I vacation for a week every summer at my aunt’s place on Prince Edward Island. I like getting out on the sandbars to dig clams, so I time my visits to coincide (as best as possible) with either a full moon or a new moon; I’ve found that the tides are lower around those times which gives me more time and access to more of the bar. Her home is not very far from Wood Islands (the ferry terminal). She told me that the rough rule of thumb for estimating tides in that area was that the low tide at the full (or new) moon was at 5pm, and would advance by 45 or 50 minutes or so every day.
The first year I started clamming it was a bit stormy the first night I was going to go out, and the rain, wind and waves made it a no go. “Fine”, I thought, “I’ll go out tomorrow morning.” I set my alarm and got up before 5am to see that the low tide hardly seemed to have receded at all. I talked to my aunt about this, and she confirmed that the low tide around there only exposes the sand bars in the evening, and never in the morning.
I checked a tide chart at this site, and sure enough, it seems to be the case. I’ll bold the low tides:
So, low tide shown for the morning is 0.9 meters, but in the evening is between 0.02 to 0.05 meters, a difference of almost 3 feet. I know that there are variances between high and low tides, and I know that there are variances that correlate with lunar cycles, but I’d never known that there would be that much difference between a morning low tide and an evening low tide.
I’ll take a stab at this, but explaining tides is notoriously difficult, and I will likely screw something up…, so take this for what it’s worth (not much)
If tides were perfectly semi-diurnal, the two low tides in the day would be the same height. When analysing the tides, one also has to take into account other constituents of the tide which are daily (diurnal) and longer (monthly, annually) When you add all of the phases together, you get a mixed semi-diurnal tide with a “high” low tide and a “low” low tide. We’re more familiar with mixed semi-diurnal tides on the Pacific coast of N. America.
I’m probably not explaining it well, so here’s a NOAA site that does a better job…
Far from it. And it gives me a start on how to do some further research on this, so I am very grateful. It appears to be a lot more common than I’d thought. But in thinking about it, I may have never paid that much attention at other areas when I’ve been to the beach, and a smaller discrepancy would have escaped my notice.