2008 REED’s East Coast Edition - Tides
The Bermuda and Bahama tide tables sonal variation, representing average freshet
marked Chart Datum are less definitive. NOS and drought conditions. Unusual freshets or
says on the subject: “For foreign coasts a da- droughts, however, will cause the tides to be
tum approximating to mean low water springs higher or lower, respectively, than predicted.
(MLWS), Indian spring low water, or the low- Number of tides: At locations with semidi-
est possible low water is generally used.” The urnal or mixed tide curves, there are usually
tide datums are supposed to match the local two high and two low waters in a day. Tides
charts for these waters, but often there are re- follow the moon more closely than they do
dundant charts originating in several different the sun, and the lunar or tidal day is about 50
countries, and they may use different chart minutes longer than the solar day. This causes
datums. For maximum accuracy, you should the tide to occur later each day, and a tide that
check the datum used on your chart and then has occurred near the end of one calendar day
scan the low-tide heights in the table you are will be followed by a corresponding tide that
using to approximate its datum. Tide tables may skip the next day and occur in the early
(like U.S.) with numerous tides less than 0 ft morning of the third day. Thus, on certain
per month use a MLLW-type datum. Tables days of each month, only a single high or a
with almost no minus tides per month use a single low water occurs.
datum like MLWS; and tables with no minus Relation of tide to current: In using these
tides probably use Lowest Astronomical Tide tide prediction tables, bear in mind that they
(LAT) or LLWLT. The Bermuda and Bahama give the times and heights of high and low
tide tables appear to use the LAT datum fa- waters and not the times of turning of the
vored by the British. current or slack water. For stations on the
Variation in sea level: Changes in winds outer coast, there is usually a small difference
and barometric conditions cause variations between the time of high or low water and
in sea level from day to day. In general, with the beginning of ebb or flood current; but for
onshore winds or a low barometer, the heights places in narrow channels, landlocked har-
of both the high and low waters will be higher bors, or on tidal rivers, the time of slack water
than predicted; while with offshore winds or a may differ by several hours from the time of
high barometer, they will be lower. There are high-or low-water stand. The relation of the
also seasonal variations in sea level, but these times of high and low water to the turning of
variations have been included in the predic- the current depends upon a number of fac-
tions for each station. At ocean stations, the tors, so no simple or general rule can be given.
seasonal variation in sea level is usually less See Currents chapter for more information.
than half a foot. At stations on tidal rivers, the Sunrise and sunset: The small table at the
average seasonal variation in river level due bottom left-hand corner of each monthly col-
to freshets and droughts may be considerably umn gives the times of sunrise and sunset for
more than a foot. The predictions for these the 15th and the last day of the month. Inter-
stations include an allowance for this sea- mediate times may be easily interpolated.
Height of the tide at any time
There are several methods to estimate the mixed or especially diurnal curves. See the
height of tide for a given time between pre- beginning of this chapter for a fuller explana-
dicted highs and lows. On these pages, we tion of tide types.
present the Rule of Twelfths, NOS’s Table 3,
The Rule of Twelfths
and a graphic solution.
Caution: All of the methods presented are
A tide rises or falls (approximately)
based on the assumption that the rise and fall
conform to simple cosine curves. The heights
1/12 of its range during the 1st hour
obtained will be approximate. The roughness
2/12 of its range during the 2nd hour
of approximation will vary as the tide curve
3/12 of its range during the 3rd hour
differs from a cosine curve. Semidiurnal tide
3/12 of its range during the 4th hour
curves tend to be closer to cosine curves than
2/12 of its range during the 5th hour
1/12 of its range during the 6th hour
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