Work out tide time estimate instantly with clear inputs, formula shown and shareable results.
Semi-diurnal tides repeat every 12 hours 25 minutes, half the 24 hour 50 minute lunar day, so successive high waters drift about 50 minutes later daily. Between them the height follows roughly a cosine, which is the basis of the rule of twelfths. Three hours after a high tide of 4.2 m range the water has fallen to about 3.15 m above low water.
Tidal period
semi-diurnal cycle = 12.42 hours
Height between tides
height = (range/2)(1 + cos(2 pi t / 12.42))
The tide moves 1, 2, 3, 3, 2 and 1 twelfths of the range in successive hours - a mental approximation to the cosine curve.
No. Parts of the Gulf of Mexico and South China Sea have one tide a day, and many coasts are mixed with unequal successive highs.