Reader eclecticca asks:
I really like the last two posts about waves, and they left me with another question… My dad had a little boat he used to take us ocean fishing on quite a bit. I always noticed that some days we just had big waves (swells) when out from the coastline and in fairly deep water (a hundred feet to hundreds of feet according to the depth sounder) and other days those swells would “break” and curl and foam and crash in on themselves, being what we called “breakers” or “white caps”. There is no shore to create the breakers in this case, so what is happening? Is it due to wind? current a combination of factors? Always been kind of curious about this really…
You’re exactly right: those open ocean white caps are due to wind. Strictly speaking, the wind is what’s causing all* of the waves out in open, deep waters. But once the wind is strong enough, it starts breaking up the crests of waves, creating those foamy white tops.
According to one study, the break-up happens when the wind transfers more energy to the wave than surface tension can withstand. When the wave crest breaks up into a mixture of air, spray, and foam, it effectively gives the wind more surface area to push against and continue transferring energy. (Image credit: M. Moers)
* With a few notable exceptions, like in the case of a tsunami.