The tide type dominated by the semidiurnal constituents M2 and S2, giving two roughly equal highs and lows about 12 hours 25 minutes apart. The US Atlantic coast, most of Europe and much of the world's coastline have semidiurnal tides. Its regular shape is what makes simple rules such as the rule of twelfths work tolerably well.
Pure semidiurnal tide here, so if you miss this low there's another just as good tonight.
Also heard as
- semi-diurnal tide
- twice-daily tide
Related terms
M2
The principal lunar semidiurnal constituent, period 12.42 hours, usually the largest part of the tide.
Diurnal tide
A tide with a single high water and a single low water in each lunar day.
Mixed tide
A tide with two highs and two lows a day of clearly unequal heights, combining semidiurnal and diurnal parts.
Form factor
A ratio of diurnal to semidiurnal constituent amplitudes used to classify a tide as semidiurnal, mixed or diurnal.