Equilibrium wood moisture content (EMC) calculated based on relative humidity and temperature in degrees Celsius.
The equilibrium moisture content (EMC) of wood is reached when the wood is neither gaining nor losing moisture. EMC of wood is dependent on temperature and the relative humidity of the place the wood is located and stored.
Air drying is the exposure of lumber to outside environmental conditions. For instance if you were to air dry your firewood it would eventually obtain a certain moisture content (EMC) based on the relative humidity and temperature of your location.