High & Low tides

[stevie m]

Sitting on the beach today, I wondered how do the people who formulate the tide timetables know,

a). How long it is between high and low tides.

b). Just how to the nearest cm how high or low its gonna be.

I've looked at various timetable and cannot see a distinct mathmatical pattern.

Also why does the Mediteranean Sea seem to be tideless?

[Clanad]

The level of the tides is determined by physical factors, mainly the relative positions of the earth, sun, and moon, as well as specific locality. After the changes in tidal level have been followed for a few weeks in a locality, they are very predictable for that locality for years into the future. Of course, local weather conditions, such as storms, can modify the extent of the tides, but not the timing. The highest and lowest tides are called spring tides (they "spring" up and down) and they occur every two weeks near the full or new moon (the dark phase of the moon). The weeks between the spring tides are periods of less extreme tides called neap tides. By tradition, the zero tide level (0.0 tide) is the mean of the low tides, and tide levels are measured in reference to the zero tide level.

Additionally, The Rule of Twelfths is a rule of thumb for estimating the height of the tide at any time, given only the time and height of high and low water. The rule assumes that the rate of flow of a tide increases smoothly to a maximum halfway between high and low tide before smoothly decreasing to zero again and that the interval between low and high tides is approximately six hours. The rule states that in the first hour after low tide the water level will rise by one twelfth of the range, in the second hour two twelfths, and so on according to the sequence

Further, the Mediterranean sea does have tides, but they are of a very low amplitude.  The low tides in the Mediterranean are mostly associated with the fact that it has a very narrow outlet/inlet into the Atlantic, although there are other factors...

[Tim Baxter]

A bit more info.  Tidal movements of water are caused by the sun's and moon's gravity pulling water around the globe, the moon's effect being the greater.  When the sun and moon are in opposition (full moon) or conjunction (new moon) then the tidal movement is at its greatest.  When the two are at right-angles (half moon waxing or waning) then the gravitational pull is diminished and the tidal movement is at its least.

The next influence is the shape of the land mass which the tidal movement is up against.  For examples, the bay of Fundy and the Severn estuary have some of the largest tidal ranges in the world because their shape funnels the tidal movement and concentrates it.  As the width gets less then the tidal water has to 'pile up' as it is dragged around the earth.  Equally, mid-ocean islands have little or no tide simply because there is nothing blocking the tidal movement and therefore nothing to make it pile-up.  Ports like Southampton get two tides at a time (one slightly after the other) because the Isle of Wight makes two channels for the tidal water to travel up.