Understanding how waves are created is the first step in understanding how to describe them, how we predict them and how you can start using our tools to make the call.
For surfing purposes almost all waves are made when strong winds blow over the ocean's surface. Initially the wind disturbs the surface and creates tiny capillary waves - the ripples you see when a gust passes over still water. Once the surface has texture the wind continues to build energy into the larger gravity waves that we choose to surf. There are three main factors that determine the size of the waves created:
The area over which winds are blowing is called the 'fetch'. The size of this fetch, and particularly the length over which winds are blowing in the same direction, is a key factor in producing larger waves. Large ocean storms may measure 2000 miles across and can produce huge waves. Even a powerful Hurricane can be limited in it's ability to produce big waves by it's small size.
The stronger the wind the more energy it can impart into a building swell. No matter how large the storm if the wind isn't strong enough it'll limit wave growth.
The longer the wind blows over the waves it's generating the larger they'll become.
Any one of these factors can limit overall wave size. An intense Hurricane blowing over a relatively small area, a large area of weaker winds or a short lived storm will all produce smaller waves than a large fetch of strong winds blowing for several days.
Because storms themselves can move over the ocean surface it is possible for a smaller storm to act like a larger one. If the storm travels in the same direction as the waves it creates at the same speed those waves spend a longer time growing under the storms influence. Because this effectively replicates the effect of a larger storm we call this phenomena virtual fetch
Magicseaweed creates your local surf forecast by first using weather agency data to track current and future winds over the whole ocean surface and then calculating, using a much more sophisticated model of the physics an idea of exactly what sort of waves will be created.