tides tutorial.pdf

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OAA National Ocean Service Education: Tides and Water Levels

NOS home NOS education home site index

Welcome to Tides and Water Levels

The rise and fall of the tides play an importantrole in the natural world and can have a

marked effect on maritime-related activities.Here, a ship's crew inspects the hull of theirvessel. It became stranded on a sandbarfollowing a rapidly receding tide.

NOAA's National Ocean Service collects,studies and provides access to thousands ofhistorical and real-time observations as well aspredictions of water levels, coastal currentsand other data. Maritime activities throughoutthe world depend on accurate tidal and currentinformation for safe operation.

In this subject, you will find three sectionsdevoted to learning about tides and waterlevels: an online tutorial, an educationalroadmap to resources, and formal lesson plans.

The Tides and Waters Levels Tutorial is anoverview of the complex systems that governthe movement of tides and water levels. Thetutorial 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 thearrows at the top or bottom of each chapter page. The tutorial includes many illustrativeand interactive graphics to visually enhance the text.

The Roadmap to Resources complements the information in the tutorial. The roadmapdirects you to specific tidal and current data offered within the NOS and NOAA family ofproducts.

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 912 andfocus on the forces that cause and effect tides, analysis of the variations in tidal patternsand what conditions may cause them, and the effect of lunar cycles on living organisms.

The National Science Teachers Association (NSTA) has included thisonline resource in its SciLinksdatabase.SciLinksprovide students and teachers access to Web-based,educationally appropriate science content that has been formallyevaluated by master teachers.

For more information about the SciLinks evaluation criteria, click here:http://www.scilinks.org/certificate.asp.

To go directly to the SciLinkslog-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 SurveyNOAAs National Ocean Service| National Oceanic and Atmospheric Administration| U.S. Department of Commercehttp://oceanservice.noaa.gov/education/kits/tides/welcome.html

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Tides and Water Levels

What Causes Tides?

The relationship between the masses of theEarth, moon and sun and their distances toeach other play critical roles in affecting tides.Click the image for a larger view.

Gravity is one major force thatcreates tides. In 1687, Sir IsaacNewton explained that ocean tidesresult from the gravitational attractionof the sun and moon on the oceans ofthe earth (Sumich, J.L., 1996).

Newtons law of universal gravitationstates that the gravitational attractionbetween two bodies is directlyproportional to their masses, andinversely proportional to the square of

the distance between the bodies(Sumich, J.L., 1996; Thurman, H.V.,1994). Therefore, the greater themass of the objects and the closerthey are to each other, the greaterthe gravitational attraction betweenthem (Ross, D.A. 1995).

Tidal forces are based on the gravitational attractive force. With regard to tidalforces on the Earth, the distance between two objects usually is more criticalthan their masses. Tidal generating forces vary inversely as the cube of thedistance from the tide generating object. Gravitational attractive forces onlyvary 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 relationshipbetween the sun, the moon, and the Earths waters.

Our sun is 27 million times larger than our moon. Based on its mass, the sun'sgravitational attraction to the Earth is more than 177 times greater than that ofthe moon to the Earth. If tidal forces were based solely on comparativemasses, the sun should have a tide-generating force that is 27 million timesgreater than that of the moon. However, the sun is 390 times further from theEarth than is the moon. Thus, its tide-generating force is reduced by 3903, orabout 59 million times less than the moon. Because of these conditions, thesuns 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, andBulges

Changing Angles and

Tides


Tidal Variations

Types and Causes of

Tidal Cycles

What Else Affects Tides?


