1. ‘Significant Wave Height’ A closer look at wave forecasts Based on work by Tom Ainsworth, NWS Juneau, AK November 16, 2009 Great Wave off of Kanagawa by Japanese artist Katsushika Hokusai

2. What do you know about waves? wave A wave is a periodic vertical disturbance of the sea surface.

3. Wave Formation Wave height is dependent on: Wind speed Fetch length Duration of time the wind blows over the fetch Questions 1 & 2 Graphic courtesy of Tammy Pelletier, WA State Dept of Ecology

4. Seas vs. Swell Seas – wind pushes water into steep crests or peaks Swell – when waves move away from the wind their crests become smoother Question 3

5. Parts of a Wave crest trough Wave height wavelength

6. Wave Steepness Wave steepness is the ratio of wave height to wave length.

7. Significant Wave Height The average height of the highest one- third waves in a wave spectrum. Question 4

8. Interpreting Significant Wave Height If you received a marine weather forecast predicting “SEAS 10 FT” in the coastal or offshore waters, what is really being conveyed in that forecast? H s = 10 ft H (mean) = 0.64 times H s = 6.4 ft H (most probable) (slightly less that H mean) = 6 ft H 1/10 (10% highest waves) = 1.27 times H s = 12.7 ft H 1/100 (1% highest waves) = 1.67 times H s = 16.7 ft H max (highest wave you should look out for) = approximately 2 times H s = 20 ft!

9. Work Cited Ainsworth, Tom. Significant Wave Height: A closer look at wave forecasts. National Oceanic and Atmospheric Association.