抄録
Periods of sea waves are irregular as well as in their wave height. Usually the mean value of intervals of wave amplitudes in records is always shorter than the period of the maximum energy density of spectrum. From the fact that the higher the amplitudes become, the rarer they occur, we notice that if we choice suitable heights of successive two waves and observe this time interval, this may correspond to the above-mentioned most prevailing wave period. This problem is theoretically analysed, and it is found that the significant wave height must be preferred. If we develope the Rice's theory of distribution of zeros to that of arbitrary heights, the results are as follows: Let ξ(t) be wave height, and ψ(τ) be correlation function, w(f) be spectral distribution, ψ(τ)=∫^∞_0w(f)cos2πfτdf, then the probability density of intervals that ξ(t) passes ξ_1 with positive and ξ_2 with negative gradient, [numerical formula] so, distribution function becomes when ξ_1=ξ_2=ξ, [numerical formula] where [numerical formula] and μrs=χ_rχ_s,r,s=1,2,3,4. M_<rs> is cofactor of μrs [numerical formula] Here, if we take the expression at the minimum value of τ which satisfies ψγ'=0. [numerical formula] This formula shows good accordance with the results of actual observation in extensive range of τ. This leads the following results: [numerical formula] whith tells us that the time interval of significant wave is approximately the most predominant wave period.
