Directional Recording of Swell from Distant Storms - Department of ...

570 W. H. MUNK, G. R. MILLER, F. E. SNODGRASS AND N. F. BARBER1000 times the low-frequency energy (note the difference in scale). The two quantitieswere also plotted against time with the object of establishing any phase lags between thevariable swell energy and the variable low-frequency energy. None was found. If the lowfrequencies had originated in the same source region as the swell it would have preceded theswell by a week.10010cm2c/ks0-1L287 AUGUSTMEAN AUGUST0.0130 40 50 60 70c/ksFIGURE 43. The mean spectrum for August and the 'ridge cut' associated withthestorm of 28 7 August.10,00oI1000LOCAL100 GENERATIONcm2c/ks 10-01 _0 0 1 USI//o20 40 60 80 lOO 120c/ksFIGURE 44. Schematic presentation of a mean monthly spectrum.Above 35 c/ks the low-frequency tail of the swell spectrum rises sharply out of the flatspectrum, increasing by a factor of 103~ in one octave (figure 41) . Ultimately the recording ofthe low-frequency, low-energy forerunners of the swell is limited by the flat spectrum. ForMay and October the rising curve shows some peaks. These are the residuals of some strongspectral peaks on one particular day; such peaks would have been blurred out had records