NOAA National Ocean Service Education: Tides and Water Levels
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Welcome to Tides and Water Levels
NOAA's National Ocean Service collects,
studies and provides access to thousands of
historical and real-time observations as well as
predictions of water levels, coastal currents
and other data. Maritime activities throughout
the world depend on accurate tidal and current
information for safe operation.
In this subject, you will find three sections
devoted to learning about tides and water
levels: an online tutorial, an educational
roadmap to resources, and formal lesson plans.
The Tides and Waters Levels Tutorial is an
overview of the complex systems that govern
the movement of tides and water levels. The
tutorial is content rich and presented in easy-
to-understand language. It is made up of 11
"chapters" or pages (plus a reference page)
that can be read in sequence by clicking on the
arrows at the top or bottom of each chapter page. The tutorial includes many illustrative
and interactive graphics to visually enhance the text.
The Roadmap to Resources complements the information in the tutorial. The roadmap
directs you to specific tidal and current data offered within the NOS and NOAA family of
products.
The Lesson Plans integrate information presented in the tutorial with data offerings from
the roadmap. These lesson plans have been developed for students in grades 9–12 and
focus on the forces that cause and effect tides, analysis of the variations in tidal patterns
and what conditions may cause them, and the effect of lunar cycles on living organisms.
The National Science Teachers Association (NSTA) has included this
online resource in its SciLinks database.
SciLinks provide students and teachers access to Web-based,
educationally appropriate science content that has been formally
evaluated by master teachers.
For more information about the SciLinks evaluation criteria, click here:
http://www.scilinks.org/certificate.asp.
To go directly to the SciLinks log-on page, click here:
http://www.scilinks.org/.
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This site NOAA
Tides Roadmap
Tides Lesson Plans
Welcome
What are Tides?
What Causes Tides?
Gravity, Inertia, and
Bulges
Changing Angles and
Tides
The Frequency of Tides
Tidal Variations
Types and Causes of
Tidal Cycles
What Else Affects Tides?
Monitoring the Tides
How are Tides
Measured ? Pt. I
How are Tides
Measured? Pt. II
References
The rise and fall of the tides play an important
role in the natural world and can have a
marked effect on maritime-related activities.
Here, a ship's crew inspects the hull of their
vessel. It became stranded on a sandbar
following a rapidly receding tide.
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NOAA National Ocean Service Education: Tides and Water Levels
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Tides are one of the most reliable phenomena
in the world. As the sun rises in the east and
the stars come out at night, we are confident
that the ocean waters will regularly rise and
fall along our shores. The following pages
describe the tremendous forces that cause the
world’s tides, and why it is important for us to
understand how they work.
Basically, tides are very long-period waves that
move through the oceans in response to the
forces exerted by the moon and sun. Tides
originate in the oceans and progress toward
the coastlines where they appear as the
regular rise and fall of the sea surface. When
the highest part, or crest of the wave reaches
a particular location, high tide occurs; low tide
corresponds to the lowest part of the wave, or
its trough. The difference in height between
the high tide and the low tide is called the tidal range.
A horizontal movement of water often accompanies the rising and falling of the tide. This
is called the tidal current. The incoming tide along the coast and into the bays and
estuaries is called a flood current; the outgoing tide is called an ebb current. The
strongest flood and ebb currents usually occur before or near the time of the high and
low tides. The weakest currents occur between the flood and ebb currents and are called
slack tides. In the open ocean tidal currents are relatively weak. Near estuary entrances,
narrow straits and inlets, the speed of tidal currents can reach up to several kilometers
per hour (Ross, D.A., 1995).
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This site NOAA
Tides Roadmap
Tides Lesson Plans
Welcome
What are Tides?
What Causes Tides?
Gravity, Inertia, and
Bulges
Changing Angles and
Tides
The Frequency of Tides
Tidal Variations
Types and Causes of
Tidal Cycles
What Else Affects Tides?
Monitoring the Tides
How are Tides
Measured ? Pt. I
How are Tides
Measured? Pt. II
References
Tides and Water Levels
What are Tides?
As the tides rise and fall, they create flood and
ebb currents. Click the image for an animated
view.
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NOAA National Ocean Service Education: Tides and Water Levels
The relationship between the masses of the
Earth, moon and sun and their distances to
each other play critical roles in affecting tides.
Click the image for a larger view.
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Tides and Water Levels
What Causes Tides?
Gravity is one major force that
creates tides. In 1687, Sir Isaac
Newton explained that ocean tides
result from the gravitational attraction
of the sun and moon on the oceans of
the earth (Sumich, J.L., 1996).
