# Why is there a daily inequality for low tides?

I understand why there is a daily inequality for high tides. What I do not understand is why the low tides are not equal. The data all seems to be somewhere along the lines of
but the theory seems to point to
That is, a greater daily inequality for high tide and none for low tide. I know that there are things to consider besides the moons influence, but the data pretty clearly rises above the noise in its consistency.

-

## migrated from astronomy.stackexchange.comMay 1 '14 at 13:09

This question came from our site for astronomers and astrophysicists.

The water for high tides needs to come from somewhere; the mean sea level should stay approximately constant, as long as wind is neglected. That way tides are a kind of oscillations.

In the first of the two diagrams a low low tide is followed by a high high tide, and a high low tide is followed by a low high tide. That way the mean sea level, averaged over one low tide - high tide period remains constant:

In the second diagram you need to average over two low tide - high tide periods to get a constant mean:

The first diagram can be explained by the superposition of semi-diurnal tides, while the second diagram needs an additional diurnal constituent. Depending on the amplitude and phase shift of the diurnal constituent relative to the semi-diurnal constituent we can get different resulting oscillations.

-
Thank you, that makes a lot of sense! What I'm looking at is the tidal force, not the height. I was afraid it would be something like this, though; I'm going to end up messing around with my simulation for days trying to figure out how to compensate for this without eating up too much processing power, and then suddenly I'll find a solution that would have taken 10 minutes if I'd thought of it first. Better get started, heh – stellatedHexahedron Apr 2 '14 at 18:04

You need to be careful about which port or beach you take the data from. It is not the same to measure tides on the eastern side of a gulf than on the western side, due to Moon's motion, and also it is not the same to take data at the far end of a gulf than on its mouth.

I think this last thing is what happens to your data.

-