Present data year 
97. Tides at the FRF are semidiurnal. The effect of the Moon's 
declination produces an average difference of 10 cm in daily high tides and 
5 cm in the daily lows. The mean range during the year was 99 cm with a 4-cm 
difference between times near Aphelion (July), when the range was lowest, and 
Perihelion (January) (Table 14 and Figure 26). The monthly averages show a 
3- and 6-month periodicity which in part is due to the inclination of the Sun 
(Coast and Geodetic Survey 1971), a long-period astronomical tidal consti- 
tuent. Additionally, astronomical forces with annual periodicity, seasonal 
variability of the specific volume of the sea water due to temperature changes 
(the Steric effect), and meteorological conditions may also contribute to the 
observed variations. 
Extreme water levels for 1982 
98. Highest water levels generally occur when strong and persistent 
onshore winds and associated high waves produce setup of nearshore waters 
against the shoreline. This condition may be enhanced when the astronomical 
forces of the Moon and Sun are aligned (syzygy); the astronomical forces are 
even greater when the moon is also at Perigee (i.e. nearest the Earth). Ex- 
treme tidal heights at the FRF during 1982 were not coincident with the times 
of perigee-syzygean alignment that occurred on 9 January, 21 July, and 1 and 
30 December. However, extremely high levels in October and December were a 
result of low pressure systems that produced high waves and strong onshore 
winds (Section VI). Strong offshore winds caused very low tides such as those 
of 8 February 1982 (-108 cm), the lowest of the year. 
1982 versus past years 
99. The distribution of daily high and low water levels and hourly 
tidal heights during the year were typical of those in previous years (Fig- 
ure 27), except that apparently milder meteorological conditions produced 
fewer tidal heights in excess of 100 cm. In 1982, MHW, MLW, mean range (MR), 
and MTL data were consistent with those in previous years (Table 14 and 
Figure 26). 
All years combined 
100. Based on the distribution of tide heights for 1980 through 1982 
(Figure 28), 1.25 percent of the time (110 hr) the tide height can be expected 
to exceed 90 cm. Likewise, 1.1 percent of the time (98 hr) the heights can be 
expected to be less than -70 cm. It can also be seen from Figure 28 that the 
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