Newton’s law of universal gravitation
states that the gravitational attraction
between two bodies is directly
proportional to their masses, and
inversely proportional to the square of
the distance between the bodies
(Sumich, J.L., 1996; Thurman, H.V.,
1994). Therefore, the greater the
mass of the objects and the closer
they are to each other, the greater
the gravitational attraction between
them (Ross, D.A. 1995).
Tidal forces are based on the gravitational attractive force. With regard to tidal
forces on the Earth, the distance between two objects usually is more critical
than their masses. Tidal generating forces vary inversely as the cube of the
distance from the tide generating object. Gravitational attractive forces only
vary inversely to the square of the distance between the objects (Thurman, H.
V., 1994). The effect of distance on tidal forces is seen in the relationship
between the sun, the moon, and the Earth’s waters.
Our sun is 27 million times larger than our moon. Based on its mass, the sun's
gravitational attraction to the Earth is more than 177 times greater than that of
the moon to the Earth. If tidal forces were based solely on comparative
masses, the sun should have a tide-generating force that is 27 million times
greater than that of the moon. However, the sun is 390 times further from the
Earth than is the moon. Thus, its tide-generating force is reduced by 390
3
, or
about 59 million times less than the moon. Because of these conditions, the
sun’s tide-generating force is about half that of the moon (Thurman, H.V.,
1994).
This site NOAA
Tides Roadmap
Tides Lesson Plans
Welcome
What are Tides?
What Causes Tides?
Gravity, Inertia, and
Bulges
Changing Angles and
Tides
The Frequency of Tides
Types and Causes of
Tidal Variations
Tidal Cycles
What Else Affects Tides?
Monitoring the Tides
How are Tides
Measured ? Pt. I
How are Tides
Measured? Pt. II
References
(top)
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NOAA's National Ocean Service: Diagram of distances between earth, sun and moon
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The relationship between the masses of the Earth, moon and sun and their
distances to each other play a critical role in affecting the Earth's tides. Although
the sun is 27 million times more massive than the moon, it is 390 times further
away from the Earth than the moon. Tidal generating forces vary inversely as the
cube of the distance from the tide-generating object. This means that the sun’s
tidal generating force is reduced by 390
3
(about 59 million times) compared to the
tide-generating force of the moon. Therefore, the sun’s tide-generating force is
about half that of the moon, and the moon is the dominant force affecting the
Earth’s tides.
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NOAA National Ocean Service Education: Tides and Water Levels
Two tidal bulges are created on opposite sides
of the Earth due to the moon's gravitational
force and inertias counterbalance. Click the
image for a larger view.
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Tides and Water Levels
Gravity, Inertia, and the Two Bulges
Gravity is a major force responsible
for creating tides. Inertia, acts to
counterbalance gravity. It is the
tendency of moving objects to
continue moving in a straight line.
Together, gravity and inertia are
responsible for the creation of two
major tidal bulges on the Earth
(Ross, D.A., 1995).
The gravitational attraction between
the Earth and the moon is strongest
on the side of the Earth that
happens to be facing the moon,
simply because it is closer. This
attraction causes the water on this “near side” of Earth to be pulled toward
the moon. As gravitational force acts to draw the water closer to the moon,
inertia attempts to keep the water in place. But the gravitational force
exceeds it and the water is pulled toward the moon, causing a “bulge” of
water on the near side toward the moon (Ross, D.A., 1995).
On the opposite side of the Earth, or the “far side,” the gravitational attraction
of the moon is less because it is farther away. Here, inertia exceeds the
gravitational force, and the water tries to keep going in a straight line, moving
away from the Earth, also forming a bulge (Ross, D.A., 1995).
In this way the combination of gravity and inertia create two bulges of water.
One forms where the Earth and moon are closest, and the other forms where
they are furthest apart. Over the rest of the globe gravity and inertia are in
relative balance. Because water is fluid, the two bulges stay aligned with the
moon as the Earth rotates (Ross, D.A., 1995).
The sun also plays a major role, affecting the size and position of the two tidal
bulges. The interaction of the forces generated by the moon and the sun can
be quite complex. As this is an introduction to the subject of tides and water
levels we will focus most of our attention on the effects of the stronger
celestial influence, the moon.
This site NOAA
Tides Roadmap
Tides Lesson Plans
Welcome
What are Tides?
What Causes Tides?
Gravity, Inertia, and
Bulges
Changing Angles and
Tides
The Frequency of Tides
Tidal Variations
Types and Causes of
Tidal Cycles
What Else Affects Tides?
