The dynamics of large gravity waves are known to be modified from the linear model by non-linear physics. In this paper we analyse Eulerian surface elevation time histories measured from two sites, Lake George and the North Sea, to examine how weak non-linearity has modified the shape of extreme wave-groups relative to linear theory. We analyse the asymmetry of the extreme wave-groups and find that, on average, the wave in front of an extreme wave is smaller than the wave following it. We also observe a contraction in the envelope width of the wave-group relative to linear theory. The departures from linear theory are strongly correlated with the steepness of the underlying sea-state and are generally consistent with theoretical expectations providing strong evidence that such non-linear phenomena occur in naturally occurring water waves.
