Form Number

The form number F is the dimensionless ratio that classifies a tidal regime, defined as the sum of the two main diurnal amplitudes over the two main semidiurnal amplitudes, F = (K1 plus O1) / (M2 plus S2). The thresholds are standard: F below 0.25 is semidiurnal, 0.25 to 1.5 is mixed mainly semidiurnal, 1.5 to 3.0 is mixed mainly diurnal, and above 3.0 is diurnal. Computed from the harmonic constants for a station, F summarizes whether a port sees one or two tides a day and how large the diurnal inequality will be. It is one of the first numbers a hydrographer reads off a constituent table.