Wave energy period is a sea state parameter, derived from the zeroth and first negative moments of the frequency spectrum. It is the period corresponding to the weighted average of the wave energy. Wave energy period values in this dataset are represented in seconds. The Wave Energy Resource Assessment project is a joint venture between NREL and Virginia Tech. NREL is the prime contractor and Virginia Tech is responsible for development of the models and estimating the wave resource. NREL also serves as an independent validator and also develops the final GIS-based display of the data. Data represent monthly summaries for the time period from January 1980 to December 2009 (30 years) for the following parameters: significant wave height (ssh), wave energy period (wep), and wave hindcast direction (dfp). For complete details regarding this study please read the full report, available here: https://openei.org/datasets/dataset/mapping-and-assessment-of-the-united-states-ocean-wave-energy-resource. A note about the data: Bathymetric effects are known to have a large effect on wave characteristics at depths shallower than approximately 20m (\~65 ft) on the Atlantic coast and 50 m (\~160 ft) on the Pacific coast. A variance between depths exists due to the feature differences for each continental shelf. The methodology used in this resource assessment precludes providing site-specific information to such developers. Reliable site-specific information in shallow waters can only be produced using results from models with higher spatial resolution that include shallow-water physics. The wave resource assessment group acknowledges that its results will not be accurate in the shallower waters of the inner continental shelf. These shallow water regions are located within the dark gray boundaries on the map.