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NOAA National Ocean Service Education: Tides and Water Levels
 
 
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Welcome to Tides and Water Levels
 
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.
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 
Revised January 26, 2005 | Questions, Comments? Contact Us | Report Error On This Page | Disclaimer | User Survey 
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http://oceanservice.noaa.gov/education/kits/tides/welcome.html 
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NOAA National Ocean Service Education: Tides and Water Levels
 
 
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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.
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 
Revised December 02, 2004 | Questions, Comments? Contact Us | Report Error On This Page | Disclaimer | User Survey 
NOAA’s National Ocean Service | National Oceanic and Atmospheric Administration | U.S. Department of Commerce 
http://oceanservice.noaa.gov/education/kits/tides/tides01_intro.html 
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NOAA National Ocean Service Education: Tides and Water Levels
 
 
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Tides and Water Levels 
What Causes Tides?
 
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.
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  
 
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/tides02_cause.html 
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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

(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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Tides and Water Levels 
Gravity, Inertia, and the Two Bulges
 
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.
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) 
 
 
 
Revised December 01, 2006 | Questions, Comments? Contact Us | Report Error On This Page | Disclaimer | User Survey 
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http://oceanservice.noaa.gov/education/kits/tides/tides03_gravity.html 
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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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Revised December 01, 2006 | Questions, Comments? Contact Us | Report Error On This Page | Disclaimer | User Survey

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Tides and Water Levels 
Changing Angles and Changing Tides
 
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.
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) 
 
 
Revised December 02, 2004 | Questions, Comments? Contact Us | Report Error On This Page | Disclaimer | User Survey 
NOAA’s National Ocean Service | National Oceanic and Atmospheric Administration | U.S. Department of Commerce 
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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Revised December 02, 2004 | Questions, Comments? Contact Us | Report Error On This Page | Disclaimer | User Survey

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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) 
 
 
Revised December 02, 2004 | Questions, Comments? Contact Us | Report Error On This Page | Disclaimer | User Survey 
NOAA’s National Ocean Service | National Oceanic and Atmospheric Administration | U.S. Department of Commerce 
http://oceanservice.noaa.gov/education/kits/tides/tides05_lunarday.html 
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NOAA's National Ocean Service: Animation of lunar day

Back

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
 
 
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Tides and Water Levels 
Tidal Variations - The Influence of Position and 
Distance
 
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 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). 
 
 
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.
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). 
 
 
(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 
http://www.oceanservice.noaa.gov/education/kits/tides/tides06_variations.html (1 of 2)9/26/2007 4:20:52 PM

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).

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NOAA's National Ocean Service: Diagram of earth's elliptical orbit

Back

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).

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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 
(Sumich, J.L., 1996; Thurman, H.V., 
1994; Ross, D.A., 1995).
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
  
   
 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 
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NOAA National Ocean Service Education: Tides and Water Levels

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

Back

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.

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NOAA's National Ocean Service: Low tide

Back

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
 
 
 
Tides and Water Levels 
What Affects Tides in Addition to the Sun and Moon?
 
 
The shape of bays and estuaries, geographic 
location and weather patterns all can affect 
local tidal intensity. Click the image for a 
larger view.
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. 
 
 
 
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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 
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NOAA's National Ocean Service: Other effects on tides

Back

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

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Tides and Water Levels

The Importance of Monitoring the Tides and Their

Currents

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.

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

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.

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.

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

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

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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.

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NOAA's National Ocean Service: Cranes passing under bridge

Back

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.

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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.

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NOAA National Ocean Service Education: Tides and Water Levels

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Tides and Water Levels

How are Tides Measured? - The Old System

This is one of the earliest mechanical

pen and ink strip recorders for

measuring tidal levels. Click the image

for a larger view.

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.

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

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

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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.

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.

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.

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NOAA's National Ocean Service: Tide strip recorder

Back

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.

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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.

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NOAA National Ocean Service Education: Tides and Water Levels
 
 
NOS home
 
NOS education home 
 
site index
 
 
 
Tides and Water Levels 
How are Tides Measured? - The New System
 
Tide houses continue to be built and 
used to protect equipment from the 
elements. Click the image for a larger 
view.
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. 
 
 
 
A monitoring station attached directly 
to a pier. Click the image for a larger 
view.
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. 
 
 
 
 
 
 
 
  
   
 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 
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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.

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NOAA's National Ocean Service: Modern tide house diagram

Back

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.

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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

Back

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.

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NOAA National Ocean Service Education: Tides and Water Levels
 
NOS home
 
NOS education home 
 
site index
 
Tides and Water Levels 
References
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 
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