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October 25, 2001

Wild Waves

You can predict ocean waves by applying standard weather information to a formula.

Q: While fishing offshore after a storm last winter, I saw a wave I estimated at 30 feet high. I've heard that fishermen off Alaska sometimes experience waves 100 feet high. Is this possible? Is there a maximum size for ocean waves? - Rick Ondine, Boston, Massachusetts

A: Winds blowing over the ocean's surface raise waves varying over a wide range of wave heights for any specific wind speed. The average height of the tallest waves at a given time and place, known as "significant wave height," depends not only on the wind speed, but also on "fetch" (the length of water over which the wind is blowing) and how long the wind has been blowing.
For any given wind speed, there is a specific length of time and fetch length that allow seas to grow to their maximum height. For a specific wind, the wave height is limited because large waves move faster than smaller waves, and if a wave grows big enough to move faster than the wind, its growth stops when the wind accelerates it.
To estimate the significant wave height for a particular wind, square the wind speed, double the answer and divide by 100. For example: If the wind speed is 10 knots, the square of 10 is 100, 2 multiplied by 100 equals 200, and 200 divided by 100 equals 2. So, wave heights in 10-knot winds should reach an average of 2 feet. Twenty-knot winds increase wave heights to 8 feet, according to this formula, and 60-knot winds can raise 72-foot waves.
It takes time for wind to raise waves to their maximum height. Gale-force winds over three days can generate wave heights near 100 feet. Rogue waves - perhaps the result of two large waves merging together - may reach twice that height, but scientists can't be sure exactly what the maximum wave height may be, because they have no measuring instrument that can survive such a wave.