On the open sea, the changing tides 

 are barely discernible. But where the 

 edges lap at shorelines, the change in 

 water level — or tidal range — 

 is marked. In North Carolina, the 

 variance is only a few feet; but along the 

 funnel-shaped Bay of Fundy in Canada, 

 the tide may vary as much as 50 feet 

 from low to high. Tidal range is also 

 affected by 

 seasonal 

 variations — 

 such as 

 atmospheric 

 pressure, rain- 

 fall and wind 

 direction — 

 and man-made 

 alterations in 

 certain harbors. 



higher high tide is commonly thought to 

 follow the full moon, but in truth it may 

 occur after either the full or new moon. 



At the first and third quarter, when 

 the sun and moon form a right angle to 

 the Earth, scant or "neap" tides occur. 

 The tide-producing forces of the moon 

 and sun cancel each other out, causing 

 lower high tides and higher low tides. 



Sun 



Sun 



Sun 



Moon 



Gravitational 

 pull of Moon 

 plus Sun 



Spring 

 Tide 



o 



New Moon 



Here 

 Comes 

 the Sun 



The sun 

 also plays a 

 role in tidal 

 flux, though its 

 influence is less 

 than half that of 

 the moon. The 

 sun's gravity is most remarkable when it 

 pulls in concert with the moon. Twice 

 each month, the sun, moon and Earth 

 are aligned. The combined pull of the 

 sun and moon produces higher and 

 lower tidal ranges known as "spring 

 tides." Spring tides rise on the full 

 moon, when the Earth is flanked by the 

 moon and sun, and at the "new" moon, 

 when the moon is between the Earth 

 and sun. 



The term "spring" refers to the leap 

 in tidal range, not the season. Spring 

 tides may add 1 or 2 feet to the mean 

 tide levels along the North Carolina 

 coast, causing slightly higher high tides 

 and lower low tides. A common 

 misconception along Tar Heel shores is 

 that spring tides occur only once 

 monthly, says Sea Grant specialist 

 Spencer Rogers. This may be because in 

 a given month, one of the two spring 

 tides is more pronounced, he says. The 



Gravitational 

 pull of Sun 



Gravitational 

 pull of Moon 

 plus Sun 



J 



Gravitational 

 pull of Sun 



Moon 



Gravitational 

 pull of Moon 



Neap 

 Tide 



First Quarter 



c 



Full Moon 



Adapted from Marine Biology 



Tie term "spring" refers to the leap 

 in tidal range, not the season. 

 Spring tides may add 1 or 2 feet 

 to the mean tide levels along 

 the North Carolina coast, 

 causing slightly higher high tides 



and lower low tides. 

 A common misconception along 

 Tar Heel shores is that 

 spring tides occur only once monthly. 



Other Extenuating Factors 



The moon's do-si-do with the Earth 

 takes an elliptical course. Once each 

 month, only 216,000 miles separate our 

 planet from the moon. At this close 

 range, the moon is said to be at "peri- 

 gee," and the increased gravitational pull 

 causes higher tides than normal. About 



two weeks later, at "apogee," the moon is 

 farthest away from Earth and its gravita- 

 tional grip on the tides slackens. 



Tidal ranges have even greater 

 variance when two or more of these 

 monthly events overlap. For instance, a 

 spring tide coinciding with the moon's 

 perigee would cause more extreme 

 fluctuations. Local weather conditions 

 may either 

 compound or 

 dampen the 

 intensity of the 

 predictable 

 astronomical 

 events, says 

 Rogers. Add 

 to the equation 

 a coastal storm 

 such as a north- 

 easter, he says, 



©and you get 

 flooding and 

 accelerated 

 erosion in beach 

 communities. 

 The notorious 

 Hurricane Hazel 

 skirted a spring 

 tide to produce a 

 storm of ferocious 

 proportion. 

 The fierce and famous Ash Wednes- 

 day Storm, which swept the East Coast 

 from the Carolinas to New England in 

 March 1962, also attacked on a spring tide 

 at the "dark of the moon." Coincidentally, 

 the moon reached perigee within a half- 

 hour of this astronomical alignment of the 

 moon, sun and Earth. The result was a 

 three-day storm that killed more than 40 

 people, toppled homes and businesses 

 and submerged streets. 



The next simultaneous perigee and 

 spring tide — or "proxigean spring tide" 

 — is predicted to occur Dec. 21. Keep 

 your eye on the weather. □ 



Helpful sources for this article 

 included Marine Biology by Peter 

 Castro and Michael E. Huber; 

 Oceanography: An Invitation to 

 Marine Science by Tom Garrison; 

 and The Weather Book ( USA Today) 

 by Jack Williams. 



Gravitational 

 pull of Moon 



<D 



Third Quarter 



COASTWATCH 19 



