The ratio of the sum of the K1 and O1 amplitudes to the sum of the M2 and S2 amplitudes. Below about 0.25 the tide is semidiurnal; from 0.25 to 1.5 it is mixed, mainly semidiurnal; from 1.5 to 3 it is mixed, mainly diurnal; above 3 it is diurnal. Planners use it to decide which prediction methods and datums behave sensibly and how much the daily highs and lows will differ.
Form factor comes out at 1.8, so treat this as mixed, mainly diurnal, and don't trust twelfths.
Also heard as
- form number
- F ratio
- tidal form factor
Related terms
Mixed tide
A tide with two highs and two lows a day of clearly unequal heights, combining semidiurnal and diurnal parts.
Semidiurnal tide
A tide with two high waters and two low waters of similar height in each lunar day.
Diurnal tide
A tide with a single high water and a single low water in each lunar day.
K1
The lunisolar diurnal constituent, period about 23.93 hours, a main driver of once-daily tides.
M2
The principal lunar semidiurnal constituent, period 12.42 hours, usually the largest part of the tide.