Sound travels faster in warmer air than in cooler air.
Sound travels faster in denser media.
Speed of sound in a gas is almost independent of the density of the gas.
On a cold night, you can hear sound from farther on because of refraction.
Refraction is also why sometimes thunder for faraway lightning goes missing in action.
References:
http://paws.kettering.edu/~drussell/Demos/refract/refract.html
http://www.usatoday.com/weather/resources/basics/sound-refraction.htm
http://en.wikipedia.org/wiki/Speed_of_sound
Editorializing:
Did this simple fact fell through the cracks of my education and my juvenile curiosity? How intuitive is it? A new measure of the sense of wonder at old scientists goes as follows: If you have to read wikipedia to figure out somebody’s work, they’re a genius. For example, can you prove Pythagoras’ theorem? Or apply Archimedes’ theorem (after recalling what it is and why it is so)?
And what good is all this understanding now? Can we accept existing science as a given, and move on with practical knowledge of only what matters to us, and what we can apply? So for argument’s sake, let’s say now I know this bit about sound. Where do I apply it right now? Shall I use it to make drafts of warm air (flowing up in thin sheets) in front of a stage so the performers don’t hear the noise of the crowds? Or noise insulation in city windows? Dang, I’m convinced now, dude’s got to know classical physics.