THE AIR IN SECTION · A CLEAR NIGHT IN AUGUST
The Sound Comes Back Down
On a clear, still night the ground gives its heat away to the sky, and the air lying on it
turns colder than the air a hundred metres up. Meteorologists call that an inversion; it is
also how the fog is made. Sound moves faster through warm air than through cold —
about half a metre a second for every degree — so a wave leaving the ground travels
faster at its top than at its foot, and tips over. It bends back down.
Nothing is lost upward. What would spread away into the sky is turned and returned,
and skips along the ground for miles: a goods train, a church clock, a dog four fields off,
arriving with the odd nearness of a foggy morning. The fog does the other half. Its
droplets take the high frequencies and leave the low, so what arrives has come further
and lost its edges. It is why a foghorn is pitched low.
Rays traced through a linear inversion of 4 °C in the first 120 metres;
the fog forty metres deep, the source at 1.6. Vertical scale exaggerated 9.5×;
each path drawn fainter with the way it has come and with every return.
TOP OF THE INVERSION · 120 m
1 KM
2 KM
3 KM
A SOUND MADE HERE
AND STILL ARRIVING
nocturne · edition lxviii · 17 august 2026 · by noon the air is mixed again
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