The earth isn’t flat, but your surf forecast chart is. This means a straight line on the surface of the earth isn’t a straight line on your chart. Swell likes to travel in straight lines which leaves us some issues in understanding where a particular swell will end up.
Swell travels in straight lines across the curved surface of our globe. Granted it can bend around corners with a bit of refraction but, fundamentally, the consistency of your local spot is directly related to how much unobstructed ocean lies directly away from the coast. Figuring this out on a map, or even a MSW swell chart can be an issue as you can see above.
Check out your world chart here. Draw a straight line between Indo and Brazil. You’re going to find a large part of Africa in the way. This is because, like most map makers, we use the Mercator projection to convert the curved surface of the earth to the flat monitor attached to your computer. We use it primarily because it makes maps you’re familiar with. The problem is a straight line on this form of map ISN’T a direct line between two points. In fact if you grabbed a globe, a piece of string, and a couple of pins, you could happily mark a straight line between Bali and Brazil through open ocean. Swell can happily propagate this kind of distance which means a storm off the coast of Brazil can create waves 8,700 miles away at Kuta.
In fact we can make a chart that shows this. Using an Azimuthal projection we can draw a straight line through the centre of the chart between any two points. Don’t believe storms west of Africa can hit the beach in Indo? This one should make it a little clearer
The best solution? For us the answer lies in a map that blends a familiar projection with a plot of these direct routes, also known as ‘Great Circles’. These lines are curved on the map, but represent direct lines on the surface of the earth - meaning that swells propagate along the lines. While this concept is nothing unique what we’ve also done is take 15 years of historic swell information for each of the 4000 locations we forecast for and worked out the most likely swell window for each one. The result is a chart familiar to most surfers, but with Great Circle lines highlighting the swell window and giving the ability to spot swells likely to make surf in the long range and understand at what angle they’ll arrive on the beach. Additionally the concentric circles indicate the distance swell will travel in a day (in our example for 15 seconds period swell). In this first chart below you can see a modest storm west of Africa that’ll send a solid blast of swell to South African surfers, but will also create waves that’ll hit the beach in Indo 8-12 days later from around 225 degrees:
You’ll also start to appreciate that, as this storm is already in progress, our long range forecast for Indonesia can operate with confidence approaching 100% as far ahead as 12-14 days depending on the location of the storm. Once the storm has made waves tracking their travel is the easiest part of surf forecasting. As this storm travels into the Indian Ocean, however, it’s forecast to intensify and send the bulk of it’s swell from nearer 210 degrees.
You can clearly see this on the forecast for Padang Padang below. While the later stages of this storm are far enough ahead that we need a little caution that things may change there’s a guaranteed smaller more westerly component in play right now as the storm moves past Cape Horn.
To enable Great Circle lines for any spot on any large area chart you need to sign up for magicseaweed pro which also gives you a full 16 day forecast for every location we cover. For us it’s an invaluable tool for the serious surfer lucky enough to be hunting for long range groundswell.