‘Significant Wave Height’

A closer look at wave forecastsBased on work by Tom Ainsworth, NWS Juneau, AK

November 16, 2009

Great Wave off of Kanagawa by Japanese artist Katsushika Hokusai

What do you know

about waves?

A wavewave is a periodic vertical disturbance of the sea surface.

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

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

Parts of a WaveParts of a Wave

crest

trough

Wave height

wavelength

Wave Steepness

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

Significant Wave Height

The average height of the highest one-third waves in a wave spectrum.

Question 4

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?

• Hs = 10 ft• H (mean) = 0.64 times Hs = 6.4 ft• H (most probable) (slightly less that H mean) = 6 ft• H1/10 (10% highest waves) = 1.27 times Hs = 12.7 ft• H1/100 (1% highest waves) = 1.67 times Hs = 16.7 ft• Hmax (highest wave you should look out for) = approximately 2 times Hs = 20 ft!

Work CitedAinsworth, Tom. Significant Wave Height: A closer look at wave forecasts.

National Oceanic and Atmospheric Association.