Though a good guide in the center of high seas and fetch areas, sea vectors can become less meaningful once the seas escape their source. In relatively clear waters, the sea vector can become heavily influenced by the direction of non-contributory surface winds in that area. For example, a solid long-period ground swell that is 1,000 nmiles away from it's source heading north might encounter a weak fetch area that is blowing south, completely contrary to the heading of the swell. This condition makes wave models that only produce a sea height chart less useful for tracking swells that have moved substantially away from their source. The second image that can be produced by a wave model is the Swell Period chart.
Period is indicated in seconds.
Like the Sea Height chart, it use either color-coding or lines to indicate concentrations of swells and seas of equal period. An arrow is used to indicate the average heading of the swell for a specific location and is called a 'swell vector '. Waves of equal period join and travel together across the ocean. These waves become clearly visible on the Swell Period chart. They appear like a wall of water moving out from the storms center.