Rarefaction's the reduction of a medium's density, or the opposite of compression. A natural example of this's as a phase in a sound wave or phonon. Half of a sound wave's made up of the compression of the medium, and the other half's the decompression or rarefaction of the medium. Another natural example of rarefaction's in the layers of our atmosphere. Because what constitutes our atmosphere has mass, it's definite that most of the atmospheric matter will be nearer to the Earth. Therefore, a… (More on Rarefaction)