In the second of our regular series of articles on how the weather affects surfing, we look at what happens out at sea.

Waves out at sea

If there was no wind the sea would probably be like millpond, calm and relatively motionless, but thank goodness for us surfers, this is not the case!

As we learned earlier, low pressure systems moving around the Atlantic create strong winds. These winds really whip up the sea, much more than just the choppy waves we see on a windy day down at Fistral or Watergate.

The low pressure systems in the North Atlantic Ocean spin in an anti-clockwise direction, and tend to be much more powerful in the winter and weaker in summer, hence in the UK we have much more surf in winter time.

All waves that arrive on our shores are different. Waves created from a particluar low are not thought of as individuals, but rather as a group, or ’swell’. The size of each swell depends on the low pressure that creates them. All low pressure systems are different because they travel in different directions, at different speeds, and each has a different central pressure and total size . All of these parametres will fluctuate as the low pressure interacts with the atmosphere and meets other weather systems.

As a rule the deeper the central pressure and the bigger the diametre of the depression (the low pressure system), the more swell will be produced. If it is a slow moving low, it will creat even more swell. Where the swell arrives and its duration depends on the position of the low in the Atlantic and on its direction and speed.

But how do low pressure systems actually create waves? Well very simply the wind blows on the sea, moving it to create wavses. As the winds can be very strong, they can cause very large waves to form. To imagine how this might work, think of the film, ‘The perfect storm’ with George Clooney. In the film the boat he was in was hit by huge waves that were created from the winds of the storm. Waves out at sea can be as big as 50feet high. We wouldn’t want that to hit our shores! Luckily due to many other factors (we’ll learn more about this later) mean that waves are much smaller when we see them from land.

So, newly-formed waves are pushed along by the prevaling winds. As they travel away from their source the waves naturally spread out from each other, this is called ‘circumferential dispersion’. They also tend to lose their height as they travel, so that waves that have travelled twice as far, will reduce in height by about a third. Also, the waves tend to organise themselves. This means they change from being many different waves, to coming together to form swell lines, with the faster waves travelling ahead and the ‘chop’ at the back. You see swell lines on a good clean day at your  surf spots.

And finally for today some physics about waves. Wave properties can be determined by recalling your school physics lessons…….

Remember this equation? Speed = Distance

Time

You can modify it to refer to waves…

Speed =  Wavelength

Period

Period is the time from each crest of the wave to the next; the time for the wave to complete one cycle. Wavelength is the length between each wave crest.

The strength of the wind, the distance the wind blows (this is called the fetch) and the length of time the wind blows (its duration) determine the size of the waves.

When we look later at synoptic charts you will be able to use this theory to determine what the swell will be like at your favourite surf spot.