Paul: It's to do with the temperature layers in the air at different times of the day. The different temperatures create different air densities, and when these are "stacked right" you get a reflection effect that bends some of the sound waves back towards the ground (whereas in other conditions it will travel straight, or actually bend upwards into the sky). It's the same sort of idea as how the straw in your lemonade looks like it has a bend in it.
This is also the reason they typically do blasting and demolitions at certain times of the day, so that the sound will not be louder at ground level compared to other times.
And yes, I used to teach physics.