How are Tides

Measured ? Pt. I

How are Tides

Measured? Pt. II

References

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Revised December 02, 2004 | Questions, Comments? Contact Us| Report Error On This Page| Disclaimer| User SurveyNOAAs National Ocean Service| National Oceanic and Atmospheric Administration| U.S. Department of Commercehttp://oceanservice.noaa.gov/education/kits/tides/tides02_cause.html


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OAA's National Ocean Service: Diagram of distances between earth, sun and moon

Back

The relationship between the masses of the Earth, moon and sun and theirdistances to each other play a critical role in affecting the Earth's tides. Althoughthe sun is 27 million times more massive than the moon, it is 390 times furtheraway from the Earth than the moon. Tidal generating forces vary inversely as thecube of the distance from the tide-generating object. This means that the suns

tidal generating force is reduced by 3903(about 59 million times) compared to thetide-generating force of the moon. Therefore, the suns tide-generating force isabout half that of the moon, and the moon is the dominant force affecting the

Earths tides.

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Revised December 02, 2004 | Questions, Comments? Contact Us| Report Error On This Page| Disclaimer| User SurveyNOAAs National Ocean Service| National Oceanic and Atmospheric Administration| U.S. Department of Commercehttp://oceanservice.noaa.gov/education/kits/tides/media/supp_tide02.html


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Gravity, Inertia, and the Two Bulges

Two tidal bulges are created on opposite sidesof the Earth due to the moon's gravitationalforce and inertias counterbalance. Click theimage for a larger view.

Gravity is a major force responsiblefor creating tides. Inertia, acts tocounterbalance gravity. It is thetendency of moving objects tocontinue moving in a straight line.Together, gravity and inertia areresponsible for the creation of twomajor tidal bulges on the Earth(Ross, D.A., 1995).

The gravitational attraction betweenthe Earth and the moon is strongest

on the side of the Earth thathappens to be facing the moon,simply because it is closer. Thisattraction causes the water on this near side of Earth to be pulled towardthe moon. As gravitational force acts to draw the water closer to the moon,inertia attempts to keep the water in place. But the gravitational forceexceeds it and the water is pulled toward the moon, causing a bulge ofwater on the near side toward the moon (Ross, D.A., 1995).

On the opposite side of the Earth, or the far side, the gravitational attractionof the moon is less because it is farther away. Here, inertia exceeds thegravitational force, and the water tries to keep going in a straight line, movingaway 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 wherethey are furthest apart. Over the rest of the globe gravity and inertia are inrelative balance. Because water is fluid, the two bulges stay aligned with themoon as the Earth rotates (Ross, D.A., 1995).

The sun also plays a major role, affecting the size and position of the two tidalbulges. The interaction of the forces generated by the moon and the sun canbe quite complex. As this is an introduction to the subject of tides and waterlevels we will focus most of our attention on the effects of the strongercelestial influence, the moon.

This site NOAA

Tides Roadmap

Tides Lesson Plans

Welcome

What are Tides?

What Causes Tides?


Bulges

Changing Angles and

Tides


Tidal Variations

Types and Causes of

Tidal Cycles



How are Tides

Measured ? Pt. I

How are Tides

Measured? Pt. II

References

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Revised December 01, 2006 | Questions, Comments? Contact Us| Report Error On This Page| Disclaimer| User SurveyNOAAs National Ocean Service| National Oceanic and Atmospheric Administration| U.S. Department of Commercehttp://oceanservice.noaa.gov/education/kits/tides/tides03_gravity.html


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OAA's National Ocean Service: Diagram of tidal bulges

Back

Gravity and inertia act in opposition on the Earths oceans, creating tidal bulges on

opposite sites of the planet. On the near side of the Earth (the side facing themoon), the gravitational force of the moon pulls the oceans waters toward it,creating one bulge. On the far side of the Earth, inertia dominates, creating asecond bulge.

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Changing Angles and Changing Tides

The Earths tidal bulges track, or follow, theposition of the moon and to a lesser extent,the sun. As these two celestial bodies increaseand decrease their angles to the Earth, so dothe tidal bulges. Click the image for ananimated view.

As weve just seen, the Earth's twotidal bulges are aligned with thepositions of the moon and the sun.Over time, the positions of thesecelestial bodies change relative to theEarths equator. The changes in theirrelative positions have a direct effecton daily tidal heights and tidal currentintensity.