Monitoring the Tides
How are Tides
Measured ? Pt. I
How are Tides
Measured? Pt. II
References
(top)
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NOAA's National Ocean Service: Diagram of tidal bulges
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Gravity and inertia act in opposition on the Earth’s oceans, creating tidal bulges on
opposite sites of the planet. On the “near” side of the Earth (the side facing the
moon), the gravitational force of the moon pulls the ocean’s waters toward it,
creating one bulge. On the far side of the Earth, inertia dominates, creating a
second bulge.
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NOAA National Ocean Service Education: Tides and Water Levels
The Earth’s tidal bulges track, or follow, the
position of the moon and to a lesser extent,
the sun. As these two celestial bodies increase
and decrease their angles to the Earth, so do
the tidal bulges. Click the image for an
animated view.
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Tides and Water Levels
Changing Angles and Changing Tides
As we’ve just seen, the Earth's two
tidal bulges are aligned with the
positions of the moon and the sun.
Over time, the positions of these
celestial bodies change relative to the
Earth’s equator. The changes in their
relative positions have a direct effect
on daily tidal heights and tidal current
intensity.
As the moon revolves around the
Earth, its angle increases and
decreases in relation to the equator.
This is known as its declination. The
two tidal bulges track the changes in
lunar declination, also increasing or
decreasing their angles to the
equator. Similarly, the sun’s relative
position to the equator changes over
the course of a year as the Earth
rotates around it. The sun’s
declination affects the seasons as well
as the tides. During the vernal and
autumnal equinoxes—March 21 and September 23, respectively—the sun is at
its minimum declination because it is positioned directly above the equator. On
June 21 and December 22—the summer and winter solstices, respectively—the
sun is at its maximum declination, i.e., its largest angle to the equator
(Sumich, J.L., 1996).
This site NOAA
Tides Roadmap
Tides Lesson Plans
Welcome
What are Tides?
What Causes Tides?
Gravity, Inertia, and
Bulges
Changing Angles and
Tides
The Frequency of Tides
Tidal Variations
Types and Causes of
Tidal Cycles
What Else Affects Tides?
Monitoring the Tides
How are Tides
Measured ? Pt. I
How are Tides
Measured? Pt. II
References
(top)
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http://oceanservice.noaa.gov/education/kits/tides/tides04_angle.html
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NOAA's National Ocean Service: Animation of earth's declination
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The Earth’s tidal bulges track, or follow, the position of the moon, and to a lesser
extent, the sun. As the angles of these two celestial bodies in relation to the Earth
increase and decrease, so do the tidal bulges. Here we observe the moon's
changing declination to the equator and the effect that this has on the positions of
the Earth’s tidal bulges.
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NOAA National Ocean Service Education: Tides and Water Levels
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Tides and Water Levels
Frequency of Tides - The Lunar Day
Unlike a 24-hour solar day, a lunar day lasts
24 hours and 50 minutes. This occurs because
the moon revolves around the Earth in the
same direction that the Earth rotates around
its axis. Click the image to see an animation.
Most coastal areas, with some
exceptions, experience two high tides
and two low tides every lunar day
(Ross, D.A., 1995). Almost everyone
is familiar with the concept of a 24-
hour solar day, which is the time that
it takes for a specific site on the Earth
to rotate from an exact point under
the sun to the same point under the
sun. Similarly, a lunar day is the time
it takes for a specific site on the Earth
to rotate from an exact point under
the moon to the same point under the
moon. Unlike a solar day, however, a
lunar day is 24 hours and 50 minutes.
The lunar day is 50 minutes longer
than a solar day because the moon
revolves around the Earth in the same
direction that the Earth rotates
around its axis. So, it takes the Earth
an extra 50 minutes to “catch up” to
the moon (Sumich, J.L., 1996;
Thurman, H.V., 1994).
Because the Earth rotates through two tidal “bulges” every lunar day, coastal
areas experience two high and two low tides every 24 hours and 50 minutes.
High tides occur 12 hours and 25 minutes apart. It takes six hours and 12.5
minutes for the water at the shore to go from high to low, or from low to high.
This site NOAA
Tides Roadmap
Tides Lesson Plans
Welcome
What are Tides?
What Causes Tides?
Gravity, Inertia, and
Bulges
Changing Angles and
Tides
The Frequency of
Tides
Tidal Variations
Types and Causes of
Tidal Cycles
What Else Affects Tides?
Monitoring the Tides
How are Tides
Measured ? Pt. I
How are Tides
Measured? Pt. II
References
(top)
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http://oceanservice.noaa.gov/education/kits/tides/tides05_lunarday.html
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NOAA's National Ocean Service: Animation of lunar day
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Unlike a 24-hour solar day, a lunar day lasts 24 hours and 50 minutes. This occurs
because the moon revolves around the Earth in the same direction that the Earth
is rotating on its axis. Therefore, it takes the Earth an extra 50 minutes to “catch
up” to the moon. Since the Earth rotates through two tidal “bulges” every lunar
day, we experience two high and two low tides every 24 hours and 50 minutes.
