Water condensation above dew point

Anyone who stores rust-prone instruments knows that even in temperature-controlled air spaces rust can form on uncoated steel. Since [rust requires water](https://chemistry.stackexchange.com/q/5134/5354), this suggests that water will condense out of air onto surfaces even above its dewpoint.


How is surface condensation of water above the dew point characterized and quantified?


Or is "condensation" not the right term for this? There does seem to be a non-linear relationship between relative humidity and the observed effects of that airborne moisture on exposed solids.


