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When I stand outside at 2 am I can hear everything, a car 2 miles away, a train 10 miles away. At 2 pm, even when there's no traffic, it seems noisy. What is it? I know everyone's awake, I know people are talking and shouting, and dishwashers are going, and factories are running all their equipment... but what else? Does night time do something to the sound waves?
Actually, yes, there IS something about night air that effects sound waves. Specifically, night air is cooler than air during the daytime. Sound waves travel faster through warm air than cold air, and at night, there is usually a layer of cold air trapped close to the ground with a layer of warmer air higher up. When a sound wave tries traveling through these two layers at once, the part lower to the ground travels more slowly, literally bending the higher part of the wave down to the ground.
What this means for your ears is that all the sound waves that normally would be passing over your head are being pulled down to you, letting you hear things that are much further away. This will also happen if you're standing in front of a cold body of water or on a snow field, to some degree.
A page with some graphs and animations showing this effect at work can be found here: http://paws.kettering.edu/~drussell/Demos/refract/refract.html
No there's just more people moving maybe not around you but the sound can travel a couple of miles so there's always noise and at night time there's a lot less people and machines moving most people are home sleeping or just a lot more quieter like you said you can hear a train 10 miles away well during the day I'm sure that in between those 10 miles are a lot of cars moving which is why you can hear noise.
Thanks, Mike. You're right... just remembering the same effect after a snow storm. I always thought it was the snow creating some kind of sound absorber.
And I always thought sound traveled faster in cold air, which was what was causing it, so that was a fun little bit of research.