As the moon revolves around theEarth, its angle increases and

decreases in relation to the equator.This is known as its declination. Thetwo tidal bulges track the changes inlunar declination, also increasing ordecreasing their angles to theequator. Similarly, the suns relativeposition to the equator changes overthe course of a year as the Earthrotates around it. The sunsdeclination affects the seasons as wellas the tides. During the vernal andautumnal equinoxesMarch 21 and September 23, respectivelythe sun is atits minimum declination because it is positioned directly above the equator. On

June 21 and December 22the summer and winter solstices, respectivelythesun 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?


Changing Angles and

Tides


Tidal Variations

Types and Causes of

Tidal Cycles



How are Tides

Measured ? Pt. I

How are Tides

Measured? Pt. II

References

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Revised December 02, 2004 | Questions, Comments? Contact Us| Report Error On This Page| Disclaimer| User SurveyNOAAs National Ocean Service| National Oceanic and Atmospheric Administration| U.S. Department of Commercehttp://oceanservice.noaa.gov/education/kits/tides/tides04_angle.html


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OAA's National Ocean Service: Animation of earth's declination

Back

The Earths tidal bulges track, or follow, the position of the moon, and to a lesserextent, the sun. As the angles of these two celestial bodies in relation to the Earthincrease and decrease, so do the tidal bulges. Here we observe the moon'schanging declination to the equator and the effect that this has on the positions ofthe Earths tidal bulges.

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Frequency of Tides - The Lunar Day

Unlike a 24-hour solar day, a lunar day lasts24 hours and 50 minutes. This occurs becausethe moon revolves around the Earth in thesame direction that the Earth rotates aroundits axis. Click the image to see an animation.

Most coastal areas, with someexceptions, experience two high tidesand two low tides every lunar day(Ross, D.A., 1995). Almost everyoneis familiar with the concept of a 24-hour solar day, which is the time thatit takes for a specific site on the Earthto rotate from an exact point underthe sun to the same point under thesun. Similarly, a lunar day is the timeit takes for a specific site on the Earthto rotate from an exact point under

the moon to the same point under themoon. Unlike a solar day, however, alunar day is 24 hours and 50 minutes.The lunar day is 50 minutes longerthan a solar day because the moonrevolves around the Earth in the samedirection that the Earth rotatesaround its axis. So, it takes the Earthan extra 50 minutes to catch up tothe 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 12.5minutes 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?


Changing Angles and

Tides

The Frequency of

Tides

Tidal Variations

Types and Causes of

Tidal Cycles



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 SurveyNOAAs National Ocean Service| National Oceanic and Atmospheric Administration| U.S. Department of Commercehttp://oceanservice.noaa.gov/education/kits/tides/tides05_lunarday.html


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OAA'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 occursbecause the moon revolves around the Earth in the same direction that the Earthis rotating on its axis. Therefore, it takes the Earth an extra 50 minutes to catchup to the moon. Since the Earth rotates through two tidal bulges every lunarday, 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 tidesoccur 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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Tidal Variations - The Influence of Position and

Distance

Together, the gravitational effects of the moonand the sun affect the Earths tides on amonthly basis.Click the image to see ananimation.

The moon is a major influence on theEarths tides, but the sun alsogenerates considerable tidal forces.Solar tides are about half as large aslunar tides and are expressed as avariation of lunar tidal patterns, notas a separate set of tides. When thesun, moon, and Earth are inalignment (at the time of the new orfull moon), the solar tide has anadditive effect on the lunar tide,

creating extra-high high tides, andvery low, low tidesboth commonlycalled spring tides. One week later,when the sun and moon are at rightangles to each other, the solar tidepartially cancels out the lunar tideand produces moderate tides knownas neap tides. During each lunarmonth, two sets of spring tides and two sets of neap tides occur (Sumich, J.L.,1996).

