only shores

How a low-tide window is worked out

Every number on Only Shores comes from published predictions or a published formula. Nothing is guessed, and no AI is involved.

1. Tide predictions from NOAA

The tide times and heights are NOAA's own predictions from its Center for Operational Oceanographic Products and Services (CO-OPS), fetched from its public data service for each station: every predicted high and low from yesterday to about a year ahead. Heights are in feet relative to Mean Lower Low Water (MLLW), which NOAA defines as "the average of the lower low water height of each tidal day observed over the National Tidal Datum Epoch". A low below 0.0 ft is lower than that average; Only Shores calls it a minus tide.

Predictions are refreshed at least every 20 days. If NOAA does not answer, the site uses the predictions it already has and says when they were fetched.

NOAA is clear that predictions cover the astronomical tide only: "We cannot predict the effect that wind, rain, freshwater runoff, and other short-term meteorological events" have on the water, and such events can "delay or accelerate the arrival of the predicted tides by an hour or more". Always look at the water and the weather when you get there.

2. Sunrise and sunset, computed

Sunrise and sunset are calculated in code for each station's latitude and longitude with the solar equations published by NOAA's Global Monitoring Laboratory (after Jean Meeus, Astronomical Algorithms), to the top of the sun's disc with standard refraction. They agree with the U.S. Naval Observatory to about a minute. In the far north, days when the sun never sets or never rises are shown as such.

3. Grades

A low that is good at one shore can be ordinary at another, so every grade is measured against that shore's own year of predicted lows:

Among the year's lowest
At or below the 10th percentile of all the predicted lows at this shore over the fetched year (the lowest tenth). At many shores these are minus tides too; the height shows it.
Minus tide
The predicted low is below 0.0 ft MLLW but not in the lowest tenth.
Good low
At or below the 33rd percentile (the lowest third).
Ordinary low
Everything else.

Heights are compared as they are shown, to a tenth of a foot, so a low shown as 0.0 ft is never called a minus tide.

4. The walking window

Acadia National Park's guidance is that tidepooling works "between the window of time 1.5 hours before to 1.5 hours after low tide", and that "it is best to start tidepooling before low tide and to leave as the tide is rising". Only Shores is a little more cautious: its window runs from 90 minutes before the low to 60 minutes after it, so you head back while the water is still low. That is this site's own rule.

Only a low that falls between sunrise and sunset gets a window, and the window is then cut to daylight: it never starts before sunrise or runs past sunset. A low before sunrise or after sunset gets no window and is shown as "in the dark", even when there is light before it, because the walk back would be in the dusk. A low is never picked as a best walk without a window. When a window ends at sunset, plan to be back on dry land by then.

5. Best walk

Among lows that have a window, the best walk is the one with the highest grade; between equal grades, the lower water; between equal heights, the sooner.

6. The tide curve

The day page draws the tide between NOAA's predicted highs and lows with a cosine curve, the usual way to sketch a tide from its turning points. It is an approximation for seeing the shape of the day; the times and heights of the highs and lows themselves are NOAA's.

Time zones

All times are converted from UTC to the clock used at the station, daylight saving included, for US states and territories. A few places NOAA lists outside the US are shown at a fixed offset from UTC, and the page says so.

Sources