Both relative and absolute humidity matter, as does the temperature profile.
For static charge the humidity ratio (absolute humidity) is the dominant factor, grams water per kg of air, when this goes too low the leakage current through the air drops too low and large charges can build up.
For corrosion (my specific knowledge is from electronics but should be approximately true for linear rails) we are interested in several environmental factors.
The basics are that you need a water and an Oxygen molecule to be hanging around at the surface of the ferrous metal, the speed at which the ferrous metal becomes rust then depends on the local temperature, chemical reaction rate rises with temperature (subject to a few caveats for corrosion https://mti.memberclicks.net/assets/documents/TAC-Bulletins/Non-Member-Egnlish/Bulletin%2013%20-%20non-member%20English.pdf )
( I just noticed the typo in that URL, must not make a joke about American Egnlish spellings… )
In terms of water presence, this can be as water vapour in the air measured as relative humidity (relative to the saturation humidity at that temperature), above 80% is generally considered to be corrosion triggering for steels, it does not need to condense on the surface for this and there is no water layer to reduce available Oxygen.
The water can also be supplied as condensation on the surface which is triggered whenever the temperature and humidity profiles align such that the metal part temperature is below the dew-point of the local humid air. This can occur in the morning when the metal is at overnight temperature but the air warms up and humidifies, you do not need saturation humidity for this to occur. It can also occur in the evening if the cooling air reaches saturation humidity (fog) in which case the water just falls out onto the nearest available surface.
To deal with this humidity driven corrosion you can use a barrier of some sort (such as the way oil that slowly seeps out of the linear rail blocks) to keep the water off the steel surface, or you can use a corrosion inhibitor which chemically obstructs the corrosion process.
I use both, my linear rails got a coating of this stuff
https://www.arceurotrade.co.uk/Catalogue/Protection-Cutting-Aids-Adhesives/Shield-Technology/MetalGuard-Ultra
and I also, obviously, regularly re-oil my linear rail blocks so I also have an oil film on top, and wherever the linear rails have abraded away my film coating.
The shed the Shapeoko is in is disgustingly damp having been built by morons, designed by an incompetent and commissioned by a total and utter Johnson. The floor is below ground level with no membrane and water leaks through the walls and floor after it rains, everything in there is damp and grows mould unless treated. My linear rails are doing fine so far.
