Why is it possible to hear noises from far away clearly on cool damp evenings? Is it because sound travels better in cool, damp air than in dry air?

The most likely explanation for this phenomenon is that warm, damp air at ground level was sitting below a layer of drier, hotter air above. This results in refraction of the sound back down towards the ground and also the unusually high propagation distances – you hear sound which, under normal circumstances would pass over your head.

Also another explanation could be that sound travel better at night, but the reduction of daytime noise means that continual sounds become more apparent. In the evening it is usual for the temperature near the ground to fall more quickly than that higher up, causing a temperature inversion. It is not uncommon for there to be a change in the air’s moisture content at the top of this inversion layer.

The change in temperature and moisture lead to a density discontinuity which refracts sound waves much as the refraction of light causes a mirage. Sounds are trapped near the ground rather than dispersed. The wind speed in this low-level inversion layer is also normally lighter and this adds to the effect.