Does the full moon bring big surf? Almost definitely not. But why to people still keep talking about it?
It was July 1995 and I was in Punta Hermosa, Peru. I was sitting watching the ocean with a few locals, most of whom surfed, but all of whom fished, every day in rickety wooden rowing boats. The swell was largish and lumpy but not really solid. Pico Alto – the big-wave spot I had specifically come here to ride – was not breaking. “Don’t worry”, one of them said, “next week is the full moon”, and the rest of them nodded. They explained to me that big waves always arrive with the full moon. Didn’t I know?
The following week there was a big swell, Pico Alto broke for about three days in a row and I was stoked. I was sure it was just a coincidence because how could the arrival of a swell generated by a low pressure in the Pacific Ocean have anything to do with the full moon? Or was I missing something?
Fast-forward to 18th May 2007: A deep low pressure system off South Africa sent one of the biggest swells of all time to Indonesia. You might have been lucky enough to have surfed it; and you might have even seen the statement released by The Bali Meteorology and Geophysics Agency: “The Moon is in line with the Sun, and this results in higher tidal waves.”
So does the full moon really bring big waves? Or is it just some rumour started by somebody years ago, now a meme repeated meaninglessly by everybody in Bali, El Salvador and a host of other places?
To throw some light on the issue, Ben Freeston did a systematic comparison of the height of the waves and the phase of the moon, using 35-years of MSW data from Indonesia (see HERE). Results showed that there was no correlation whatsoever between the full moon and the size of swells arriving at the coast.
Ben’s analysis was unequivocal. But I still couldn’t help thinking about it. Maybe there was something that might make people think that the full moon brings big waves, even though it doesn’t really.
I had an idea. We all know that the full moon on its own can’t bring swell, but the moon does of course affect the tides. Every time there is a full moon, there is a spring tide. Now, if the spots in question have some sort of tidal modulation of wave heights like you get around southwest England, you might get bigger waves at certain stages of the tide, typically on the rising tide. Crucially, this effect would be greater during spring tides. Therefore, people might perceive the swell to be bigger when there is a full moon.
The extra boost in wave height would only occur during a small time within the tidal cycle, but might still give the impression of bigger overall surf
The extra boost in wave height would only occur during a small time within the tidal cycle, but might still give the impression of bigger overall surf. Importantly, Ben’s analysis was based on wave heights that were averaged over several days, so the data couldn’t ‘see’ those peaks inside a tidal cycle.
Perhaps there really is an effect, albeit small, that people have noticed during times when the swell has been small for several weeks, and then gets an extra boost during spring tides. Maybe that was enough to make people jump from hearing about something that happened once or twice, to predicting that it will happen every time. I’ll leave you to think about it.
Cover image, the Telo Islands from way up high by Andrew Shield.