"Sound bounces back not only off solid obstacles but also off clouds.  Even absolutely transparent air is able to reflect sound waves in certain circumstances, when for some reason it differs in sound-carrying capacity from the rest of the air mass.  [...]  The famous British physicist gave the name of acoustic clouds to those areas of transparent air which reflected sound, thus producing an 'aerial echo'.  This is what he has to say about it:  'Acoustic clouds, in fact, are incessantly floating or flying through the air.  They have nothing whatever to do with ordinary clouds, fogs or haze.  The most transparent atmosphere may be filled with them; converting days of extraordinary optical transparency into days of equally extraordinary acoustic opacity...  The existence of these aerial echoes has been proved both by observation and experiment.  They may arise either from air currents differently heated, or from air currents differently saturated with vapour.'" —Thank you, Yakov Perelman, for Physics for Entertainment, Book Two, 2008, pp.312-312.