As the wind blows over the ocean's surface, ripples begin to form. With sustained wind, gradually these small ripples become larger ripples, in turn the larger ripples become waves. The size of ocean waves depends on three factors: the wind speed, the distance over which the wind blows (known as the fetch) and the length of time it blows for.

However, wind doesn't blow in straight lines. The weather is dominated by pressure systems. Wind blows along lines of constant pressure (the isobars you see on a weather chart). The rotation of the earth causes what is known as the Coriolis effect, which, in the northern hemisphere means that wind blows anti-clockwise round lows and clockwise around highs and vice versa in the southern hemisphere.

Low-pressure systems don't make waves; it's the wind blowing around them that produces the waves and high-pressure systems also produce waves. The closer the isobars are, the stronger the wind. However, low-pressure systems generally have tighter isobars than high-pressure systems and therefore have greater wind speeds and create larger waves. The shape of the pressure system also matters. Elongated pressure systems have a larger fetch and hence larger waves travelling in the direction of the wind.

The time, in seconds, between wave crests is called the period. In deep water, waves with a large period travel faster. Hence, the first sign of a new swell on our beaches is usually small, but very long period (12-16 secs) waves.

Swell can originate from storms far across the ocean, and at any one time the sea contains a spectrum of waves, of different sizes, travelling in a variety of directions. Fortunately, for swell forecasting, we now have sophisticated computer models that can calculate and keep track of the many different patterns of swell travelling around our oceans at any one time.

As swell propagates over the continental shelf and enters the shallow waters around our coasts, it undergoes a number of transformations. ItsGoingOff.com have used a set of regional computer models to calculate the propagation of swell in coastal areas, from the shelf edge to our beaches. These models give us a better understanding of the wave characteristics along the coast and give us greater accuracy in our surf forecasts.