Here, we see the relationship between the tidal cycle and the lunar day. High tides
occur 12 hours and 25 minutes apart, taking six hours and 12.5 minutes for the
water at the shore to go from high to low, and then from low to high.
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NOAA National Ocean Service Education: Tides and Water Levels
NOS home NOS education home site index
This site NOAA
Tides Roadmap
Tides Lesson Plans
Welcome
What are Tides?
What Causes Tides?
Gravity, Inertia, and
Bulges
Changing Angles and
Tides
The Frequency of Tides
Tidal Variations
Types and Causes of
Tidal Cycles
What Else Affects Tides?
Monitoring the Tides
How are Tides
Measured ? Pt. I
How are Tides
Measured? Pt. II
References
Tides and Water Levels
Tidal Variations - The Influence of Position and
Distance
The moon is a major influence on the
Earth’s tides, but the sun also
generates considerable tidal forces.
Solar tides are about half as large as
lunar tides and are expressed as a
variation of lunar tidal patterns, not
as a separate set of tides. When the
sun, moon, and Earth are in
alignment (at the time of the new or
full moon), the solar tide has an
additive effect on the lunar tide,
creating extra-high high tides, and
very low, low tides—both commonly
called spring tides. One week later,
when the sun and moon are at right
angles to each other, the solar tide
partially cancels out the lunar tide
and produces moderate tides known
as neap tides. During each lunar
month, two sets of spring tides and two sets of neap tides occur (Sumich, J.L.,
1996).
Just as the angles of the sun, moon
and Earth affect tidal heights over the
course of a lunar month, so do their
distances to one another. Because the
moon follows an elliptical path around
the Earth, the distance between them
varies by about 31,000 miles over the
course of a month. Once a month,
when the moon is closest to the Earth
(at perigee), tide-generating forces
are higher than usual, producing
above-average ranges in the tides.
About two weeks later, when the
moon is farthest from the Earth (at
apogee), the lunar tide-raising force
is smaller, and the tidal ranges are
less than average. A similar situation
occurs between the Earth and the
sun. When the Earth is closest to the
sun (perihelion), which occurs about
January 2 of each calendar year, the
tidal ranges are enhanced. When the
Earth is furthest from the sun (aphelion), around July 2, the tidal ranges are
reduced (Sumich, J.L., 1996; Thurman, H.V., 1994).
Together, the gravitational effects of the moon
and the sun affect the Earth’s tides on a
monthly basis. Click the image to see an
animation.
The elliptial orbits of the moon around the
Earth and the Earth around the sun have
substantial effects on the earth’s tides. Click
the image for a larger view.
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NOAA's National Ocean Service: Animation of spring and neap tides
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Together, the gravitational pull of the moon and the sun affect the Earth’s tides on
a monthly basis. When the sun, moon, and Earth are in alignment (at the time of
the new or full moon), the solar tide has an additive effect on the lunar tide,
creating extra-high high tides, and very low, low tides — both commonly called
spring tides. One week later, when the sun and moon are at right angles to each
other, the solar tide partially cancels out the lunar tide and produces moderate
tides known as neap tides. During each lunar month, two sets of spring and two
sets of neap tides occur (Sumich, J.L., 1996).
(top)
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NOAA's National Ocean Service: Diagram of earth's elliptical orbit
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The elliptial orbits of the moon around the Earth and the Earth around the sun
have a substantial effect on the the Earth’s tides. Once a month, at perigee, when
the moon is closest to the Earth, tide-generating forces are higher than usual,
producing above average ranges in the tides. About two weeks later, at apogee,
when the moon is farthest from the Earth, the lunar tide-raising force is smaller,
and the tidal ranges are less than average. When the Earth is closest to the sun
(perihelion), around January 2 of the calendar year, tidal ranges are enhanced. At
aphelion, when the Earth is furthest from the sun, around July 2, tidal ranges are
reduced (Sumich, J.L., 1996; Thurman, H.V., 1994).
(top)
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NOAA National Ocean Service Education: Tides and Water Levels
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Tides and Water Levels
Types and Causes of Tidal Cycles –
Diurnal, Semidiurnal, Mixed Semidiurnal;
Continental Interference
Depending upon your location on the Earth you
may experience Diurnal, Semidiurnal or Mixed
Semidiurnal tidal cycles. Click the image for a
larger view.
