Several factors influence the shape and quality of breaking waves. These include the bathymetry of the surf break, the direction and size of the swell, the direction and strength of the wind and the ebb and flow of the tide.
Swell is generated when wind blows consistently over a large area of open water, called the wind's fetch. The size of a swell is determined by the strength of the wind, the length of its fetch and its duration. So, surf tends to be larger and more prevalent on coastlines exposed to large expanses of ocean traversed by intense low pressure systems.
Local wind conditions affect wave quality, since the rideable surface of a wave can become choppy in blustery conditions. Ideal surf conditions include a light to moderate strength "offshore" wind, since this blows into the front of the wave.
The factor which most determines wave shape is the configuration of the seabed directly behind and immediately beneath the breaking wave. The contours of the reef or sand bank influence wave shape in two respects. Firstly, the steepness of the incline is proportional to the resulting upthrust. When a swell passes over a sudden steep slope, the force of the upthrust causes the top of the wave to be thrown forward, forming a curtain of water which plunges to the wave trough below. Secondly, the alignment of the contours relative to the swell direction determines the duration of the breaking process. When a swell runs along a slope, it continues to peel for as long as that configuration lasts. When swell wraps into a bay or around an island, the breaking wave gradually diminishes in size, as the wave front becomes stretched by diffraction. However, it is more common to see waves cross into the shallower water and finally close out.
Tidal variation is the factor that most determines when to go surfing. Wave shape and the whole pattern of the surf changes with the tide, more or less hour to hour. On successive days, the tides are an hour later each day because the moon orbits the planet in the same direction as the planet spins. Since the moon advances in its orbit during the course of a day, it takes about an hour extra for your location to catch up to the moon and the tide that follows it.
Since the phases of the moon concern its position relative to the lit side of the earth, the timing of the tide reflects the lunar cycle. In the course of a week following a full or new moon, high tides occur during the middle of the day and the middle of the night. For instance, at the beginning of the week following a full moon, high tide is in the middle of the morning. Since the tide returns about an hour later than it did the day before, by the end of the week, high tide is late in the afternoon, as a low tide week begins.
Tidal variations vary greatly across the globe and their effect on surf breaks can vary greatly among the spots within each area. Locations such as Western Australia , Panama, and Ireland experience 2-3 meter tide fluctuations, whereas in Hawaii the difference between high and low tide is typically less than one meter.
You have to be sensitive to all these factors to get to know a surf break, and each break is different, since the underwater topography of one place is unlike any other. At beach breaks, even the sandbanks change shape from week to week, so it takes commitment to get good waves (a skill dubbed "broceanography" by California surfers). That's why surfers have traditionally regarded surfing to be more of a lifestyle than a sport. Of course, you can sometimes be lucky and just turn up when the surf is pumping. But, it is more likely that you will be greeted with the dreaded: "You should have been here yesterday." Nowadays, however, surf forecasting is aided by advances in information technology, whereby mathematical modeling graphically depicts the size and direction of swells moving around the globe.
The regularity of swell varies across the globe and throughout the year. During winter, heavy swells are generated in the mid-latitudes, when the north and south polar fronts shift toward the Equator. The predominantly westerly winds generate swells that advance eastward. So, waves tend to be largest on west coasts during the winter months. However, an endless train of mid-latitude cyclones causes the isobars to become undulated, redirecting swells at regular intervals toward the tropics.
East coasts also receive heavy winter swells, when low pressure cells form in the sub-tropics, where their movement is inhibited by slow moving highs. These lows produce a shorter fetch than polar fronts, however they can still generate heavy swells, since their slower movement increases the duration of a particular wind direction. After all, the variables of fetch and duration both influence how long the wind acts over a wave as it travels, since a wave reaching the end of a fetch is effectively the same as the wind dying off.
During summer, heavy swells are generated when cyclones form in the tropics. Tropical cyclones form over warm seas, so their occurrence is influenced by El Niño & La Niña cycles. Their movements are unpredictable. They can even move westward, which is unique for a large scale weather system. In 1979, Tropical Cyclone Kerry wandered for 3 weeks across the Coral Sea and into Queensland, before dissipating.
The quest for perfect surf has given rise to a field of tourism based on the surfing adventure. Yacht charters and surf camps offer surfers access to the high quality surf found in remote, tropical locations, where trade winds ensure offshore conditions. Since winter swells are generated by mid-latitude cyclones, their regularity coincides with the passage of these lows. So, the swells arrive in pulses, each lasting for a couple of days, with a couple of days between each swell. Since bigger waves break in a different configuration, a rising swell is yet another variable to consider when assessing how to approach a break.
The value of good surf has even prompted the construction of artificial reefs and sand bars to attract surf tourism. Of course, there is always the risk that one's holiday coincides with a "flat spell". Wave pools aim to solve that problem, by controlling all the elements that go into creating perfect surf, however there are only a handful of wave pools that can simulate good surfing waves, owing primarily to construction and operation costs and potential liability.
The availability of free model data from the NOAA has allowed the creation of several surf forecasting websites. These automatically combine the above variables into a presentation of how good the surf will be.
- Interactive tidal graphs
- Wave shape
- www.stormsurf.com Surfing tutorials.
- www.cdlib.org The Science of Surfing Waves and Surfing Breaks.
- www.exploratorium.edu Science Wire articles on surfing.