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Name: Michael
Status: Other
Grade: Other
Location: FL
Country: United States
Date: March 2008


Question:
Recently, I was in Fort Myers Beach, Florida and noticed, next early morning, really strong unusual low tide which happened, literally, over night, that the beach water level changed from less than 5 feet deep to less than 6 inches (barely cover your feet) and as far as for 300-400 feet away from shore line. What is causing it and can it be forecasted? Does it has to do with a moon phase? The moon was full at that time. I saw low tides before in Connecticut, but I never see the water level get down like this one.



Replies:
Mike:

The size of tides depends upon: the location of the moon, the shape of the bottom of the ocean near shore and the amount of land surrounding the area where the tides work.

In a full moon, the moon, sun and earth are lined up so the gravity of sun and moon add up to make high tides higher and, as you would expect, low tides lower.

Another thing is how shallow is the shore? If the ocean floor is nearly flat, a small change in the height of the water means a real big difference in how much of the shore is exposed.

Finally, how much land is around the water? You came from Connecticut so you were looking at Long Island sound. Long Island and the Connecticut shore are close at their eastern end and there is not much room for the water to rush in and out of the Sound. That means the tides never really reach their full height before it is time to "turn around". In Florida, the shore is open and there is plenty of room for the water to move.

So, you had all three things working together, I will bet: full moon, shallow shore and open ocean. No wonder they were big tides.

Robert Avakian


Michael,

I believe that you witnessed a "spring" tide (not associated with the season), which occurs when the Sun and Moon are lined up with the Earth. The Earth would have to be at new or full Moon phase for this to happen and you mention that the Moon was full, which probably confirms a spring tide.

The alignment of the Sun and Moon (Sun on one side of the Earth and Moon on the other side, or both on one side) cause a larger bulging of the water on the two sides of the Earth that are aligned with the Sun and Moon (causing higher high tides) and less bulging on the perpendicular sides (causing lower low tides). the effect should be slightly greater for the new Moon, as the Moon and Sun are on the same side of Earth and thus their gravitational force effect on the ocean is combined.

If the moon is at perigee (its closest point to Earth in its elliptical orbit around Earth), spring high tides are even higher and low tides are even lower. The moon was recently full on Feb 21 and at perigee on Feb 14, so if you were at Fort Meyers around Feb 21, the tidal height range would have been fairly large.

You can find a good Moon phase, perigee, and apogee calculator at http://www.fourmilab.ch/earthview/pacalc.html

David R. Cook
Meteorologist
Climate Research Section
Environmental Science Division
Argonne National Laboratory



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