> "What we managed to show is the combination of the crosswind and the temperature gradient can cause sound that's going to go vertically (to) get refracted back down," he said. "The sound was literally bent back downwards."
> The effect often happens at dawn and dusk because sound bends from hotter air into cooler air. During the day, the ground is hotter than the air above it so sound bends vertically upward.
> "Its only when the sun stops falling on the ground that the ground cools down. Then the air gets hotter above," he said. "That's when sound can bend towards the ground and become trapped."
https://www.nbcnews.com/id/wbna43154773
> "What we managed to show is the combination of the crosswind and the temperature gradient can cause sound that's going to go vertically (to) get refracted back down," he said. "The sound was literally bent back downwards."
> The effect often happens at dawn and dusk because sound bends from hotter air into cooler air. During the day, the ground is hotter than the air above it so sound bends vertically upward.
> "Its only when the sun stops falling on the ground that the ground cools down. Then the air gets hotter above," he said. "That's when sound can bend towards the ground and become trapped."