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Swell Calculator 'How To'


Welcome to the new Swell Arrival Calculator. This tool can be used to calculate when a swell will arrive at your break assuming you know a storm starting position and the approximate size of the seas it has generated. This tool is far more advanced than our previous swell calculator but still retains much of old layout and style. Not only are great circle distances and swell arrival times calculated (like the old calculator), but now, pure swell size is calculated (using the swell decay tables published elsewhere on the site) and a rough estimate of when that 'peak size' will be reached for each storm position. For storm greater than 4000 nmiles out, decay sizes are automatically reduced (size X 0.75). This is to correct a deficiency found previously in the decay tables.

Interpolation between travel distances in the static tables is automatically performed. For example, if the decay rate for a storm 5000 nmiles out is 0.15 (per the tables) and for 5500 nmiles out is 0.10, the software automatically calculates the decay rate as 0.125 for a storm 5250 nmiles out.

But some manual interpretation is still required. One must analyze swell producing fetch areas and determine if wind/swell energy is pushing directly up the great circle paths to your target. If greater than 30 degrees off axis, reduce swell size by 25% (final swell height X 0.75). Likewise if the fetch area is smaller than 'average', reduce resulting swell size by 25%. If both situations are occurring simultaneously, reduce by 25% two times. Conversely, if the fetch is within 1000 nmiles of the coast, the resulting swell size defined by the tables might be understated. So it takes some practice in a variety of situations to build and tune your forecasting skills using the calculator.

Also a graph is provided that plots the time of 'peak swell'. When multiple storm positions are entered and if multiple peaks occur in a cluster (within a few hours of each other for a nearby storm or within 6-8 hours of each other for a long distance storm), then that suggests that virtual fetch is occurring and the number of waves per set will be enhanced.

Nothing is guaranteed, and this remains a work in progress. But it does provide more guidance about how big any given swell will be, and when it's peak will occur. And of course the mixture of swell size and swell period determined surf size.

To use this tool:

In the Target Location fields enter:

  1. The coordinates (latitude [N or S] and longitude [E or W]) of your beach (or any destination/buoy). Use a negative sign (-) in front of coordinates that are S and/or W. For example, for 37S 122W, enter -37 -122. N and E are automatically assumed to be positive (+) and no sign is required.
  2. Check in the GMT box if you are using GMT time.

In the 'Storm Coordinates' area enter:

  1. The coordinates (latitude and longitude) of the center of the fetch area for the first position of the storm aimed at your destination from your favorite wave model. Again use the negative sign (-) for coordinates that are S or W.
  2. The date and time of that first position (Assumes your "local time" unless you check the GMT box above).
  3. The maximum sea height (forecast or confirmed) aimed at your location.
  4. Click on 'Add Row' to add additional storm positions. Repeat steps 1-3 above for each additional storm position.
  5. Click on 'Calculate Results' to view Swell Arrival Times and Swell Heights.
  6. The results display below the data entry location. A individual result (line) is calculated for each storm position entered above. Swell Arrival times are calculated for each swell period that should result (i.e. 20 sec period swells will start arriving at XXXX, followed by 18 sec period swells at XXXX, followed by 17 sec ....). 'End Height' is the maximum size of the decayed swell upon arrival at the Target Destination. 'Peak Period' is the period when that size will occur. Size will typically start fading off after that peak period is reached (i.e. when period falls below the indicated peak period). Results for each period include swell travel duration (days, hours, minutes DD:HR:MIN followed the arrival date and the time. All arrival times are in 24 hr format.
  7. The graph plots all peak periods for the corresponding storm positions entered - One plot point per storm position. They are color coded so you can figure out which peak corresponds to what storm position.
  8. Some practice and interpretation skill is required. Read the HELP section for more info.

Many thanks to for doing a great job of building this application from our requirements and adding a few great features of his own.


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