mean lower low water (MLLW)— A tidal datum. The arithmetic mean of the lower low water 

 heights of a mixed tide observed over a specific 19-year metonic cycle (the National 

 Tidal Datum Epoch). See epoch (2). Only the lower low water of each pair of low waters 

 of a tidal day is included in the mean. For stations with shorter series, simultaneous 

 observational comparisons are made with a primary control tide station to derive the 

 equivalent of a 19-year value. See datum and type of tide. 



mean lower low water line (MLLWL)— The intersection of the land with the water surface at 

 the elevation of mean lower low water. 



mean range of tide (Mn)— The difference in height between mean high water and mean low 

 water. 



mean river level— A tidal datum. See datum. The average height of the surface of a tidal river 

 at any point for all stages of the tide observed over a 19-year metonic cycle (the National 

 Tidal Datum Epoch), usually determined from hourly height readings. In rivers subject to 

 occasional freshets, the river level may undergo wide variations, and for practical 

 purposes certain months of the year may be excluded in the determination of tidal 

 datums. For charting purposes, tidal datums for rivers are usually based on observations 

 during selected periods when the river is at or near low water stage. 



mean sea level (MSL)— A tidal datum. The arithmetic mean of hourly water elevations 

 observed over a specific 19-year metonic cycle (the National Tidal Datum Epoch). See 

 epoch (2). Shorter series are specified in the name; e.g., monthly mean sea level and 

 yearly mean sea level. See datum. 



mean tide level (MTL)— Same as half-tide level. 



mean water level (MWL)— The mean surface elevation as determined by averaging the heights 

 of tlie water at equal intervals of time, usually hourly. 



mean water level line (MWLL)— The Une formed by the intersection of the land with tlie 

 water surface at an elevation of mean water level. 



meteorological tides— Tidal constituents having their origin in the daily or seasonal variations 

 in weather conditions which may occur with some degree of periodicity. The principal 

 meteorological constituents recognized in the tides are Sa, Ssa, and Si . See storm surge. 



metonic cycle— A period of 19 years or 235 lunations. Devised by Meton, an Athenian 

 astronomer who Uved in the fifth centrury (B.C.), for the purpose of obtaining a period 

 in which new and full Moon would recur on the same day of year. Taking the Julian year 

 of 365.25 days and the synodic month as 29.530588 days, gives the 19-year period of 

 6,939.75 days as compared with the 235 lunations of 6,939.69 days, a difference of only 

 0.06 day. 



03 



