Ocean & Coastal Phenomena
Tides are the regular rise and fall of sea level caused mainly by the gravitational pull of the Moon and the Sun.
They shape coastlines, expose tidal flats, drive powerful currents, flood low-lying shores and sometimes produce
dramatic waves known as tidal bores.
What Are Tides?
Tides are long-period ocean waves that move around the planet as sea level rises and falls. Unlike wind waves,
which are created by local weather, tides are controlled by astronomy: the Moon, the Sun, Earth’s rotation and
the shape of ocean basins and coastlines.
Most coastal areas experience two high tides and two low tides each lunar day, but the exact pattern depends on
local geography. Some places have one main high tide per day, while others have mixed tides with unequal highs
and lows.
How Do Tides Work?
The Moon’s gravity pulls ocean water toward it, creating a tidal bulge on the side of Earth facing the Moon.
A second bulge forms on the opposite side because of inertia within the Earth-Moon system. As Earth rotates
through these bulges, coastlines move from high tide to low tide and back again.
The Sun also affects tides, although its tidal influence is weaker than the Moon’s because it is much farther
away. When the Sun, Moon and Earth line up, tides become stronger. When the Sun and Moon pull at right angles,
tidal range becomes smaller.
Tides Explained: Main Types and Concepts
These child pillars break tides into the key ideas people search for most: why some tides are higher, why others
are weaker, how the Moon and Sun control the ocean, and why some rivers suddenly form powerful upstream waves.
Spring Tides Explained
Spring tides are the highest high tides and lowest low tides of the regular tidal cycle. They happen when the
Sun, Moon and Earth line up during the new moon and full moon phases.
Neap Tides Explained
Neap tides occur when the Sun and Moon pull at right angles to each other. The result is a smaller tidal range:
lower high tides and higher low tides.
Tidal Forces Explained
Tidal forces are differences in gravitational pull across Earth. They deform the oceans, influence the solid
Earth and help explain why tides are not just simple water movement.
Lunar vs. Solar Tides Explained
The Moon is the main driver of tides, but the Sun modifies them. This page explains why lunar tides dominate,
how solar tides reinforce them, and why alignment matters.
Tidal Cycles Explained
Tidal cycles include daily high and low tides, the lunar day, spring-neap cycles, monthly patterns and longer
tidal variations caused by orbital geometry.
Tidal Bores Explained
Tidal bores are waves that surge upstream in rivers and estuaries when a strong incoming tide is funneled
into a narrow, shallow channel.
Why Tides Matter for Coastal Hazards
Tides are not dangerous by themselves, but they can amplify coastal hazards. A storm arriving at high tide can
push water farther inland. Spring tides can worsen coastal flooding. Strong tidal currents can trap swimmers,
move boats, expose mudflats and rapidly cut off access to beaches, islands and causeways.
Tides also interact with other ocean phenomena, including
coastal erosion,
extreme ocean waves,
tsunamis and
ocean currents.
Tides vs. Waves vs. Tsunamis
Tides are caused by gravitational forces and follow predictable astronomical cycles. Wind waves are created by
wind blowing over water. Tsunamis are usually triggered by sudden seafloor movement, landslides or volcanic
activity. Although all three affect sea level, they are produced by very different mechanisms.
Where Tides Become Extreme
Some of the world’s largest tidal ranges occur where coastal geography funnels and amplifies tidal water. Bays,
estuaries and narrow channels can dramatically increase the difference between high and low tide. In these places,
tidal currents can become powerful enough to resemble fast rivers.
FAQ: Tides Explained
What causes tides?
Tides are caused mainly by the Moon’s gravity, with additional influence from the Sun. Earth’s rotation then
carries coastlines through tidal bulges, producing high and low tides.
Why are some tides higher than others?
Tides become higher when the Sun, Moon and Earth align during new moon and full moon phases. These are called
spring tides.
What is the difference between spring tides and neap tides?
Spring tides have the largest tidal range. Neap tides have the smallest tidal range and occur when the Sun and
Moon pull at right angles.
Are tides the same everywhere?
No. Local coastline shape, seafloor depth, ocean basin geometry and latitude all affect the timing and height
of tides.
Can tides cause flooding?
Yes. High tides, especially spring tides or king tides, can worsen coastal flooding when combined with storms,
low pressure, strong winds or sea-level rise.
What is a tidal bore?
A tidal bore is a wave that travels upstream in a river or estuary when a strong incoming tide is forced into
a narrowing channel.
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