The stratosphere is the second layer of Earth’s atmosphere, extending up to 50 kilometers (30 miles). It is relatively calm – but through it, it is possible to hear faint sounds coming from the troposphere, the part we inhabit. But not all the sounds detected have a clear origin.
Scientists have used microbarometers, attached to hot air balloons that fly thanks to passive solar power, to monitor the sounds. These instruments can measure small changes in pressure. First designed to monitor volcanoes, they can now be used to detect sounds lower than the human ear can perceive.
Sources of stratospheric sounds have been traced back to natural events such as thunderstorms and collisions between ocean waves. Human activities can also be picked up, such as wind turbines or explosions. What’s more peculiar is a sound that currently has no explanation.
“[In the stratosphere,] there are mysterious infrasound signals that occur a few times per hour on some flights, but the source of these is completely unknown,” research lead Daniel Bowman of Sandia National Laboratories, said in a statement.
A fascinating aspect of this research is that while it discovered something mysterious about the stratosphere, it used a very simple setup. The balloons are made from over-the-counter material, so they are quite cheap. As these balloons and instruments travel for hundreds of miles, they can land in hard-to-reach places – unless the US Air Force shoots them down first.
"Our balloons are basically giant plastic bags with some charcoal dust on the inside to make them dark. We build them using painter’s plastic from the hardware store, shipping tape, and charcoal powder from pyrotechnic supply stores. When the sun shines on the dark balloons, the air inside heats up and becomes buoyant. This passive solar power is enough to bring the balloons from the surface to over 20 km (66,000 ft) in the sky,” Bowman explained. “Each balloon only needs about $50 worth of materials and can be built in a basketball court.”
Studying the stratosphere is very important. This is where the ozone layer is located, which protects life on Earth from the most dangerous ultraviolet light from the Sun. Humans are also affecting this portion of the atmosphere beyond the ozone layer and its famous hole: Greenhouse gases have been shrinking the stratosphere.
The findings of the stratospheric sounds including the unidentified one were presented at the 184th Meeting of the Acoustical Society of America.