It is very important to know properties of long period waves from the point of view of ship mooring criteria in a harbour facing to the open sea. A lot of studies related long period waves are carried out, and some relations among long period waves and moored ship motions are obtained.However, there are very few studies conceming wave periods and the correlation of long period waves considering the wave periods, as well as wave heights. In this study, wave data of NOWPHAS system are statistically analyzed, when it becomes stormy weather conditions in a harbour facing to the Pacific Ocean. Therefore, main conclusions are obtained as follows.
1) Observed long period waves are mostly explained by bound waves when typhoons are close to observation point offshore harbour facing to the Pacific Ocean. On the other hand, the theory of bound waves can not apply when typhoons are passed far from observation point.
2) Long period wave heights can be regressed with high accuracy by some kinds of polynomials consisted on significant wave heights and periods.
3) The accuracy of reproduction becomes better if orders of frequency in the equation of JONSWAP type are changed for observed long period waves.
4) It can be possible to estimate spectrums of long period waves by analyzing spectrums of short wave periods below 20s due to the theory of group waves.