The rollercoaster of wind on Windfinder

Articles: MagicSeaweed on everything to do with weather and swell

One of the approximately three hundred and forty five weather and wave forecasting websites that Tony consults before planning dives is MagicSeaweed. He told me about this website the first time I ever spoke to him on the phone (he was in a reed hut in Mozambique, and I was in the only room in my house in Kenilworth that had proper cellphone signal). I laughed like a drain and wondered what this “magic weed” he was describing was good for.

One of the things MagicSeaweed turns out to be good for is explaining the intricacies of swell forecasting, summing up why your experience of the swell on a day might turn out to be a bit different from the (single) forecasted number. Imagine a situation where waves from two storms converge, travelling in slightly different directions. What does the resulting swell that comes ashore look like? What is its height and direction?

The website has recently added a feature that allows you to view the forecast for the primary swell on a day, or the secondary swells too. Visit the Muizenberg forecast page, and in the forecast table at the top of the Primary Swell column click on the blue and white button (looks like a lightning bolt, and when you hover over it it says “Toggle Showing All Swells”) to see the secondary swells contributing to the total forecast.

The MagicSeaweed website also has articles on how ocean swells are created and propagated, how global weather systems work, the physics of waves, and what happens to swells when they get close to shore. In this section there’s a pertinent article for South African users of the Atlantic ocean, explaining the phenomenon of upwelling and how it affects the accuracy of local sea surface temperature forecasts.

If you’re interested in the nitty gritty of swell forecasting you should visit the section on swell models. That’s where the first article that I linked to above (on swell forecasting) is found. This article, from the same section of the MagicSeaweed website, is an excellent place to begin your understanding of the mathematical models used to produce weather and swell forecasts. You can probably end your understanding there too, but perhaps your interest will be piqued.

MagicSeaweed is technically a surfing website, but that shouldn’t prevent the interested scuba diver from learning as much as possible about how forecasts are created, and what they mean. Swell and wind can dramatically influence the conditions on a dive. The safest divers are those who are able to look at weather and swell information, apply their prior experience and knowledge of how it all fits together, and decide for themselves – without having to rely on someone who has an economic interest in the matter – whether it’s safe to go diving on a day. This is specially important in Cape Town, where the weather is variable.

Read lots of weather forecasts, and read lots of dive charter newsletters to understand the reasoning behind an Atlantic or False Bay lauch. Be a safe diver! Educate yourself.

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Lapsed mathematician, creator of order, formulator of hypotheses. Lover of the ocean, being outdoors, the bush, reading, photography, travelling (especially in Africa) and road trips.

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