If the Earth were a perfect sphere
without large continents, all areas
on the planet would experience two
equally proportioned high and low
tides every lunar day. The large
continents on the planet, however,
block the westward passage of the
tidal bulges as the Earth rotates.
Unable to move freely around the
globe, these tides establish complex
patterns within each ocean basin
that often differ greatly from tidal
patterns of adjacent ocean basins or
other regions of the same ocean
basin (Sumich, J.L., 1996).
Three basic tidal patterns occur
along the Earth’s major shorelines.
In general, most areas have two
high tides and two low tides each
day. When the two highs and the
two lows are about the same height, the pattern is called a semi-
daily or semidiurnal tide. If the high
and low tides differ in height, the
pattern is called a mixed semidiurnal
tide. Some areas, such as the Gulf of
Mexico, have only one high and one
low tide each day. This is called a
diurnal tide. The U.S. West Coast
tends to have mixed semidiurnal
tides, whereas a semidiurnal pattern
is more typical of the East Coast
This site NOAA
Tides Roadmap
Tides Lesson Plans
Welcome
What are Tides?
What Causes Tides?
Gravity, Inertia, and
Bulges
Changing Angles and
Tides
The Frequency of Tides
Tidal Variations
Types and Causes of
Tidal Cycles
What Else Affects Tides?
Monitoring the Tides
How are Tides
Measured ? Pt. I
How are Tides
Measured? Pt. II
References
(Sumich, J.L., 1996; Thurman, H.V.,
1994; Ross, D.A., 1995).
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NOAA National Ocean Service Education: Tides and Water Levels
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This map shows the geographic distribution of
different tidal cycles. Coastal areas
experiencing diurnal tides are yellow, areas
experiencing semidiurnal tides are red and
regions with mixed semidiurnal tides are
outlined in blue. Click the image for a larger
view.
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NOAA's National Ocean Service: Tide cycle variations
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Diurnal tide cycle (upper left). An area has a diurnal tidal cycle if it experiences
one high and one low tide every lunar day. Many areas in the Gulf of Mexico
experience these types of tides.
Semidiurnal tide cycle (upper right). An area has a semidiurnal tidal cycle if it
experiences two high and two low tides of approximately equal size every lunar
day. Many areas on the eastern coast of North America experience these tidal
cycles.
Mixed Semidiurnal tide cycle (lower middle). An area has a mixed semidiurnal tidal
cycle if it experiences two high and two low tides of different size every lunar day.
Many areas on the western coast of North America experience these tidal cycles.
(top)
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NOAA's National Ocean Service: Low tide
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Tides establish complex patterns within each ocean basin that often differ greatly
from tidal patterns of adjacent ocean basins or other regions of the same ocean
basin (Sumich, J.L., 1996). This map shows the geographic distribution of different
tidal cycles along the earth's coastlines. Areas experiencing diurnal tides are
marked in yellow, areas experiencing semidiurnal tides are drawn in red and
regions with mixed semidiurnal tides are outlined in blue.
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NOAA National Ocean Service Education: Tides and Water Levels
NOS home NOS education home site index
This site NOAA
Tides Roadmap
Tides Lesson Plans
Welcome
What are Tides?
What Causes Tides?
Gravity, Inertia, and
Bulges
Changing Angles and
Tides
The Frequency of Tides
Tidal Variations
Types and Causes of
Tidal Cycles
What Else Affects
Tides?
Monitoring the Tides
How are Tides
Measured ? Pt. I
How are Tides
Measured? Pt. II
References
Tides and Water Levels
What Affects Tides in Addition to the Sun and Moon?
The relative distances and positions of the sun,
moon and Earth all affect the size and
magnitude of the Earth’s two tidal bulges. At a
smaller scale, the magnitude of tides can be
strongly influenced by the shape of the
shoreline. When oceanic tidal bulges hit wide
continental margins, the height of the tides can
be magnified. Conversely, mid-oceanic islands
not near continental margins typically
experience very small tides of 1 meter or less
(Thurman, H.V., 1994).
The shape of bays and estuaries also can
magnify the intensity of tides. Funnel-shaped
bays in particular can dramatically alter tidal
magnitude. The Bay of Fundy in Nova Scotia is
the classic example of this effect, and has the
highest tides in the world—over 15 meters
(Thurman, H.V., 1994). Narrow inlets and
shallow water also tend to dissipate incoming
tides. Inland bays such as Laguna Madre,
Texas, and Pamlico Sound, North Carolina,
have areas classified as non-tidal even though
they have ocean inlets. In estuaries with
strong tidal rivers, such as the Delaware River
and Columbia River, powerful seasonal river
flows in the spring can severely alter or mask
the incoming tide.
