The root atmo- means air or vapor, originating from Greek ἀτμός (New Latin atmosphaera).
The atmosphere has this root, being the layer of gases surrounding a planet, or some other body of sufficient mass, which is held in place by the body's gravity. On Earth, the atmosphere protects life on Earth from ultraviolet solar radiation, retains heat though the greenhouse effect, and reduces temperature extremes between day and night. The composition of the Earth's atmosphere is 78.1% nitrogen, 21.0% oxygen, 0.9% argon, and 0.04% carbon dioxide with small amounts of other gases.
The term atmo has also developed meanings as the Internet developed. It is an acronym for 'according to my opinion', which can be used in place of the more common 'IMO' and 'IMHO'. It can also be used as a short-hand of 'at the moment'.