National Weather Service United States Department of Commerce





Significant Wave Height
Commonly Referred to as Seas in the Marine Forecast


This is the average of the highest one-third (33%) of waves (measured from trough to crest) that occur in a given period. This is measured because the larger waves are usually more significant than the smaller waves. For instance, the larger waves in a storm cause the most beach erosion, or the larger waves can cause navigation problems for mariners. Since the Significant Wave Height (Seas) is an average of the largest waves, you should be aware that many individual waves will probably be higher. On average, about 15% of waves will equal or exceed the significant wave height. Occasional seas are the average height of the highest 10 percent of the waves. The occasional seas could be 25-30% higher than the significant wave height.

If we take a sample forecast of Seas Beyond the Reef of 2 to 4 feet, this implies that the average of the highest one-third waves will have a Significant Wave Height of 2 to 4 feet. But mariners need to keep in mind that roughly one of every ten waves will be greater than 4 feet; one in every one hundred waves will be greater than 5 feet; and one in every 1000 waves will be greater than 6 feet.

As a general rule, the largest individual wave one may encounter is approximately twice as high as the Significant Wave Height (or Seas).

Note: Seas can refers to the combination or interaction of wind waves and swells (combined seas) in which the separate components are not distinguished. This includes the case when swells are negligible or are not considered in describing sea state.