Local wind and weather patterns also can
affect tides. Strong offshore winds can move
water away from coastlines, exaggerating low
tide exposures. Onshore winds may act to pile
up water onto the shoreline, virtually
eliminating low tide exposures. High-pressure
systems can depress sea levels, leading to
clear sunny days with exceptionally low tides.
Conversely, low-pressure systems that
contribute to cloudy, rainy conditions typically
are associated with tides than are much higher
than predicted.
(top)
The shape of bays and estuaries, geographic
location and weather patterns all can affect
local tidal intensity. Click the image for a
larger view.
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NOAA's National Ocean Service: Other effects on tides
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The shape of bays and estuaries, geographic location and weather patterns all can
affect local tidal intensity. This image of a tidal monitoring station in Alaska taken
at high and low tide illustrates the dramatic effect that geographic location can
have on tidal range. At increasing lattitudes (as one moves further from the
equator and closer to the poles) there often is a dramatic increase in tidal range—
in this case, approximately 45 feet.
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NOAA National Ocean Service Education: Tides and Water Levels
NOS home NOS education home site index
This site NOAA
Tides Roadmap
Tides Lesson Plans
Welcome
What are Tides?
What Causes Tides?
Gravity, Inertia, and
Bulges
Changing Angles and
Tides
The Frequency of Tides
Tidal Variations
Types and Causes of
Tidal Cycles
What Else Affects Tides?
Monitoring the Tides
How are Tides
Measured ? Pt. I
How are Tides
Measured? Pt. II
References
Tides and Water Levels
The Importance of Monitoring the Tides and Their
Currents
Predicting tides has always been
important to people who look to the
sea for their livelihood. Commercial
and recreational fishermen use their
knowledge of the tides and tidal
currents to help them improve their
catches. Depending on the species
and water depth in a particular area,
fish may concentrate during ebb or
flood tidal currents. In some areas,
strong tidal currents concentrate bait
and smaller fish, attracting larger fish.
In addition, knowledge of the tides
has also been of interest to
recreational beachgoers and surfers.
Navigating ships through shallow
water ports, intracoastal waterways
and estuaries requires knowledge of
the time and height of the tides as
well as the speed and direction of the
tidal currents. This was particularly
critical to sailing ships because they
had to take advantage of the tides
The ability to predict tides and currents is
essential for people who rely on the sea for
their livelihood. Knowledge of the marine
conditions was critical in transporting these
four marine cranes, each 220 feet tall and
worth approximately $1.25 million, beneath
the Oakland Bridge in San Francisco Bay. Click
the image for a larger view and detailed
description.
and currents to manuever correctly. Knowledge of tides and currents is still
critical because today’s vessels are much larger than the old sailing ships. The
depths and widths of the channels in which they sail, and the increased marine
traffic leaves very little room for error. Real-time water level, water current,
and weather measurement systems now are being used in many major ports to
provide mariners and port operators with the latest conditions.
Coastal zone engineering projects,
including the construction of bridges,
docks, etc., require engineers to
monitor fluctuating tide levels.
Projects involving the construction,
demolition or movement of large
structures must be scheduled far in
advance if an area experiences wide
fluctuations in water levels during its
tidal cycle. Habitat restoration
projects also require accurate
knowledge of tide and current
conditions.
Scientists are concerned with tides,
water levels and tidal currents as
well. Ecologists may focus on the tidal
mixing of near-shore waters, where
pollutants are removed and nutrients
are recirculated. Tidal currents also
Marine commerce is one area in which tide
and current predictions are critical. In June
2002, these four marine cranes valued at $5
million cleared the Oakland Bridge in San
Francisco Bay by approximately 6 feet. Click
the image for a larger view.
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NOAA National Ocean Service Education: Tides and Water Levels
move floating animals and plants to
and from breeding areas in estuaries
to deeper waters. Oceanographers or
atmospheric scientists may study tidal fluctuations to better understand the
circulation of the ocean and its relationship to world climatic changes.
(top)
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NOAA's National Ocean Service: Cranes passing under bridge
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On June 14, 2002, these four industrial cranes, valued at approximately $1.25
million each, arrived in San Francisco Bay from Shanghai, China. Designed to
rapidly hoist 40-foot-long containers from super-sized cargo ships, they had to be
transported beneath the Oakland Bridge to reach their final destination, the Port of
Oakland. The tidal range of San Francisco Bay when these cranes were transported
was 4.1 feet and the bridge had a motion of approximately 6 inches. With light
chop on the bay and winds blowing at around 10 mph, there was little room for
error. With detailed knowledge of the tidal cycle and skillful piloting of the vessel,
the cranes cleared the bottom of the bridge by about 6 feet.
