MEAK aEA LEVEL 



maximum — The greatest value attained (or at- 

 tainable) by a function; the opposite of mini- 

 mum. (5) 



maximum ebb — The greatest speed of an ebb tidal 

 current. (66) 



maximum flood — The greatest speed of a flood 

 tidal current. (66) 



maximum sound pressure — For any given cycle 

 of a periodic wave, the maximum absolute value 

 of the instantaneous sound pressure occurring 

 during that cycle. 



mean — ^ee arithmetic mean. 



mean chart — (or mean map). Any chart, on 

 which isopleths of the mean value of a given 

 oceanographic element are drawn. 



mean current — (or average current, scalar mean) . 

 The current speed and direction determined to 

 be the average of a total niunbeir of observations 

 for a specified area. 



meander — A deviation of the flow pattern of a 

 current. 



mean diurnal high water inequality — (abbrev- 

 iated DHQ). Half the average difference be- 

 tween the heights of the two high waters of each 

 tidal day over a 19-year period, obtained by sub- 

 tracting the mean of all high waters from the 

 mean of the higher high waters. ( 68 ) 



mean diurnal low water inequality — (abbrevi- 

 ated DLQ). Half the average difference be- 

 tween the heights of the two low waters of each 

 tidal day over a 19-year period, obtained by sub- 

 tracting the mean of the lower low waters from 

 the mean of all low waters. ( 68 ) 



mean higher high water — (abbreviated 

 MHHW) . The average height of all the daily 

 higher high waters recorded over a 19-year pe- 

 riod, or a computed equivalent period. It is 

 usually associated with a tide exhibiting mixed 

 characteristics. See mixed tide. 



mean higher high water springs — (abbreviated 

 MHHWS). The average height of all higher 

 high waters recorded during syzygy over a 19- 

 year period, or a computed equivalent period. 



mean high water — (abbreviated MHW). The 

 average height of all the high waters recorded 

 over a 19-year period, or a computed equivalent 

 period. (,S'eefigurefor tide range.) 



mean high water lunitidal interval — See luni- 

 tidal interval. 



mean high water neaps — (abbreviated MHWN). 

 The average height of all high waters recorded 

 during quadrature over a 19-year period, or a 

 computed equivalent period. {See figure for 

 tide range.) 



mean high water springs — (abbreviated 

 MHWS). The average height of all high 

 waters recorded during syzygy over a 19-year 

 period, or a computed equivalent period. {See 

 figure for tide range.) 



mean horizontal sound speed — The mean veloc- 

 ity along the horizontal for one cycle of a soimd 

 ray path. - 



mean lower low water — (abbreviated MLLW). 

 The average height of all the lower low waters 

 recorded over a 19-year period, or a computed 

 equivalent period. It is usually associated with 

 a tide exhibiting mixed characteristics. See 

 mixed tide. {See figure for surf zone.) 



mean lower low water springs — (abbreviated 

 MLLWS). The average height of all lower 

 low waters recorded during syzygy over a 19- 

 year period, or a computed equivalent period. 



mean low water — (abbreviated MLW). The 

 average height of all the low waters recorded 

 over a 19-year period, or a computed equivalent 

 period. {See figure for tide range.) 



mean low water lunitidal interval — See luni- 

 tidal interval. 



mean low water neaps — (abbreviated MXiWN). 

 The average height of all low waters recorded 

 during quadrature over a 19-year period, or a 

 computed equivalent period. {See figure for 

 tide range.) 



mean low water springs — (abbreviated MLWS) . 

 The average height of all low waters recorded 

 during syzygy over a 19-year period, or a com- 

 puted equivalent period. {See figure for tide 

 range.) 



mean map — See mean chart. 



mean neap range — (abbreviated Np) . The aver- 

 age semidiurnal range occurring at the time of 

 quadrature. It is smaller than the mean range 

 where the type of tide is either semidiurnal or 

 mixed and is of no practical significance where 

 the type of tide is diurnal. {See figure for tide 

 range.) 



mean neap rise — The height of mean high water 

 neaps above the chart datum. (68) {See 

 figure for tide range.) 



mean range — (abbreviated Mn). The difference 

 in height between mean high water and mean 

 low water, measured in feet or meters. {See 

 figure for tide range.) 



mean rise — The height of mean high water above 

 chart datum. (50) {See figure for tide 

 range.) 



mean rise interval — (abbreviated MRI). The 

 time interval in hours and minutes between the 

 transit of the moon and the height of the tide 

 measured above chart datum. The mean rise 

 interval may be referred either to the local or 

 Greenwich meridian. 



mean river level — The average height of the sur- 

 face of a river at any point for all stages of the 

 tide over a 19 -year period, usually determined 

 from hourly lieight readings. Unusual varia- 

 tions of river level due to discharge or runoff 

 may be excluded in computation. 



mean sea level — (abbreviated MSL; or sea level 

 datum). The mean surface water level deter- 

 mined by averaging heights at all stages of the 

 tide over a 19-year period. Mean sea level is 

 usually determined from hourly height readings 



103 



