Surf Predictions

## Question:

Hello, I am a surfer @ heart, I live near the gulf coast and every so often get a decent swell. Often I must plan my excursions around a swells formation. I would be interested in a formula to calculate a swells arrival time, using whatever factors are relevant. I use the web to it's fullest potential for prediction purposes, so I'll find the data. An example would be; on july 11 99 I went to the beach @ Fourchon Louisiana, S. of New Orleans (yes we do get surf in Louisiana) a small Ground swell was coming in from a tropical wave passing over the Yucatan. I was wondering when it would peak in size, using the wind strength and direction factoring the distance the swell had to travel. I would be most gratefull. Thanks in Advance.

## Answer:

It turns out that there is a pretty good rule of thumb which says that large, wind generated, ocean surface waves average about 25 miles per hour, pretty much independent of the wind speed at the origin. I have verified this up in the Gulf of Maine by matching boat speed to wave speed and reading velocity from the GPS.

So, if you know the location of a storm at sea and the distance, d, in miles from the edge of storm to your beach, you can predict that the maximum surf will occur d/25 hours after the peak wind velocity in the storm.

Of course this prediction is subject to complications. As a storm progresses the maximum wave height may not correspond to the time of the maximum wind speed. Waves lose energy as they travel so very distant storms may not have significant impact where you are. A wind from the opposite direction can knock down waves before they get to your beach. Probably lots of other uncertainties will crop up. Still the 25mph rule gives you a starting point.