The sound can be reflected not only on solid objects, but also on such delicate formations, as the clouds. Moreover, even completely transparent air may under certain conditions to reflect sound waves - it is in the case when it is in the ability to sound different reason from the rest of the air mass. Here is a phenomenon, similar to that in optics is called "total reflection". Sound is reflected from the invisible obstacles, and we hear the mysterious echo, coming from nowhere.
Tyndale accidentally discovered this interesting fact, when had been experimenting with sound signals by the sea. "From completely transparent air was an echo, " he writes. Echo walked towards us, as if by magic, from the invisible sound clouds".
Sound clouds, the famous English physicist called the portions of the transparent air, which causes the sound to be reflected, creating an "echo from the air." Here's what he says about this:
Sound cloud is constantly floating in the air. They do not have the slightest relation to ordinary clouds, fog or haze. The transparent atmosphere can be full of them. Thus can get airy echo; contrary to the prevailing opinion, they can happen and when the clear atmosphere. The existence of such air echo proven by observation and experience. They may generate air currents, different hot or containing different amount of steam".
The existence of sound clouds, opaque to sound, explains some mysterious phenomenon, often observed during battles. Tyndale cites the following passage from the memoirs of an eyewitness about the Franco-Prussian war of 1871:
"The morning of the 6th the number was a complete contrast to yesterday morning. Yesterday was freezing cold and the fog, not allowing anything to see the next mile or so. And the 6th was clear, bright and warm. Yesterday the air was filled with sounds, and today he reigned the silence of Arcadia, not knowing the war. We look with amazement looked at each other. Really disappeared Paris, its FORTS, guns, bombing?... I went to Montmorency, where my eyes were opened a vast panorama of the North side of Paris. However, there was dead silence... I met three soldiers, and we began to discuss the situation. They were willing to admit that he talks about the world, as the morning had not heard a single shot...
I went further in Gonesse. With amazement I learned that the German batteries vigorously shot from 8 o'clock in the morning. On the South side of the bombardment began at about the same hour. However, in Montmorency we have not heard a single sound!... It all depended on air: today he spent his sound as bad as he had yesterday."
A similar phenomenon was repeatedly observed during large battles of 1914 - 1918