The average wave steepness may be defined as the ratio between the significant wave height Hs and the wavelength associated to either the zero-up-crossing mean period Lm or the peak period Lp (Bitner Gregersen et al., 1998). This parameter may be calculated from wave data at fixed locations, as well as by starting from theoretical spectra. In this paper the average wave steepness is firstly analyzed by considering a JONSWAP spectrum. It is shown that for this spectrum the ratio Sm = Hs/Lm, as well as Sp = Hs/Lp, depends upon the values of the spectrum parameters. The theoretical values are then compared with wave data in the Mediterranean Sea, Pacific Ocean, Atlantic Ocean and North Sea. The values of Sm and Sp are deeply investigated for severe sea states, with large values of Hs. It is obtained that in severe sea state the observed values of wave steepness, defined as either Sm or Sp, are always in the range defined by theoretical spectra; therefore, the extreme values of Sm and Sp, which are of interest for naval architecture, may be obtained from theoretical analysis, as a function of extreme values of Hs.

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