Refraction is exactly what is happening:
A google image search for “atmospheric sound refraction” provides numerous illustrations of what’s happening.
From this site:
At night or during periods of dense cloud cover, a temperature inversion occurs; the temperature of the air increases with elevation, and sound waves are refracted back down to the ground. Temperature inversion is the reason why sounds can be heard much more clearly over longer distances at night than during the day—an effect often incorrectly attributed to the psychological result of nighttime quiet.