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.
Speed
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.
