Wave Height Explanation - National Weather Service

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Explanation of Wave Height in the Nearshore Marine Forecast

Noticing this in our Nearshore Marine Forecast?

Thursday Southwest winds to 30 knots becoming northwest 15 to 25 knots. Snow likely during the day, then a chance of snow showers thursday night. Waves 5 to 9 feet subsiding to 3 to 6 feet. Waves occasionally to 11 feet.
How is Wave Height measured?

Wave height is the vertical distance between the crest (peak) and the trough of a wave.

Some other definitions:

Still-Water Line is the level of the lake surface if it were perfectly calm and flat.

Crest is the highest point on the wave above the still-water line.

Trough is the lowest point on the wave below the still-water line.

Typical Distribution of Wave Heights

Explanation of the arrows being pointed to on the graph above:

  • H is the most probable or the most frequent wave height, this is not displayed in the forecast. The most frequent wave height is approximately half the value of the significant wave height.
  • H (with the line above the H) is the average wave height. This is not displayed in the forecast. The average wave height is estimated to be about 5/8 the value of the significant wave height
  • Hs is the significant wave height. This is the average of the highest one third of waves. This is currently what is displayed in the forecast.
  • H1/10 is the average of the highest 10% of waves observed. This is what is newly being displayed in our forecast. This is indicated by "Waves occasionally to xx feet".

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