The hours of darkness, by and large, are more silent than the day. Paradoxically, however, this nocturnal impression of pervasive quietness is enhanced by the fact that distant sounds, though few and far between, are often very clearly heard. And there are sound reasons, if I may say so, why this is.
Sound travels relatively slowly through air that is cold and dense. This results in the general rule that sound waves tend to be diverted, or refracted, towards a zone of lower temperature.
On a summer's day, when the air near the surface of the Earth is warmer than aloft, the waves bend upwards and sounds are therefore inaudible at a relatively short distance from their source.
But on a clear and cloud-free night, as the Earth loses heat by radiation, the air temperature near the ground is usually much lower than that at some distance up above.
This has the effect of diverting sound waves, originally headed upwards, back again in the direction of the ground, so distant noises accentuate the sound of silence.
Acoustic refraction of this kind also explains a related phenomenon that has been known for centuries. It was noticed that the sound of gunfire was often heard, not only in the immediate vicinity, but also in an outer ring 60 miles or more away from the source, with a zone of silence in between.
It was also noticed that the sound appeared to take an unusually long time to reach this outer region.
The reason for this anomaly was deduced in the early years of this century. Listeners in the inner zone hear sound waves which have travelled directly towards them through the lower atmosphere; the existence of the outer zone can only be explained by the hypothesis that waves originally moving upwards from the source are subsequently redirected downwards again to reach the earth a great distance away.
And the only reason sound waves should so dramatically change their direction of propagation would be the existence of a region of relatively high temperature high above the Earth, again refracting the sound in the direction of the colder air.
Early meteorologists put this hypothesis to good use. During the 1920s accurate measurements were taken of the dimensions of the "silent" and "noisy" zones associated with prearranged loud explosions.
By measuring the length of time taken by the sound to reach a selection of different sites, and the angle from which it appeared to approach them, researchers calculated that a warm layer existed in the atmosphere between 20 and 40 miles above our heads a conjecture that was subsequently confirmed.