This started initially with an observation that the summer highest tides seemed to be always right around lunchtime, while the winter highest tides were around midnight.
Here in Seattle we have a 16 foot daily tidal range -- large enough to be very noticeable. We are on a fjord called The Salish Sea or Puget Sound; it is teaming with life: at low tide it is fascinating to wander the intertidal zone and see the crabs, mollusks, fish, plants and even octopi that live at the edge of the underwater world. A biologist friend and i often gave guided tide tours to travelers at the lowest tides, so i was alert to what time these tides happen.
The extremes happen during the 2 or 3 full or new moons closest to the solstice. That means that once in May, once in June and once in July. And also once in November, once in December and once in January.
The lowest and highest tides happen on the same day! It's pretty amazing to walk a couple hundred meters from 'shore' on the sand and rocks, knowing that in just 6 hours the water will be three times my height deep where i am! The entire fjord fills up really fast. That's a lot of water.
I tend to think of the highest high tides being in the winter and the lowest low tides being in the summer, but that may have more to do with what i see in daylight. And THAT gets me to the question i had...
Tides vary significantly from tide height to tide height and from day to day, month to month. A detailed tide table, made using very complex math, is required to predict a tide for any day.
There are four tides a day, a high high and a lower high, a low low and a higher low, although because the cycle is not exactly timed to 24 hours (i think it's 28 hours) every few days one tide is missed. The tide tables are sine waves, showing the tide rising a falling.
Every year when we would plan our low tide walks, we would consult the tide tables to find out at what time the lowest tides each month would be. And after about 10 years i noticed that those super low tides in May, June & July were always right around mid-day.
So i wondered, is that a rule of the system? Is it just a coincidence of Seattle's location (47 degrees latitude), of era (1990s & 2000s), or is there something that governed that.
Tides are caused by a gravity interplay of the three body system of the sun, the moon and the earth. They effect our bodies and the rocks and mountains, but in such tiny amounts that it is not noticeable. However water is both fluid and heavy and there's a lot of it. The moon's pull, the sun's pull and the force of the earth swinging around the sun sloshes the oceans around, keeping big bumps of water underneath the moon and the sun, and opposite the sun (from the swing).
Tide is also effected by latitude (because the earth is tilted, tides are smaller near the equator) and by the depth and shape of various inlets.
I have several good tide prediction websites that i regularly use, notablyNOAA and the WWW Tide Predictor at the University of South Carolina. I
Initially i got data from both places, but i wanted to be consistent and i really like the US National Oceanic an Atmospheric Administration, so i used only data from there. I picked 28 sites around the world. I tried to get a good spread, but i can't say this is random in any way; i leaned towards places i had visited or knew of and i only picked 28 data points.
I used the tides data for two years, from both 2010 and 2011 superimposed on each other to get fuller and more consistent data. That's a couple thousand tides (about 4 per day 730 days) for each site! I graphed only the most extreme tides, and color coded them according to season.
The graphs helped me to answer my question. This phenomenon is pretty much world wide, it's not unique to Seattle. The highest highs in the winter and summer are about 12 hours apart, and the lowest lows the same. Some places it's not noon and midnight, but the pattern is the same. (Studying the physics of tidal motion further i was able to determine those super low tides in the summer will in fact always be around mid-day, and the super-highs in the winter around mid-day.)
I was finished with the graphs, but i found that the images were so beautiful i wanted to keep them.
Here they are, arranged from south to north, regardless of longitude. I hid the spring and fall and only plotted the summer (pink) and winter (blue) tides. You'll notice a solid line through each graph, that's the mean low tide line, or zero. The scale changes, often enormously, from graph to graph. I am not trying to compare the amplitude of tides from place to place, only the patterns, so i chose a different scale for each graph so they are all comparable in size.
