How Storms Generate Swell

If you’ve read the articles on global weather you’ll have some idea how a storm is created, the next step is to understand how storms create swell. Anyone who’s watched high winds blow up chop on the surface of a lake or river will appreciate the first stage in this process and you might actually be more surprised to learn that the exact science of it is still only partially understood. Exactly how it happens really isn’t important to the surf forecaster, we just need to know that the wind initially blows up small chop called capillary waves. Once the surface has these bumps and dips the wind blowing over it starts to get disturbed, these turbulent areas of air flow then spin over the small waves already created, pressure differences at the top and bottom of these waves cause them to grow larger still.

So the bigger the waves the bigger the turbulence and the faster they grow. This is pretty important for us as surfers and particularly those used to surfing more local wind swells, this ‘exponential’ rather than ‘linear’ growth means that the second hour of a storm blowing will create a bigger increase in wave size than the first hour.
So we understand that wind makes waves – what sort of wind makes what sort of waves? There are three main factors:

  • Wind Strength All other factors being equal the stronger the wind the larger the waves it’ll create and limiting factor on the maximum size of the waves will be the strength of the wind.
  • Duration The longer a storm blows the larger the waves it can create.
  • Fetch The larger the area over which the wind is blowing the bigger waves it can create. This assumes the wind is blowing over the area in broadly the same direction. Fetch is complicated because a storm can move and if, as it does so, it travels in the same direction as the swell it’s creating it can continue to increase it’s size, this is called virtual fetch.