Surf Forecasting 101: How to read a chart

Learning to read weather charts can be a bit daunting… what are all those colours and lines anyway and why, when I can easily check a cam, should I pay any attention to them? The short answer is that, by understanding the charts and how swell is generated, you’re going to be able to plan your life around the coming swells… its much less obvious to tell your boss that you have a dentist’s appointment on Thursday than to have a sickie every time there’s swell! It also means you wont have to spend so much time looking for the best surf, you’ll already have a good idea of where it will be.

Luckily, we teamed with Surfline’s Surfology® forecasters to make life easier.

Let’s take a look.


On Surfline, the day and time that the chart is effective, as well as the region the chart is focused on is clearly labeled at the top of the map.  The chart we’re looking at here is for the Gold Coast on the 13th May. “00Z” time is midnight GMT. The Goldy is GMT +10 so the 12Z time for this chart is 10pm local time.

This chart is a sea surface pressure chart, which displays areas of low and high pressure. The red lines, known as isobars, connect areas of equal pressure.

Low pressure systems (storms), can generally be identified by tightly packed isobars. The closer the isobars are packed together, the stronger the wind is in within the storm. In this particular chart, we have three areas of low pressure to keep an eye on (noted by the red ‘L’): A rather weak looking low under New Zealand (L1); a stronger storm to the south-east of New Zealand (L2); and a relatively strong storm peaking up from Antarctica (L3).

Regions of high pressure can generally be identified by the more loosely packed isobars and are labeled hear with a blue ‘H’. One ridge of high pressure is located over central Australia (H1), while another high is roughly half way between New Zealand and South America (H2).

The lines extending away from the Gold Coast throughout the Pacific are known as great circle lines. Great Circle lines are the shortest distance between two points on our spherical earth. When you see isobars lining up parallel to any of these great circle lines, you know that wind and swell are being pointed at you area of interest (the Gold Coast, in this case). The longer the isobars parallel the great circle lines, the longer the fetch is!

The great circle lines can also tell you the direction from which a swell is approaching in true degrees, with the lines spaced at 10 degree intervals. On this map the straight up and down line (no bend) represents 180 degrees.

*In this case, storm L1 is generating a swell that will approach the Gold Coast from 160-150 degrees.

The coloured areas on the map show the strength of the wind that is lining up along the great circle lines. The legend is on the right hand side of the map with the blue colors representing 20 knots of wind, while the pink colors on the opposite end of the spectrum represent 60-70 knot wind.  Think of it like an equation.  Strong winds over a long distance of the ocean for a long duration of time = solid swell!

The concentric half rings that bisect the great circle lines represent the number of days away a swell is when it has a swell period of 17 seconds (swells with period of 17 seconds will travel roughly 1130 kilometres per day, so each ring represents a distance of 1130 kilometres from the Goldy.)

*Looking at this chart, we should see a bit of new southerly swell from storm L1 roughly 2.5 days from the time that this chart was effective and from L3 in roughly 4.5 days.

And there you have it: the basics to identifying storms, fetch size, and swell direction on the charts. It may look a bit complicated at first, but the more you watch these charts, the better you’ll get at understanding the data and you’ll be able to plan your next surf mission based on the coming swell.

Why not head on over the Surfline forecasts now and put your newfound knowledge to use!

Thanks to Surfline for this article. If you’re interested in going even more in depth into Surf Science, check out Surfline’s Surfology® pages.  

Thanks too to Shey from The Froth Lab for the beautiful image of the storm front at North Curl Curl.