Pierson-Moskowitz spectrum

The Pierson-Moskowitz spectrum describes a fully developed sea, one in which wind has blown steadily over a long fetch and duration so the waves reach equilibrium with the wind. Its frequency form is S(omega) = alpha times g squared times omega-to-the-minus-5 times exp of minus 0.74 times (omega_0 over omega) to the fourth, with the Phillips constant alpha = 0.0081 and omega_0 = g over the 19.5 m wind speed. A two-parameter version in significant wave height and peak period is the standard for fully arisen seas in classification work. JONSWAP extends it with a peak-enhancement factor for fetch-limited, growing seas; with gamma = 1 JONSWAP collapses to this spectrum.