Sitting at the beach, I observe waves crashing onto the shore with some periodicity (although not quite mono-periodic). Since wind is the source of ocean waves (not considering tides), how can we understand this apparent periodicity?
For some periodicities. the positive feedback from the wind on the waves is strongest. Those are the waves that will grow.
The speed of water waves depends on their period. Water waves can only gain energy (height) from the wind when they are slower than the wind. The height of the waves then depends on the fetch, the distance over which the waves traveled in a certain wind (speed and direction).
It is quite interesting. Wikipedia has some data, and many references: https://en.wikipedia.org/wiki/Wind_wave