Air drying is a traditional method of bringing timber that has been sawn to different sizes down to air dried moisture content that is suitable for its use. An example of air dried would be for planking which is 100mm thick would dry down to 18-24% average moisture content, which would make the air dried timber dry enough for use internally. Oak beams would need to be air dried for up to 10 years to stabilize and reduce movement. Air dried timber cut to thinner sizes would reach equilibrium of 18/24% average moisture content.
This is the relative humidity of the UK. Further drying would be produced by kilning.