eli5: Why are tide highs and lows not uniform?



Some places, like the Bay of Fundy, have extreme high vs low tides, but other places (I can’t think of one) have almost invisible tide level changes. I have wondered this for years: since all tides have the same cause, why aren’t all tides the same? For the record, I do understand why tides vary through the month, but no idea why they vary geographically.

In: Physics

Things like wind also affect the tide, also high and low pressure weather systems. It’s not just the moon and sun’s gravity.

The shape of the Bay of Fundy is the cause of extreme tides there. Water has inertia, and a lot of water is very heavy. When the Sun/Moon gets water moving in a bulge (the actual cause of the tide), and this bulge comes in contact with the shoreline, all the water has to stay together until it’s given up all it’s inertial energy. Focus all that energy in a funnel, and you get a very high tide at the tip.

The ocean has boundaries with complicated shapes, so the water can’t move totally freely. In the same way that water in a container will slosh around it differently when it’s moved depending on the shape of the container, or in the same way a drum’s vibration is affected by the shape of its edge, the ocean’s up and down movement forms complicated patterns that ultimately derive from the shape of coastlines.

You can see those patterns [here](https://en.wikipedia.org/wiki/Tide#/media/File:M2_tidal_constituent.jpg) – the points where the lines meet in blue regions are the spots in the up and down “vibration” that stay still, while the red regions have very large tides. You can see a few patterns: tides often ‘rotate’ around intervening land (see [this video](https://upload.wikimedia.org/wikipedia/commons/f/f6/Global_surface_elevation_of_M2_ocean_tide.webm)), mostly have high and low points in each large ocean basin (since there’s not enough time for major flow between oceans in a tidal cycle), tend to be highest in areas where the water is tightly constrained by a curved coastline (e.g. between Madagascar and Africa or near the English Channel), and rotate in opposite directions around the stable points.

There are other reasons, too. There are multiple contributing pulls (primarily Moon vs Sun) on the Earth that don’t have the same frequency. The rotation of the Earth adds effects that make currents flow (the Coriolis force). Even in an open ocean, water takes some time to flow, and won’t make a full lap around the world in a day. The ocean floor isn’t uniformly deep, which interferes with the flow and adds the shape of the seafloor to the equation.

In general the theory of tides is quite complex, with the map above accounting for only about half of the total tidal range at any given place; the full story requires reasonably advanced fluid dynamics and partial differential equations.

There are a variety of factors that all play roles. Gravity if nearby celestial bodies The axis of the Earth is at about 23° to the sun, so the Earths poles point towards the sun at during the summer and winter, the earths orbit around the sun is elliptical not perfectly round. The moon is on a 27 day orbit around the earth. These factors are predictable and tug at the water of the ocean slightly differently every day. The unpredictable factor is the weather. Wind makes waves that shoves water through inlets and precipitation events from large drainages raise near shore tidal estuaries.
These are some of easy to explain reasons that tides are not uniform.