homing pigeon tests, Cornell to Jersey Hill, 1968 - 1987
via The Atlantic
Acoustic shadow zones — also known as “zones of silence” — are a normal phenomenon: Sound waves bend up through the troposphere (the lower 10 km of the atmosphere), reach the stratosphere where the temperature gradient changes, and are bent back down. The space in between where the rays go up and where they come back down is mostly silent. This is why in big explosions, such as the eruption of Mt. Saint Helens, people closer in often don’t hear anything, while those further out do. Released in a shadow zone, the Cornell pigeons were lost.
But why, then, on August 13, 1969, were they able to navigate so well? When Hagstrum modeled the atmospheric conditions from that date, he found an anomaly: Something — a wind shear or a temperature inversion at about a kilometer’s height — bent the sounds back down. “It bent it down early enough that it hit at Jersey Hill,” Hagstrum said.
No one on the ground would have ever noticed anything odd weather-wise.