Surfing Waves and Swell Forecasting

Now days forecasting waves, tides, swell and surfing conditions is quite easy, there are many websites offering from daily to hourly surf conditions.

Read on for an overview of how waves are made and how wave predictions are made for your local surf report.

How Are Waves Made?

Waves are formed by wind blowing over the water surface. The three things needed to make waves are the wind speed; it's duration and fetch (the size of the wind area). If the wind duration is more than 24 hours, the swell is fully developed and even if it blows for a longer time at that speed, the waves will not get any bigger. If the fetch is more than 200 miles or so, then the wave becomes fairly well developed. If the wind blows for less than 24 hours or the fetch is smaller than 200 miles the waves will be smaller.

Lastly but most importantly is the wind speed. The stronger the wind the bigger the wave. Also waves made in a stronger wind will last over a longer period of time.

How to Recognise a Swell Making Area

On a weather chart there are high and low pressure systems. They are separated by isobars (the lines on the chart). If you see on a weather map an area with the isobar lines close together and straight for more than 200 miles and the pattern is not moving very quickly, then this would probably mean it is a good swell making weather pattern.

How Does The Swell Move?

Waves move away from the swell making area in 'wave groups'. If you have winds stronger than 35 knots the waves will have a bigger wave period and move quickly through the water.The swell will move in roughly the same direction as the wind, but will spread out at an angle of 15 degrees. This spreading out is why swells die out. After a swell travels about a 1000 miles it will be half its original size. Over 1000 miles the swell holds it size for longer.

In the above weather chart you can see that the isobar lines at the bottom are close together and stay straight for more than 200 miles. This is a perfect swell making pattern, with strong winds and big waves.

Deep and Shallow Water Waves

As waves pass from the oceans to the shallow continental shelf they will change from being deep water waves to shallow water waves. This causes a drag on the wave and slows it down. When the water becomes less than one and a half times the wave height, it is likely to break, which is what happens on a beach.

Shallow Water Waves

If a wave approaches a beach at an angle, the part closest to the beach is in shallower water and therefore travels slower than those further away so the whole wave front tends to be turned into the beach. When the wave reaches the beach, it will be parallel to it.

At a headland, waves are turned towards the point and break along its side at a much bigger size than on the beach. Also, on individual sand bars the surf will break at a bigger size than anywhere else. This makes ideal surf conditions.

How Are Rogue Waves Made?

If there are two swells coming from different areas they could become in sync and so the wave height becomes the sum of the two in sync waves. Therefore two waves which are 3 feet in height if in sync could produce a wave of 6 feet. Read more about rogue waves

Where Can I Find a Weather Chart?

The best source of charts is on the Internet. There are heaps of charts to choose from. You can also get hold of satellite pictures and weather forecasts in addition to computer generated swell forecasts. 

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The WetSand WaveCast® Guide to Surf Forecasting

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