in this week's episode of surfing explained we're going to explore how an ocean wave is formed waves are created by wind blowing over the surface of the ocean initially this creates tiny ripples known as capillary waves it is not fully understood how this formation occurs as the surface tension of water makes the process difficult to observe at a molecular level that said we're pretty sure there are two methods by which wind energy is transferred from the air to the water first small changes in air pressure created by fluctuations in the wind will either push down
on the ocean surface as high pressure or lift up on the ocean surface during lower air pressure second when the wind blows directly across the ocean surface the shear force between the stationary water molecules and the moving air molecules cause the incompressible water molecules to push over each other in the same direction as the wind water molecules strong attraction to each other creates a surface tension that limits the energy of these capillary waves which are often just a few millimeters high and as soon as the wind stops the surface tension will smooth these ripples back
to a mirror finish if the wind continues though capillary waves can grow into larger gravity waves where they have enough energy to overcome the surface tension and the controlling force then becomes gravity attempting to pull the now larger waves back down again as wind blows over the wave shape from behind air is directed upwards and compressed over the crest pulling the water and wave upwards with it low pressure and eddies in the wind on the downwind wave face then also draw the leeward side up as well making the wave even bigger which will direct even
more wind upwards there are three factors during this process that will decide whether waves will absorb enough energy to be surfable when they reach shore the wind speed the duration of time the wind blows for and the distance over which the wind blows known as the fetch and for any given wind speed duration and fetch there is a theoretical maximum wave height that can be created so in order to create the biggest waves we need a strong wind to blow over a large area for a long period of time as an example an average hurricane
is around 500 kilometers in diameter with wind speeds well over 200 kilometers per hour that can blow for many days during a storm wave height increases faster than the wave period increases creating tall narrow waves that are unstable and topple over into what are called white caps when a wave reaches this height energy can still be added but it will do so by increasing the wave energy below the surface and thus increasing the wave period and as the wave period increases so does the maximum wave height before the wave topples into a white cap as
wave period increases so does the speed at which the wave travels once the speed of the wave is faster than the wind speed normally due to the storm dying away no more energy will be added to the wave and it will start its journey out away from the fetch area waves within the fetch area are called windswell and are often bumpy and irregular as wind gusts of different strength and direction create a mix of small and large waves traveling over each other in all directions once the waves travel out the fetch area they are known
as ground swell these ground swell waves can have enough energy to cross thousands of miles of ocean before arriving at the beach which leads us nicely to next week's episode on how the waves propagate across the ocean thanks for watching and we'll see you then
