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