In physical oceanography, significant wave height (SWH) is the average wave height (trough to crest) of the one-third largest waves
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Origin & Definition
The original definition resulted from work by the oceanographer Walter Munk during World War II.[1][2] The significant wave height was intended to mathematically express the height estimated by a "trained observer". It is commonly used as a measure of the height of ocean waves. The American Meteorological Society defines SWH as Hm0.[3]
Statistical Distribution
Significant wave height, scientifically represented as Hs or Hsig, is one important level when considering the statistical distribution of ocean waves. The most common waves are less than that of Hs. This implies that encountering the significant wave is not frequent. However, statistically, when two significant waves come into phase, it is possible to encounter a wave that is much larger than the significant wave.
Generally, it is assumed that individual wave heights can be described using a Rayleigh Distribution. For example, given that Hs = 10 meters, or 33 feet, statistically:
- 1 in 10 will be larger than 10.7m (36 ft)
- 1 in 100 will be larger than 15.1m (51 ft)
- 1 in 1000 will be larger than 18.6m (62 ft)
This implies that one might encounter a wave that is roughly double the significant wave height. However, in rapidly changing conditions, the disparity between the significant wave height and the largest individual waves might be even larger.
Other Statistics
Other statistical measures of the wave height are also widely used. The RMS wave height, which is defined as square root of the average of the squares of all wave heights, is approximately equal to Hs divided by 1.4.
For example, according to the Irish Marine Institute:[4]
- "… at midnight on 9/12/2007 a record significant wave height was recorded of 17.2m at with a period of 14 seconds."
Weather Forecasts
The World Meteorological Organization stipulates that certain countries are responsible for providing weather forecasts for the world's oceans. These respective countries' meteorological offices are called Regional Specialized Meteorological Centers, or RSMCs. In their weather products, they give ocean wave height forecasts in significant wave height. In the United States, NOAA's National Weather Service is the RSMC for a portion of the North Atlantic, and a portion of the North Pacific. The Ocean Prediction Center and the Tropical Prediction Center's Tropical Analysis and Forecast Branch (TAFB) issue these forecasts.
RSMCs use wind-wave models as tools to help predict the sea conditions. In the U.S., NOAA's Wavewatch III model is used heavily.
Below is an example of a text forecast from the Ocean Prediction Center, where the term Seas represents Significant Wave Height.
...STORM WARNING... .LOW E OF AREA 60N32W 952 MB MOVING NW 05 KT. OVER FORECAST WATERS WITHIN 360 NM W SEMICIRCLE WINDS 35 TO 50 KT. SEAS 18 TO 32 KT. ELSEWHERE N OF 47N BETWEEN 35W AND 48W WINDS 25 TO 35 KT. SEAS 12 TO 24 FT. .24 HOUR FORECAST LOW 61N35W 974 MB. WITHIN 660 NM W AND 300 NM NW QUADRANTS WINDS 30 TO 45 KT. SEAS 14 TO 26 FT. ELSEWHERE WITHIN 720 NM W SEMICIRCLE WINDS 20 TO 30 KT. SEAS 12 TO 20 FT. .48 HOUR FORECAST LOW 59N40W 985 MB. WITHIN 90 NM OF THE E COAST OF GREENLAND WINDS 25 TO 40 KT. SEAS 14 TO 22 FT. ELSEWHERE N OF 58N BETWEEN 35W AND 43W WINDS 20 TO 30 KT. SEAS 8 TO 16 FT.
See also
- Sea state
- Rogue wave: a wave of over twice the significant wave height
- Ocean Prediction Center
References and notes
- ^ Denny, M.W. (1988). Biology and the Mechanics of Wave-swept Shores. Princeton, New Jersey: Princeton University Press. ISBN 0691084874.
- ^ Munk, W.H. (1944). Proposed uniform procedure for observing waves and interpreting instrument records. La Jolla, California: Wave Project at the Scripps Institute of Oceanography.
- ^ "Significant wave height". Glossary of Meteorology. http://amsglossary.allenpress.com/glossary/search?p=1&query=significant+wave+height&submit=Search. Retrieved 2007-06-30.
- ^ "Report on Weather Buoy Readings During December Storm — 6th to 11th December". Irish Marine Institute. http://www.marine.ie/home/aboutus/newsroom/news/ResultsofWeatherBuoyReadings.htm. Retrieved 2008-01-13.
External links
- Finnish Institute of Marine Research: Significant wave height
- NOAA Wavewatch III
- NWS Environmental Modeling Center
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