EXTRA TIDES 

     In most parts of the world the tides are semi-diurnal and there are 
     four extreme tides per day, two highs and two lows. Because the moon 
     is almost always the dominant factor in the tides, the day involved is 
     the lunar day of 24 hours and 50 minutes. This means that occasionally 
     one of the tides will slip over into the next day, leaving only three 
     extremes in the solar day. 

     There is also a diurnal tide generated when the moon or sun is not on 
     the equator which usually causes the two highs or lows to differ. In 
     some places, such as much of the Gulf of Mexico, the diurnal component 
     of the tides can swamp out the semi-diurnal so that there is only one 
     high and one low in the lunar day. See DIURNAL.TXT for more on this.

     On the other hand in some locations (Southampton, England; Hoek van 
     Holland, The Netherlands; Rio de Janiero and Santos, Brazil) there can 
     be more than four extremes per day. This is sometimes described as a 
     double high or low, e.g there can be a high tide followed by a slight 
     dip and another high.

     In published tide tables the extra are often filtered out by some 
     obscure procedure. In TIDES4 every extreme which comes along is 
     treated equally even when the differences in height are too small to 
     be resolved in the printed output. They can always be examined to 
     another decimal point using Option D and a general picture of what is 
     going on can be had by plotting the heights using Option C.

     While these multiple extrema can be real, at least to the extent that 
     the harmonic constants represent the actual ocean, they are usually 
     academic.

     Sometimes the extra tides are spurious and are an artifact of the 
     method of computation. In TIDES4 this may happen for a subordinate 
     station if the time differences for high and low differ greatly, say 
     an hour or more. (A half-hour difference is probably OK.) This 
     generally occurs when the tide propagates into shallow water as it 
     travels up a bay or river.


     Ed Wallner/ 1996 DEC 03/ EXTRATID.TXT