It all depends on air pressure as to how much moisture the air contains. low pressure = low moisture due to the fact there are less air/water molecules per cubic measurement.
Therefore a place at sea level will have much more moisture than say mexico city at the same temperature.
On the engine care front, I run my engine dry and if I use it again soon i don't bother with ARO. if however I may not use it for a while I add ARO.