Tides Explained
Tidal cycles are the repeating patterns of high tide and low tide caused by the Moon, the Sun and Earth’s rotation.
They explain why tides shift each day, why some tides are stronger than others, and why coastal water levels follow
predictable rhythms.
What Are Tidal Cycles?
Tidal cycles are the regular rise and fall of sea level over time. They are driven mainly by the Moon’s gravity,
modified by the Sun’s gravity and shaped locally by coastlines, seafloor depth, bays, estuaries and ocean basins.
The most familiar tidal cycle is the daily movement from high tide to low tide and back again. But tides also
follow longer cycles linked to the Moon’s orbit, lunar phases and the alignment of the Sun, Moon and Earth.
The Daily Tide Cycle: High Tide and Low Tide
Many coastlines experience two high tides and two low tides during a tidal day. This happens because
tidal forces
create two main tidal bulges around Earth: one facing the Moon and one on the opposite side.
As Earth rotates through these bulges, sea level rises toward high tide, then falls toward low tide. The exact
timing and height of each tide depend heavily on local geography.
What Is a Lunar Day?
A lunar day is the time it takes for a place on Earth to rotate back under the Moon. It is slightly longer than
a solar day because the Moon moves along its orbit while Earth rotates.
This is why tides usually occur later each day. In many places, high tide shifts by roughly 50 minutes from one
day to the next, although local tidal conditions can change the exact timing.
The Spring-Neap Tidal Cycle
Tides also change through the lunar month. The strongest and weakest tidal ranges are controlled by the changing
alignment of the Sun, Moon and Earth.
-
Spring tides:
occur near new moon and full moon, when the Sun, Moon and Earth align. They produce the largest tidal range. -
Neap tides:
occur near first quarter and third quarter moon, when the Sun and Moon pull at right angles. They produce the
smallest tidal range.
This spring-neap rhythm repeats about twice each lunar month and is one of the most important tidal cycles for
coastal flooding, boating, fishing, mudflat exposure and tidal currents.
Types of Daily Tide Patterns
Not all coastlines experience tides in the same way. Depending on local geography and ocean basin shape, daily
tide patterns can look very different.
- Semidiurnal tides: two high tides and two low tides of roughly similar height each tidal day.
- Diurnal tides: one high tide and one low tide each tidal day.
- Mixed tides: two high tides and two low tides with noticeably different heights.
Why Tide Times Change Every Day
Tide times change because the Moon is orbiting Earth while Earth rotates. A coastline has to rotate a little
farther each day to line up with the Moon again. This daily delay is why tide tables are necessary instead of
using the same high-tide time every day.
Monthly and Longer Tidal Patterns
Tides also vary over longer periods. The Moon’s changing distance from Earth, the tilt of its orbit and seasonal
solar effects can all modify tidal range. These longer cycles help explain unusually high predicted tides, often
called king tides in public forecasts.
Why Tidal Cycles Matter
Understanding tidal cycles helps explain coastal flooding risk, beach access, harbor operations, navigation,
fishing windows, exposed reefs, mudflats, tidal currents and
tidal bores.
Tidal cycles are also essential for understanding how tides interact with
coastal erosion,
ocean currents and
extreme ocean waves.
FAQ: Tidal Cycles Explained
What is a tidal cycle?
A tidal cycle is the repeating rise and fall of sea level from high tide to low tide and back again.
How many tides happen each day?
Many coastlines have two high tides and two low tides each tidal day, but some areas have one high tide and
one low tide or mixed tides with unequal heights.
Why do tide times change every day?
Tide times change because the Moon moves in its orbit while Earth rotates, making the lunar day slightly longer
than the solar day.
What is the spring-neap tidal cycle?
The spring-neap cycle is the regular shift between large tidal ranges during spring tides and small tidal ranges
during neap tides.
What are semidiurnal tides?
Semidiurnal tides are tide patterns with two high tides and two low tides of roughly similar height during each
tidal day.
What are mixed tides?
Mixed tides have two high tides and two low tides each tidal day, but the two highs or two lows are noticeably
different in height.
