In physical oceanography, the significant wave height (SWH or H_s) is defined traditionally as the mean wave height (trough to crest) of the highest third of the waves (H_1/3). Nowadays it is usually defined as four times the standard deviation of the surface elevation – or equivalently as four times the square root of the zeroth-order moment (area) of the wave spectrum. The symbol H_m0 is usually used for that latter definition. The significant wave height may thus refer to H_m0 or H_1/3; the difference in magnitude between the two definitions is only a few percent.