Mo the Caller,
Tides vary according to the strength of gravitational pull of the moon, if the moon is full it has a greater pull than when it is a crescent, I do not pretend to understand the vagaries of the science however, I doubt if many do. The rise and fall varies with the moons fullness and the time of the tide also varies each day being a slightly longer (or shorter) time between high and low tides. It is possible to preict with a fair degree of accuracy the height of the tide for years in advance, for instance on September 20 2015 the tide is predicted to be 6.4 metres, the highest tide in whitby for some 20 years or so.