You may notice sounds this time of year that you don’t during the warmer months — a train whistle from a neighboring town, the muffled humming of vehicles whizzing down a distant highway or even a house explosion tens of miles away.
This is usually caused by something called an inversion. It’s a layer in the atmosphere where the temperature increases with height, instead of naturally dropping. Most of the time, it gets colder the higher up one goes. But under certain conditions, a lid of warmth can overspread a cooler air mass, “capping” it.
The effects of this change throughout the year. In the springtime, that cap can suppress storm growth much of the day before eroding, allowing explosive storm development on the Great Plains. In the summer months, inversions tend to be higher up and weaker/more diffuse — but stronger, shallow ones can trap pollutants near the surface and lead to poor air quality.
During the autumn, the air is dry but not too cold. That allows for big-time temperature swings.
As soon as the sun sets, the ground rids itself of heat, and “radiational cooling” carries thermal energy away to space. It leaves the ground cold and the air just above it crisp, but a thousand or so feet up, the air doesn’t cool down quite as fast. That inversion acts as a wall, serving as a natural barrier for sound waves.
That’s because of two processes: refraction and reflection. The former is a bending of waves based on their speeds. If an inversion is strong enough, the change in refractive index (the amount of bending of a wave) can curve sound waves traveling at an upward angle back towards the ground. With a dramatic enough inversion in place, sound waves can reflect off the ground, bouncing upward, before refracting back to the ground again. This cycle can repeat while waves travel long horizontal distances.
If the sound is coming from above — like from a distant airplane or a rumble of thunder — it gets accelerated toward the ground and beamed downward at more vertical angle. That’s part of the reason thunder heard at the same distance will be louder in the typically chillier months than in warmer ones. It’s also why cold-season thunderstorms may seem to have more thunder. This is common with elevated thunderstorms that are rooted not near the ground but in the warm layer hovering up above.
Because the sound from the lightning ricochets between the ground and the inversion, you’ll hear increasingly faint carbon-copies of the same thunderclap at intervals. And if you listen closely enough, you’ll notice that the sound alternates its orientation every second time; with each successive flip at the inversion, it becomes the mirror-image of the sound it was previously. In most cases it’s impossible to hear enough reflections to notice this phenomenon, but it’s not out of the question under ideal circumstances. Most of the time, the reflected sound would be masked by booming from the original thunderclap’s wave being refracted back down to the surface.
Sometimes, we can even see inversions on radar in the fall. When the temperature rises with height, the ordinarily upward-aimed radar beam can be bent back to the surface and pick up “ground clutter” targets like hills, buildings and infrastructure.
But that’s not the only interesting thing the atmosphere does to sound. Ever heard an airplane pass overhead? It doesn’t stay at the same volume — instead, it may pulse a bit or fade out. That’s because of invisible waves in the atmosphere causing minute local temperature, pressure and density variations.
Think of the atmosphere like a swimming pool. You know those beautiful ripples in sunlight you see on the bottom? The same occurs invisibly in the atmosphere — but with sound waves. When waves constructively or destructively interfere, they can change the perceived volume of a sound.
Once in a while, the atmosphere throws up a cloak of invisibility where waves meet up and cancel out. This is called an acoustic shadow.
Acoustic shadows played a role in the Civil War. Union Major Gen. Don Carlos Buell found himself before a military council investigating how he was unaware troops were fighting the Battle of Perryville, less than three miles from where he was camped on Oct. 8, 1862. More than 4,000 Union soldiers were killed in the battle.
Odds are you or someone you know have experienced an acoustic shadow. But it probably wasn’t when you were waging war. Instead, they’re a common aberration on ultrasound images.
Editor’s note, 3:45 p.m. Tuesday: Based on feedback from readers, the discussion of sound wave refraction and reflection was updated and clarified.