Significant Wave Height
Water
The average height of the one-third highest waves of a given wave group.
Weather
The mean or average height of the highest one third of all waves in a swell train or in a wave generating region. It approximates the value an experienced observer would report if visually estimating sea height. When expressed as a range (e.g. Seas 2-4 ft) , indicates a degree of uncertainty in the forecast and/or expected changing conditions (not that all waves are between 2-4 ft). Generally, it is assumed that individual wave heights can be described using a Rayleigh distribution.
Example: Significant Wave Height = 10 ft 1 in 10 waves will be larger than 11 ft 1 in 100 waves will be larger than 16 ft 1 in 1000 waves will larger than 19 ft
Therefore, assuming a wave period of 8 seconds, for a significant wave height of 10 feet, a wave 19 feet or higher will occur every 8,000 seconds (2.2 hours).