Waves – the wind’s assassin

The BBC is today carrying the story of a new record wave height for the North Atlantic, nineteen metres, or about sixty-three feet, measured by an oceanographic buoy. A few years ago I heard a report by the Irish Met office of a wave off Donegal that measured in at over twenty metres, sixty-seven feet. The Irish Met Office says the buoy that measured that wave is 11km off the coast; it too was generated in deep water by persistently high winds. Whichever claim is actually the record, they’re both pretty big waves and I wouldn’t like to meet either one in a small boat.

By the way, these are wave heights measured by buoys, there is a report of a wave measured by a ship which was a staggering ninety-five feet.

The probable maximum height of wind waves is around 80% of the wind speed. So, a 50 knot wind blowing over an area of ocean with unlimited fetch would produce a maximum wave height of about forty feet. This height is achieved after it has been blowing for a day, having doubled in height since the first four or five hours of the storm. Further maximum wave height increase is more subdued, it takes two days to get that wave up to fifty feet in height.

The average wave height in a storm is about half the height of the top ten percent of waves and one third of the highest wave. So, if the maximum wave height in our 50 knot blow is forty feet, the top ten percent of waves will be about twenty feet and the overall average will be about fourteen feet. It doesn’t sound so bad when you put it like that, does it? Except that you still have to survive those forty-footers.