Air in the atmosphere will not be perfectly dry since even in very cold air there will still be a trace of water vapor that can cause damage to moisture senstive equipments, The amount of water vapor in the air depends on the dewpoint of the air. When water vapor is ignored what is left is a fairly fixed percentage of the percent by volume or percent by mass of Oxygen, Nitrogen and Argon. However, Dry air is that air has no water vapor.