The elliptial orbits of the moon around theEarth and the Earth around the sun havesubstantial effects on the earths tides. Clickthe image for a larger view.

Just as the angles of the sun, moonand Earth affect tidal heights over the

course of a lunar month, so do theirdistances to one another. Because themoon follows an elliptical path aroundthe Earth, the distance between themvaries by about 31,000 miles over thecourse of a month. Once a month,when the moon is closest to the Earth(at perigee), tide-generating forcesare higher than usual, producingabove-average ranges in the tides.About two weeks later, when themoon is farthest from the Earth (atapogee), the lunar tide-raising force

is smaller, and the tidal ranges areless than average. A similar situationoccurs between the Earth and thesun. When the Earth is closest to thesun (perihelion), which occurs aboutJanuary 2 of each calendar year, thetidal ranges are enhanced. When theEarth is furthest from the sun (aphelion), around July 2, the tidal ranges arereduced (Sumich, J.L., 1996; Thurman, H.V., 1994).

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This site NOAA

Tides Roadmap

Tides Lesson Plans

Welcome

What are Tides?

What Causes Tides?


Changing Angles and

Tides


Tidal Variations

Types and Causes of

Tidal Cycles



How are Tides

Measured ? Pt. I

How are Tides

Measured? Pt. II

References

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OAA'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 sunhave a substantial effect on the the Earths tides. Once a month, at perigee, whenthe 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. Ataphelion, when the Earth is furthest from the sun, around July 2, tidal ranges arereduced (Sumich, J.L., 1996; Thurman, H.V., 1994).

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Revised December 02, 2004 | Questions, Comments? Contact Us| Report Error On This Page| Disclaimer| User SurveyNOAAs National Ocean Service| National Oceanic and Atmospheric Administration| U.S. Department of Commercehttp://oceanservice.noaa.gov/education/kits/tides/media/supp_tide06b.html


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Types and Causes of Tidal Cycles Diurnal, Semidiurnal, Mixed Semidiurnal;

Continental Interference

Depending upon your location on the Earth youmay experience Diurnal, Semidiurnal or MixedSemidiurnal tidal cycles. Click the image for alarger view.

If the Earth were a perfect spherewithout large continents, all areason the planet would experience twoequally proportioned high and lowtides every lunar day. The largecontinents on the planet, however,block the westward passage of thetidal bulges as the Earth rotates.Unable to move freely around theglobe, these tides establish complex

patterns within each ocean basinthat often differ greatly from tidalpatterns of adjacent ocean basins orother regions of the same oceanbasin (Sumich, J.L., 1996).

Three basic tidal patterns occuralong the Earths major shorelines.In general, most areas have twohigh tides and two low tides eachday. When the two highs and thetwo lows are about the same height, the pattern is called a semi-

daily or semidiurnal tide. If the highand low tides differ in height, thepattern is called a mixed semidiurnaltide. Some areas, such as the Gulf ofMexico, have only one high and onelow tide each day. This is called adiurnal tide. The U.S. West Coasttends to have mixed semidiurnaltides, whereas a semidiurnal patternis 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?


Changing Angles and

Tides


Tidal Variations

Types and Causes of

Tidal Cycles



How are Tides

Measured ? Pt. I

How are Tides

Measured? Pt. II

References

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This map shows the geographic distribution ofdifferent tidal cycles. Coastal areasexperiencing diurnal tides are yellow, areasexperiencing semidiurnal tides are red andregions with mixed semidiurnal tides areoutlined in blue. Click the image for a largerview.

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OAA's National Ocean Service: Tide cycle variations

Back

Diurnal tide cycle (upper left). An area has a diurnal tidal cycle if it experiencesone high and one low tide every lunar day. Many areas in the Gulf of Mexicoexperience these types of tides.

Semidiurnal tide cycle (upper right). An area has a semidiurnal tidal cycle if itexperiences two high and two low tides of approximately equal size every lunarday. Many areas on the eastern coast of North America experience these tidalcycles.

