Tidal range means the difference between low tide and high tide. This can be two similar things:
1 the ‘difference’ changes with spring and neap tides
2 the ‘difference’ is a constant (avaerage) measurement asigned to a specific coatal location, other locations will have their own values.
What I want to find out is regarding number 2 above… How is it that different coastal locations which are quite close to each other can have very different tidal range values?
Mostly, it has to do with the shape of the land and ocean floor. The Bay of Fundy, in Canada, for example, has the greatest tidal variation between high tide and low tide. This is because it is long and skinny, and gets narrower away from its mouth. When high tide starts coming in, basically a large amount of water is pushed into the bay, and held there by the large mass of water at the mouth of the bay. Thus, the water level is higher at the end of the bay than at the mouth.
Imagine a tub of water that is tapered towards one end. You shove a bunch of water towards the tapered end, causing one big wave. As the wave travels towrds the small end, the water sloshes higher, being funnelled into a tighter space. Tides are like that, except huge and slow (6 hours 10 minutes from high to low) in comparison. Also, there’s all sorts of little currents and strangenesses involved everywhere.
What I want to know is why tide charts use a 19-year average to base their predictions on.
I believe the moon cycles repeat every 19 years and I forget the name of this period… Someone who knows will be here shortly I am sure.
The nineteen year cycle referred to is the precession of the plane of the Moon’s orbit with respect to the plane of the Earth’s orbit. The plane of the Moon’s orbit is tilted at 5 degrees to the plane of the Earth’s orbit. Tidal forces cause gyroscopic precession to take place and the orientation of the plane of the Moon does a nice slow wobble that takes, oh, nineteen years or so.
That difference of direction can add up to a 10 degree difference in the altitude of the Moon above a given location on Earth and must therefore be taken into account when printing predictions of tide tables.
The high tides seen at the Bay of Fundy have more to do with the natural resonant frequency of the bay. The time it takes for a tidal surge to traverse the bay up and back is approximately 12.5 hours, IIRC, and this is the same time as half a tidal cycle. The sloshing back and forth is thus built up and reinforced. If it was the narrowing of the bay only we would expect to see high tidal levels at places like San Felipe, Mexico, which is located at the top of the
Sea of Cortez.
Sidereal revolution period of the line of the nodes: 18.60 years
Think of this: The forces that create the tides travel from east to west. If the Earth were uniformly covered by water, the tide would be a wave traveling westward. Now, the Chesapeake bay runs from N to S and is quite narrow E-W. The tides on the bay are caused exclusively by the tidal wave at the mouth of the bay. The tide creates a wave at the mouth which travels up the bay and if you blocked this wave at the mouth there would be no tides on the bay. Now imagine the bay ran westward. The wave would start at the E entrance and as it traveled W it would be continually reinforced but with nowhere to exit at the other end so it would grow as it progressed.
So, let’s sea if I’ve got it!
With a little general information, it would possible to estimate (interpolate or extrapolate) a typical tidal range for a piece of coast on a macro scale eg a section of the side of a continent. One side of it being likely to have a fairly dissimilar set of values to the other. The same would be true on a micro scale, in say, a small bay, or other areas only a few metres apart, except that the values are likely to very similar.
…But when it comes to meso scale calculations, (say 10 to 100 miles, it is all so incredibly individual that to get anywhere near accurate, one would have to make an empirical study.
Thanx for all contributions.