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The Math and Science behind Wave Forecasts


Forecasting surf seems easy these days. Just hop on the computer and start clicking the models and summaries for your region. Done. However, you will find that the models and the summaries often disagree…widely. So like with most Internet-based tools, use them but don’t totally rely on someone else to do the job if you can do it better. Plus, it’s fun and brings you closer to understanding the waves you ride. Here is the deal. To properly forecast the timing, size, and quality of a swell, you need to take a few simple concepts into consideration.

Note: You must first know where to find your closest off-shore buoy. Here is a link to NOAA and you can find your buoy which will offer information on wave size, period, and direction as well as water and wind temperatures.


The speed of the swell is important if you want to be there when the waves are hitting. When I lived in Hawaii, I knew how long a swell would take from when it first hit the off-shore buoy to when it would start hitting the North Shore. If I knew a big swell would begin hitting in 8 hours and the next few days would bombing, then logic would follow that I would quickly organize a trip to Pearlridge Mall with my girlfriend. You know; lunch, a movie, maybe a little ice cream, and when we returned after a wonderful afternoon…What! A new swell and I was good to go for the next few days. Oh I know that’s horrible, but it underscores the power of understanding swell speed.

Here’s how it works. All wave buoys will indicate the period (average time between waves). This is some of your most valuable information because it shows how much time goes by between each wave. Once you know the period, use this simple formula to find how fast the swell is moving:

1.5 X Period = Speed.

To illustrate, a swell with a 16 second period will be moving at a speed of 24 knots (nautical miles per hour). One knot equals 1.2 miles on land. Bam! Once you know how far off the coast your local swell buoy is, then you are good to go in finding to the hour when the swell will hit. Now you can plan your day accordingly.

Bottom Contour

This is another aspect of surfing that is often overlooked by inexperienced surfers. The bottom contour (bathymetry) can create a variety of types of waves , but it also dictates how or if the swell will be hitting. This is a cool formula that shows at what point the waves will begin to feel the effect of the bottom.

Swell Period (squared) X 2.56 equals at what depth the wave will feel the bottom.

This is especially important when you are considering a deep water spot because regardless of the wave size, the period will dictate whether the waves will be breaking. To illustrate, a 10 second period swell will begin to feel the bottom at 256. Now that doesn’t mean that it will break at that depth, but rather feel the pull or bend of the bottom contours. So when you hear words like “long period ground swell ,” you know that it is a strong swell from far away and will be working at the deeper spots.

Swell Size

Size is a tricky business. Wave size is important as it will indicate at what depth it will break. For instance, a 4 foot wave will break in 5.2 feet of water since it’s based on the following formula:

1.3 X (wave height) = depth of breaking wave

But in all honesty, I look at period more intensely than wave height because waves with longer periods have more power and better form. And they often have (ironically) more size since there is much more water and force built up behind them. So if a long period swell hits a shallow reef or sand bar, all the moving water power behind it will stack up and break. Boom! If you like steep drops, longer rides, big barrels, you want a longer period swell.

Final Considerations

There are many factors that will affect the quality of your next session. You can easily check your local buoy and find wind information; swell direction, period, size and more in one glance. Do the math and you are good to go, but you must first get to know your local waves by riding different spots and noticing their personalities. Go diving on flat days and check out the bottoms. It takes a while to get intimate with your local waves, but just like with any relationship, it pays off in the end and will aid your goal of finding the perfect wave.

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