Hydrodynamics Of Surfing

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 ridable 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.

A surfer waits as a wave crashesThe factor which most determines wave shape is the topography 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. For specific surf spots, the state of the ocean tide can play a significant role in the quality of waves or hazards of surfing there. Tidal variations vary greatly among the various global surfing regions, and the effect the tide has on specific spots can vary greatly among the spots within each area. Locations such as Bali, 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.

In order to know a surf break one must be sensitive to each of these factors. 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 a few 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 modelling 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