Quantitative information on the stength and size distribution of whitecaps in a given wave field is very scarce. During the MARSEN field experiments, observations of surface elevation were made with a capacitance wire wave recorder attached to a free floating spar buoy. Automatic analysis of the records with a differentiating circuit and counter allowed a histogram of jump-heights to be constructed corresponding to any preset critical rise rate R of the surface elevation. Over a certain range of R the histogram was nearly independent of the precise values of R. This occurred usually when 0.6<R/c0<1.0 where c0 was the speed of the dominant waves. Records were obtained in wind speeds ranging from 1 to 14 m/s. At 14 m/s the number of ''jumps'' indicating either steep or breaking waves was of the order of 1 every 100 wave periods. It is shown that this number is consistent with previous theoretical estimates, and with visual observations of whitecap coverage. Because of the dispersive properties of waves, whitecapping in deep water is intermittent. Both theoretical calculation and laboratory experiments lead us to expect steep or breaking waves to induce separated flow, with a high local input of momentum to the wave field. It is concluded that such local events could contribute significantly to the total horizontal stress exerted by the wind.