A History of Surf Forecasting: Part I
Posted on June 20, 2007 @ 2:26 PM
Old fishermen used to say it had to do with the moon, the wind, the lay of the seaweed on a low-tide rock, but their surf forecasts weren’t always that accurate. These days we’re way ahead of ourselves, and this is – scientifically speaking – how our powers to see the future emerged.
More than ever before, a tremendous amount of scientific effort is being put into predicting the future. Climate modellers, tidal modellers, economics modellers, coastal modellers – all are concerned with describing some phenomenon, natural or otherwise, in enough detail to simulate its behaviour and then predict what it might do in the future. Most of these people know well that their task will never be fulfilled completely – the laws of chaos prevent this – but they nevertheless keep on trying. But still, imperfect predictions are useful to us, and the less imperfect they are, the more useful they are.
And now, more than ever before, we surfers want to know the future. Surf forecasting has become a major part of our lives, especially since the internet has made available to us an entire universe of information. Nowadays, any surfer can access a swell-forecasting website that will tell them where to go, what board to take, which wetsuit and what stickers to wear. No brain required.
However, aside from the increase in available forecast information for the ‘masses’ (which has obviously improved things tremendously), can we really make more accurate forecasts than we could before? Can we be any more precise about swell arrival time, swell quality, changes in local winds or how long the swell will last? Or are we still missing something? How far has wave forecasting come, how much more useful is it than it was before, and how much more useful will it be in the future?
To begin to answer those questions, I decided to have a look at the history of wave forecasting right from the very beginning, and then assess where it is at right now, and where we’re going with it in the future.
In this, the first part, we start by looking at our own particular history from a surfer’s point of view, together with the almost-parallel scientific development of wave-forecasting models. Then we look at how the two finally merged together with the advent of the internet.
EARLY DAYS OF SURF FORECASTING
Until a very short time ago, surf forecasting was shrouded in mystique – almost a ‘dark art’, certainly a murky science. It was best left to a handful of gurus possessed with that special power: the power for interpreting the isobaric chart. Before the internet came along and we were suddenly, almost overnight, given access to unimaginable amounts of information, wave forecasting was little more than blind guesswork. This was true even for those who were able to boast some knowledge of physics or oceanography.
For much of the time the only resource available was the isobaric analysis chart. This is a daily analysis of the atmospheric pressure at sea level. In those days, we normally had to rely on a chart that was published in the newspaper or shown on the television. The chart is compiled from a series of atmospheric pressure values measured by a network of buoys and ships.
These measurements are then interpolated to provide an estimate of the pressure ‘field’ over the expanse of the entire ocean, in the form of contour lines of equal pressure called isobars (iso = equal, bar = pressure). The result is a ‘snapshot’ of the situation at some time in the recent past. It is not a forecast.
A few simple rules enabled us to use the isobaric chart to make a reasonable estimate of the surf conditions in the near future. From the orientation and number of isobars we could easily identify the current position of a storm over the ocean, together with its all-important fetch (the area containing winds that generate the swell). With some experience of the likely size of swell produced by a particular fetch, how fast that swell travels and how it might be affected by the continental shelf, we would then make a guess as to the height and arrival time of the swell. Don’t forget, this might be at some beach thousands of miles away from the storm itself. Then, with some previous knowledge as to the most likely trajectory of the storms in that particular ocean at that time of year, we would make a guess at the local wind conditions expected on that beach. As you can see, that’s a lot of guesswork and extrapolation based on very little initial information. Even for the most experienced and talented wave-forecasting shaman, it was still very difficult to predict the state of affairs at the end of a long and uncertain string of physical processes connected to just one snapshot of a single parameter. There were just too many variables.
“BEFORE THE INTERNET CAME ALONG AND WE WERE SUDDENLY, ALMOST OVERNIGHT, GIVEN ACCESS TO UNIMAGINABLE AMOUNTS OF INFORMATION, WAVE FORECASTING WAS LITTLE MORE THAN BLIND GUESS WORK.”
Instead of relying on pure instinct and experience, what we really needed was access to the ‘automatic’ forecasting being done by the real experts; the meteorological research centres such as those under the National Oceanographic and Atmospheric Administration (NOAA), the European Centre for Medium-range Weather Forecasts (ECMWF) or the UK Meteorological Office. These centres were already making predictions using the most powerful supercomputers in the world, together with a great deal of clever mathematics.
At first, this information was scarcely available to the general public. To obtain predictions of local winds around the coast, or the movement of storms over the ocean, we relied on radio forecasts giving a ‘general synopsis’ and ‘area forecasts’ or perhaps, television forecasts which, if we were lucky, gave isobaric charts for the next few days ahead. One could also obtain isobaric forecast charts in the form of a fax, usually by subscribing to a special (paid) marine service. And later, a simplified ‘significant wave height’ forecast chart also started to become available by marine fax. These were generated by one of many predecessors to the computer programmes that produce the wave forecasts we all use today.
As marine faxes and other similar facilities gradually improved and became more widely available, along with the (initially incipient) growth of the internet, a few people began to realise that surfers would benefit greatly from this ‘expert’ information. For a number of years, paid subscription telephone and fax services were available whereby the caller could receive ready-interpreted swell-forecast information, often together with real-time reports from a wide range of beaches. The ‘forecaster’ would typically spend time every morning assessing the charts and making a best guess at the coming surf conditions, which would then be recorded for the clients to listen to, or sent to the clients via fax.
MEANWHILE, BACK AT THE LAB …
The idea of using mathematical equations to generate predictions for waves started way back in the 1940s. This was when swellprediction techniques were first developed to help military landing operations in the Second World War. (Sadly, most of the motivation and funding for wave-model research is still military-based). The first methods for automatic wave prediction were not fully based on the real physics behind wave generation.
Instead, they were what could be termed semi-theoretical and semi-empirical. This means that, although the method was based on mathematical relationships between the wind and the waves, the relationships contained ‘constants’ – numbers whose values could only be ascertained by feeding actual measured data into the equations.
By the early 1950s, a set of equations had been developed by Harald Sverdrup, Walter Munk and Charles Bretschneider, relating wave height and period to the three factors of windspeed, fetch and duration. This was subsequently referred to as the SMB method, after its inventors. The height and period predicted by the SMB method were just single values, called the significant wave height and the significant wave period. These were designed to correspond with those values estimated by an ‘experienced’ observer by eye, from aboard a ship. As I have mentioned many times before, the famous significant wave height, also called ‘H1/3’, is calculated by averaging the highest one-third of all the waves observed at a point over some time period.
The method assumed right from the beginning is that the entire sea state could be approximated as a single ‘significant wave’ which grew, decayed and propagated away from the storm centre as a single entity. This was, of course, highly unrealistic. However, it was the best they could do at the time. The height and period of the ‘significant wave’ were dependent upon the fetch size (bigger fetches produce bigger waves), the duration (the longer the wind blows over the same stretch of sea, the bigger the wave) and the windspeed itself (the stronger the wind, the bigger the wave).
This last factor, the windspeed, had a quadratic relationship to the wave height rather than a linear one. This meant that doubling the windspeed will make the wave four times as big. Of course, waves cannot keep growing forever. So, automatically built into Sverdrup and colleagues’ formulae were limiting states whereby the wave growth reached a saturation point. These were termed fetch-limited, duration-limited, and fully-arisen sea, the wave growth being limited in each case by the fetch, the duration or the windspeed.
Continued on page 2...
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