tides, currents, and leeway 99
strong tidal currents. Along other coastlines, local effects produce only one
high and low tide per day (a diurnal tide), or a mixed tide, in which one high
water is much higher than the other.
Here are the tide terms you’ll want to know:
Tide: Th e vertical rise and fall of water in coastal regions throughout the
world.
Semidiurnal tides: Most of the world, including the U.S. east coast and Europe,
experiences two high tides and two low tides each day.
Diurnal tide: Some places, including the U.S. Gulf Coast and Southeast
Asia, experience only one high tide and one low tide every day.
Mixed tides: Places along the U.S. and Canadian west coasts receive multiple
tides each day—with a high tide followed by a higher high tide, and a
similar pattern for low tides.
Stand: Th e tidal height “pauses” aft er reaching its highest or lowest point. Th is
high-water or low-water stand may last a few seconds or several minutes,
then the tide begins to move down or up.
Spring tides: At or near the time of a full or new moon, the moon is aligned
with the sun, and the gravitational pulls of the two bodies are additive. Th e
result is that tidal ranges (the distance a tide rises and falls) increase. High-
water heights average higher than normal. Low-water heights average lower
than normal.
Neap tides: At or near the times of the moon’s fi rst and third quarters, the
moon’s gravitational pull on the earth is exerted more or less at right
angles with the sun’s, and the sun’s pull partially cancels the moon’s. Th e
result is that tidal ranges decrease. High-water heights average lower
than normal. Low-water heights average higher than normal.
Charted depth: Th e charted depth is the “starting depth,” or low-water datum,
for the measurement of tides. (Low-water datum in the U.S. is mean lower
low water, or MLLW, the long-term average of the lower of each day’s low
tides.) Tide tables and publications show how much height (in feet or
meters) to add to or subtract from the charted depth to correct for the stage
of the tide at a specifi ed time.
Range: Th e total distance the tide rises or falls from high tide to low tide. If
the low-water height is below low-water datum (called a minus tide), add
low-water height to high-water height to fi nd the range. (For example, if
the high-water height as read from a tide table is 10.3 feet and the low-
water height is −0.7 feet, the tidal range is 11.0 feet.) If the low-water
height is above low-water datum, subtract it from the high-water height
to fi nd the range.