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Neaps

How harmonic constituents work

Tide predictions are created by adding up the effects of potentially hundreds of factors like the location of the sun, the moon, and the benthic environment. These constituents are derived from analyzing past water levels.

You can think of each tidal constituent as a sine wave. For example, below is a single constituent, M2, or "Principal lunar semidiurnal constituent" for Monterey, California during the first week of 2018:

1,514,800,000,0001,514,850,000,0001,514,900,000,0001,514,950,000,0001,515,000,000,0001,515,050,000,0001,515,100,000,000Meters-0.4-0.20.20.4

If this constituent, which measures the regular effect of the moon on water levels, were the only one that affected the tides, then tides would go up and down in a regular pattern.

However, once you add more constituents to measure more effects, it gets complicated.To simplify, let us take just four constituents and plot them for the same location:

1,514,800,000,0001,514,850,000,0001,514,900,000,0001,514,950,000,0001,515,000,000,0001,515,050,000,0001,515,100,000,000Meters-0.4-0.20.20.4

Just for fun, let's take a look at all the defined constituents:

1,514,800,000,0001,514,850,000,0001,514,900,000,0001,514,950,000,0001,515,000,000,0001,515,050,000,0001,515,100,000,000Meters-0.4-0.20.20.4

If we add all these constituents together, they cancel each other or amplify each other, leading to an accurate tide prediction:

1,514,800,000,0001,514,850,000,0001,514,900,000,0001,514,950,000,0001,515,000,000,0001,515,050,000,0001,515,100,000,000Meters-1.0-0.50.51.0

To read more about tidal constituents, the following resources are helpful:

Where tidal constituents come from

Tidal constituents are derived by running linear regressions (read: lots of maths) on observed water levels. Some agencies, like NOAA, publish tidal constituents that are avialble on their website and via an API. Other agencies do not provide constituents, but do provide data on observed levels.

We are also working on a world-wide database of tidal constiutents.

If you have at least a year of observations (one an hour will usually do), you can generate your own constituents on this website. These can even be added to a growing database of global tide stations.

Constituents supported by Neaps

Neaps supports all the major tidal constituents, and we are still adding the more esoteric ones that affect large shallow bays.

CodeDescriptionIn Neaps
M2Principal lunar semidiurnal constituentSupported
S2Principal solar semidiurnal constituentSupported
N2Larger lunar elliptic semidiurnal constituentSupported
K1Lunar diurnal constituentSupported
M4Shallow water overtides of principal lunar constituentSupported
O1Lunar diurnal constituentSupported
M6Shallow water overtides of principal lunar constituentSupported
MK3Shallow water terdiurnalSupported
S4Shallow water overtides of principal solar constituentSupported
MN4Shallow water quarter diurnal constituentSupported
NU2Larger lunar evectional constituentSupported
S6Shallow water overtides of principal solar constituentSupported
MU2Variational constituentSupported
2N2Lunar elliptical semidiurnal second-order constituentSupported
OO1Lunar diurnalSupported
LAM2Smaller lunar evectional constituentSupported
S1Solar diurnal constituentSupported
M1Smaller lunar elliptic diurnal constituentSupported
J1Smaller lunar elliptic diurnal constituentSupported
MMLunar monthly constituentSupported
SSASolar semiannual constituentSupported
SASolar annual constituentSupported
MSFLunisolar synodic fortnightly constituentSupported
MFLunisolar fortnightly constituentSupported
RHOLarger lunar evectional diurnal constituentSupported
Q1Larger lunar elliptic diurnal constituentSupported
T2Larger solar elliptic constituentSupported
R2Smaller solar elliptic constituentSupported
2Q1Larger elliptic diurnalSupported
P1Solar diurnal constituentSupported
2SM2Shallow water semidiurnal constituentSupported
M3Lunar terdiurnal constituentSupported
L2Smaller lunar elliptic semidiurnal constituentSupported
2MK3Shallow water terdiurnal constituentSupported
K2Lunisolar semidiurnal constituentSupported
M8Shallow water eighth diurnal constituentSupported
MS4Shallow water quarter diurnal constituentSupported