Like all of the Giro helmets in our test, the Atmos is available in three sizes. We found that it fit similarly to the Aeon, Savant, and Giro Xar, which all use the same RocLoc5 retention system as the Atmos. The Giro Air Attack fits a bit smaller than the Atmos due to its slightly different retention system.
The Atmos comes in at least nine different colors to help you match your kit. The Atmos has been a part of the Giro line for some time now, and has been produced in a number of discontinued colors that may still be available around the net.