Mysterious sounds found in the stratosphere

The stratosphere is a fairly cool layer of the Earth’s atmosphere. Rarely disturbed by planes or turbulence, microphones in the stratosphere pick up sounds that are not heard anywhere else, at least in that way. This includes natural sounds like thunder, artificial sounds like explosions, and even sounds of unknown origin.

All this is being verified by Daniel Bowman’s team from the National Watermelon Laboratories in the United States, which is sending solar-powered hot air balloons equipped with microphones and other equipment into the stratosphere.

The diameter of these balloons, made by Bowman and his colleagues, is between 6 and 7 meters.

Despite their large size and great data collection capacity, these balloons are quite simple.

“Our balloons are basically giant plastic bags with a little charcoal powder inside to make them black. We made them out of painter’s plastic, duct tape bought from a hardware store, and powdered charcoal bought from a pyrotechnic supply store . When sunlight comes in. The darker the balloon, the air inside heats up, and the balloon begins to float and climb up. This passive solar energy is enough to lift the balloon more than 20 kilometers into the sky from the surface ,” Bowman explains. “Each balloon only requires about $50 worth of materials and can be made on a basketball court.

With these balloons, researchers collect data. To detect low-frequency sounds, they use microbarometers, which were originally designed to monitor volcanoes.

After the balloons are released, they track their routes using GPS, a necessary function because the balloons sometimes travel hundreds of kilometers and land in inaccessible places. But because balloons are cheap and easy to make and launch, they can drop much higher and collect more data.

In addition to recognizable natural and artificial sounds, Bowman and his colleagues have detected other sounds that they could not identify.

“There are mysterious infrasound signals in the stratosphere that occur a few times an hour on some flights, but their origin is completely unknown,” confesses Bowman.

Infrasound is sound below the lowest pitch that the human ear can pick up.

Bowman’s team hopes to uncover the mystery of these stratospheric infrasounds of unknown origin through future studies.

The latest results in this line of research have been presented at the recent Congress of the ASM (Acoustical Society of America) held in the US city of Chicago.