The Pierson–Moskowitz (PM) spectra is an empirical relationship that defines the distribution of energy with frequency within the ocean. The result is given as a wave height power spectra density.
Developed in 1964, the PM spectrum is one of the simplest descriptions for the energy distribution. It assumes that if the wind blows steadily for a long time over a large area, then the waves will eventually reach a point of equilibrium with the wind.
This is known as a fully developed sea. Pierson and Moskowitz developed their spectrum from measurements in the North Atlantic during 1964.
* * *
Here is a paper: wave_height.pdf
Here is a Matlab script: ocean_wave_PSD.m
– Tom Irvine