(top)
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NOAA's National Ocean Service: Closeup of cranes passing under bridge
Back
Marine commerce is an area in which tide and current predictions are critical. In
June 2002, these four marine cranes valued at $5 million cleared the Oakland
Bridge in San Francisco Bay by approximately 6 feet. If you look carefully in the
center of the image, you can see a shadowed figure between the crane and the
bridge. This is one of the mariners standing on top of the crane and touching the
bottom of the bridge as the barge passes beneath it.
(top)
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NOAA National Ocean Service Education: Tides and Water Levels
NOS home NOS education home site index
This site NOAA
Tides Roadmap
Tides Lesson Plans
Welcome
What are Tides?
What Causes Tides?
Gravity, Inertia, and
Bulges
Changing Angles and
Tides
The Frequency of Tides
Tidal Variations
Types and Causes of
Tidal Cycles
What Else Affects Tides?
Monitoring the Tides
How are Tides
Measured ? Pt. I
How are Tides
Measured? Pt. II
References
Tides and Water Levels
How are Tides Measured? - The Old System
Since the early 1800s, NOAA and its
predecessor organizations have been
measuring, describing and predicting tides
along the coasts of the United States. The
longest continuous sea level records exists
for the Presidio, in San Francisco, California.
Records for the area date back to June 30,
1854. Today, the Center for Operational
Oceanographic Products and Services (CO-
OPS), which is part of NOAA’s National
Ocean Service (NOS), is responsible for
recording and disseminating water level
data.
In the past, most water level measuring
systems used a recorder driven by a float in
a “stilling” well. A stilling well calms the
waters around the water level sensor. A
typical stilling well consisted of a 12-inch
wide pipe. Inside the stilling well, an 8-inch
diameter float was hung by wire from the
recording unit above.
This is one of the earliest mechanical
pen and ink strip recorders for
measuring tidal levels. Click the image
for a larger view.
Special tide houses were constructed
to shelter permanent water level
recorders, protecting them from harsh
environmental conditions. Click the
image for a larger view.
Before computers were used, water level data was recorded on a continuously
running pen and ink strip chart. These records were collected by observers
once a month and mailed to headquarters for manual processing. In the 1960s,
data were recorded onto mechanically punched paper tape that were read into
a computer for processing. Water levels were recorded at 6-minute intervals.
Observers maintained and adjusted the clocks, and calibrated the gauges with
the tide readings. Tide stations were visited annually to maintain the tide
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NOAA National Ocean Service Education: Tides and Water Levels
houses and clean biological fouling from the underwater surfaces. During these
annual visits, the components and support structures also were checked for
stability.
Although these systems worked well, they
had their limitations. Stations were subject
to recording errors and marine fouling, and
were constantly in need of maintenance. In
addition, the measurement and data
processing equipment could not provide
users with information until weeks after the
data was collected.
(top)
This is a mechanical “punch” recorder
that was brought into service when
computers first became available for
analyzing tidal patterns. Click the
image for a larger view.
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NOAA's National Ocean Service: Tide strip recorder
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This is a close-up of one of the earliest mechanical pen-and-ink strip recorders. In
the upper left part of the image, we can see the stylus marking water level data
onto the paper recording strip as it slowly rotates in time with an internal clock.
These innovative devices required continuous monitoring and maintenance. All of
these mechanical recorders have been replaced with electronic devices that are
much more accurate and require less maintenance.
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NOAA's National Ocean Service: Older tide house diagram
Back
Special tide houses were constructed to shelter permanent water level recorders,
protecting them from harsh environmental conditions. In this diagram, we can see
how the analog data recorder (ADR) is situated inside the house with the float, and
the stilling well located directly beneath it. Attached to one of the piers pilings is a
tidal staff. Essentially a giant measuring stick, this device would allow scientists to
manually observe the tidal level and then compare it to the readings taken by the
analog recorder.
(top)
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NOAA's National Ocean Service: Digital data recorder
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NOAA's National Ocean Service: Digital data recorder
This is a close-up view of the early analog to digital “punch” data recorders that
replaced the earlier pen-and-ink strip recorders. These devices would literally
punch a hole into a specially marked strip of paper every six minutes, recording
the tidal level at that time. At regular intervals, the paper strips would be removed
from the devices and fed into electronic computers. The punches from the strips
would be analyzed and graphed. These devices were the precursers to today's
advanced electronic monitoring systems. Although more accurate than the older
pen-and-ink recorders, they still required frequent maintenance and adjustments.
