effect is less obvious.  But that's exactly what happened on Sunday.  A powerful lightning strike created a roll of thunder, and the sound waves propagated outward at about 343 meters per second; but when they struck the undersurface of the temperature inversion, instead of dispersing upward into the upper atmosphere, they reflected back downward.  This not only drastically increased the distance over which the sound was heard, but amplified it, changing the quality of the sound from the usual booming roll we associate with thunder to something more like an explosion -- or an earthquake.

So despite the jolt and the odd (and startlingly loud) sound, we didn't have an earthquake on Sunday.  I'm kind of disappointed, actually.  I didn't feel the one on April 6 -- although some folks in the area did -- and despite having lived in a tectonically-active part of the country (Seattle, Washington) for ten years, I've never experienced an earthquake.  I'd rather not have my house fall down, or anything, but given that the pinnacle of excitement around here is when the farmer across the road bales his hay, a mild jolt would have been kind of entertaining.

But I guess I can't check that box quite yet.  Thunder, combined with a temperature inversion, was all it was.

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