1.3.4 Duration of wave records
The optimal duration of wave records is determined by
several factors. First of all, for a correct description of the
sea state, conditions should be statistically stationary
during the sampling period. In fact, this will never be
achieved completely as wave fields are usually evolving
(i.e. growing or decaying). On the other hand, to reduce
statistical scatter, the wave record should contain at least
200 zero-downcrossing waves. Hence, the optimal time
over which waves are usually measured is 15–35 minutes,
as this reasonably accommodates both conditions.