(top)
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NOAA National Ocean Service Education: Tides and Water Levels
NOS home NOS education home site index
This site NOAA
Tides Roadmap
Tides Lesson Plans
Welcome
What are Tides?
What Causes Tides?
Gravity, Inertia, and
Bulges
Changing Angles and
Tides
The Frequency of Tides
Tidal Variations
Types and Causes of
Tidal Cycles
What Else Affects Tides?
Monitoring the Tides
How are Tides
Measured ? Pt. I
How are Tides
Measured? Pt. II
References
Tides and Water Levels
How are Tides Measured? - The New System
Advances in technology have helped solve
many of the problems associated with the
old tidal recording systems. Microprocessor-
based technologies allow for customized
data collection and have improved
measurement accuracy. While older tidal
measuring stations used mechanical floats
and recorders, a new generation of
monitoring stations uses advanced
acoustics and electronics. Today's recorders
send an audio signal down a half-inch-wide
sounding tube and measure the time it
takes for the reflected signal to travel back
from the water's surface. The sounding
tube is mounted inside a 6-inch diameter
protective well, which is similar to the old
stilling well.
In addition to measuring tidal heights more
accurately, the new system also records 11
different oceanographic and meteorological
parameters. These include wind speed and
direction, water current speed and
direction, air and water temperature, and
barometric pressure.
Like the old recorders, the new measuring
stations collect data every six minutes.
However, whereas the old recording
stations used mechanical timers to tell
them when to take a reading, timing is
controlled on the new stations by a
Geostationary Operational Environmental
Satellite (GOES). The stations also use
these satellites to transmit their data hourly
to NOAA headquarters. In the event of a
storm, the stations can be programmed to
transmit their data every six minutes. Field
teams can quickly check and maintain the
systems using laptop computers. In
addition, all of the raw and processed data
are available over the Internet.
Tide houses continue to be built and
used to protect equipment from the
elements. Click the image for a larger
view.
A monitoring station attached directly
to a pier. Click the image for a larger
view.
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NOAA National Ocean Service Education: Tides and Water Levels
Today, tide monitoring stations are
very accurate, require little
maintenance, and are part of a larger
nationwide network. Click the image
for a larger view.
(top)
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NOAA's National Ocean Service: Modern tide house diagram
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While similar in design to older tide houses, these newer enclosures are designed
to protect sensitive electronics, transmitting equipment, and backup power and
data storage devices. The older stilling well has been replaced with an acoustic
sounding tube and the tidal staff with a pressure sensor. The new field equipment
is designed to operate with the highest level of accuracy with a minimum of
maintenance, transmitting data directly back to NOAA headquarters for analysis
and distribution.
(top)
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NOAA's National Ocean Service: Tide station on pier
Back
Not all monitoring stations are housed in protective enclosures. This water level
and meteorological recorder is attached directly to a pier. On the far left is the
acoustic sounding tube and sensor. Rising up from the piling is a solar cell, and
above that, a satellite transmitter. The remainder of the recording electronics are
housed in a small weatherproof box (open).
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NOAA's National Ocean Service: Satellite tide network
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The earliest tidal monitoring stations were small and self-contained, but they
required frequent visits for maintenance and adjustment. Today, stations are still
self contained but are very accurate, require little maintenance, and are part of a
larger nationwide network. Today, data are transmitted to NOAA headquarters via
satellite shortly after they are collected. After rapid computer analysis, the data
are immediately posted to one of several Web sites where they can be universally
accessed. With these systems in place, scientists can run diagnostic checks on the
equipment without needing to travel into the field. This saves both time and
money.
(top)
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NOAA National Ocean Service Education: Tides and Water Levels
Tides and Water Levels
References
NOS home NOS education home site index
Ross, D.A. 1995. Introduction to Oceanography. New York, NY: HarperCollins. pp. 236-
242.
Sumich, J.L. 1996. An Introduction to the Biology of Marine Life, sixth edition. Dubuque,
IA: Wm. C. Brown. pp. 30-35.
Thurman, H.V. 1994. Introductory Oceanography, seventh edition. New York, NY:
Macmillan. pp. 252-276.
(top)
This site NOAA
Tides Roadmap
Tides Lesson Plans
Welcome
What are Tides?
What Causes Tides?
Gravity, Inertia, and
Bulges
Changing Angles and
Tides
The Frequency of Tides
Tidal Variations
Types and Causes of
Tidal Cycles
What Else Affects Tides?
Monitoring the Tides
How are Tides
Measured ? Pt. I
How are Tides
Measured? Pt. II
References
Revised December 02, 2004 | Questions, Comments? Contact Us | Report Error On This Page | Disclaimer | User Survey
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