Wood responds to atmospheric moisture in a predictable manner. If the relative humidity and temperature of the air are known, we can determine the equilibrium moisture content of that air, or EMC. Knowledge of the EMC is useful in estimating how fast lumber on an air drying yard will dry; how long it will take for kiln dried lumber to pick up moisture when stored in a warehouse; and the performance of finished wood products in exterior service. The National Oceanic and Atmospheric Administration publishes 30-year Climate Normals for locations in the United States. Climate Normals for 1981-2010 were used to determine the average monthly exterior EMC values for 252 locations in the US.