In between Earth and space lies an ocean of air.
That “great aerial ocean,” as biologist Alfred Russel Wallace (the other natural selection guy) called it, is an extremely thin envelope of gas andsuspended particles that encompass our planet. To get perspective on just how thin, there’s this very rough equivalency: The atmosphere is to the Earth as an onion’s wafer thin outer skin is to an onion. But while it may be just a sliver, it’s critical to life on Earth.
Source: http://visual.ly/earths-atmosphere-great-aerial-ocean