For the Safety Navigation In Japanese Coastal Waters page 18/120

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- 14 -2. WavesIn Japanese waters waves run very high when a series of northwesterly seasonal winds blowor when developed low atmospheric pressures or typhoons pass through. Northwesterlyseasonal winds....

- 14 -2. WavesIn Japanese waters waves run very high when a series of northwesterly seasonal winds blowor when developed low atmospheric pressures or typhoons pass through. Northwesterlyseasonal winds begin to blow as the barometric gradient of atmospheric pressure distribution,in which the high pressure area lies to the west and the low pressure area to the east, deepensafter the passage of low atmospheric pressures. Consequently, the sea becomes turbulet forseveral days. Some of low atmospheric pressures gain in force while passing in the vicinity ofJapan and stormy winds extend over a radius of 500 to 800 M. A high-wave region covers alarge sea area where waves reach as much as 0 metres or more in height. From summerthrough autumn, typhoons frequently pass in the vicinity of Japan causing big waves to risearound their courses and big surges to roll in the sea areas around the courses. Special careshould be paid to such atmospheric phenomena as the above.(1) Wave Height in Japanese Waters〔Winter〕In Japanese waters, the mean wind velocity is 15 to 20 kn and the mean wave height is 1.5to 2 metres. High-wave regions are widely distributed, ranging from the sea area east ofKanto to the offing far from the shore of Japan. The mean wave height in these regons ismore than 2.5 metres. In some places, it reaches as much as 3.5 metres.〔Spring〕In Japanese waters, the mean wind velocity is 10 to 17 kn and the mean wave height is 1to 1.8 metres. High-wave regions, where the mean wave height is about 2 metres, arescattered in the sea area southeast of Kamchatka and in the offing far east of Sanriku.〔Summer〕In Japanese waters, the mean wind velocity is 9 to 13 kn and the mean wave height is 0.8to 1.5 metres. This is the calmest season of the year. High-wave regions where the meanwave height is about 2 metres can only be observed in some sea areas off Kito to Izu.〔Autumn〕In Japanese waters, the mean wind velocity is 13 to 18 kn and the mean wave height is 1.3to 1.9 metres. This is the season after winter when waves are most turbulent. High-waveregions, where the mean wave height is over 2.5 metres, can be observed in the sea areaextending from the southeastern part of Kamechatka to the offing far from the shore ofSanriku and in the northern part of the East China Sea. In some places, hte wave heightreaches as much as approximately 3 metres.(2) Waves Generated by Seasonal WindsSeasonal winds continuously blow almost in a certain direction for hours at aconsiderably high velocity, and the fetch (the distance along open water over which the windblows) of each seasonal wind gains in scale in the vicinity of Japanese waters. Consequently,big waves begin to rise and develop into larger ones over a wide range. Especially after thepassage of a cold front accompanying a low atmospheric pressure, northwestward ornorthward seasonal winds blow one after another over the sea Act A at a velocity of morethan 20 m/s and waves rage furiously. Moreover, atmospheric layers become unstable due tothe approaching cold. As a result, the direction of wind suddenly changes and a violent gustof wind springs up, so that waves rolling in opposite directions gain in force, dash againstone another, and turn into pyramidlike chopping waves. Stringent precaution should betaken against chopping waves since they are so powerful as to sink even a large-sized ship