Mixed Semidiurnal tide cycle (lower middle). An area has a mixed semidiurnal tidalcycle 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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OAA's National Ocean Service: Low tide

Back

Tides establish complex patterns within each ocean basin that often differ greatlyfrom tidal patterns of adjacent ocean basins or other regions of the same oceanbasin (Sumich, J.L., 1996). This map shows the geographic distribution of differenttidal cycles along the earth's coastlines. Areas experiencing diurnal tides aremarked in yellow, areas experiencing semidiurnal tides are drawn in red andregions with mixed semidiurnal tides are outlined in blue.

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What Affects Tides in Addition to the Sun and Moon?

The shape of bays and estuaries, geographiclocation and weather patterns all can affectlocal tidal intensity. Click the image for alarger view.

The relative distances and positions of the sun,moon and Earth all affect the size andmagnitude of the Earths two tidal bulges. At asmaller scale, the magnitude of tides can bestrongly influenced by the shape of theshoreline. When oceanic tidal bulges hit widecontinental margins, the height of the tides canbe magnified. Conversely, mid-oceanic islandsnot near continental margins typicallyexperience very small tides of 1 meter or less(Thurman, H.V., 1994).

The shape of bays and estuaries also canmagnify the intensity of tides. Funnel-shaped

bays in particular can dramatically alter tidalmagnitude. The Bay of Fundy in Nova Scotia isthe classic example of this effect, and has thehighest tides in the worldover 15 meters(Thurman, H.V., 1994). Narrow inlets andshallow water also tend to dissipate incomingtides. Inland bays such as Laguna Madre,Texas, and Pamlico Sound, North Carolina,have areas classified as non-tidal even thoughthey have ocean inlets. In estuaries withstrong tidal rivers, such as the Delaware Riverand Columbia River, powerful seasonal riverflows in the spring can severely alter or maskthe incoming tide.

Local wind and weather patterns also canaffect tides. Strong offshore winds can movewater away from coastlines, exaggerating lowtide exposures. Onshore winds may act to pileup water onto the shoreline, virtuallyeliminating low tide exposures. High-pressuresystems can depress sea levels, leading toclear sunny days with exceptionally low tides.Conversely, low-pressure systems thatcontribute to cloudy, rainy conditions typicallyare associated with tides than are much higherthan predicted.

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This site NOAA

Tides Roadmap

Tides Lesson Plans

Welcome

What are Tides?

What Causes Tides?


Bulges

Changing Angles and

Tides


Tidal Variations

Types and Causes of

Tidal Cycles

What Else Affects

Tides?


How are Tides

Measured ? Pt. I

How are Tides

Measured? Pt. II

References

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OAA's National Ocean Service: Other effects on tides

Back

The shape of bays and estuaries, geographic location and weather patterns all canaffect local tidal intensity. This image of a tidal monitoring station in Alaska takenat high and low tide illustrates the dramatic effect that geographic location canhave on tidal range. At increasing lattitudes (as one moves further from theequator and closer to the poles) there often is a dramatic increase in tidal rangein this case, approximately 45 feet.

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The Importance of Monitoring the Tides and TheirCurrents

The ability to predict tides and currents isessential for people who rely on the sea fortheir livelihood. Knowledge of the marineconditions was critical in transporting thesefour marine cranes, each 220 feet tall andworth approximately $1.25 million, beneaththe Oakland Bridge in San Francisco Bay. Clickthe image for a larger view and detaileddescription.

Predicting tides has always beenimportant to people who look to thesea for their livelihood. Commercialand recreational fishermen use theirknowledge of the tides and tidalcurrents to help them improve theircatches. Depending on the speciesand water depth in a particular area,fish may concentrate during ebb orflood tidal currents. In some areas,strong tidal currents concentrate bait

and smaller fish, attracting larger fish.In addition, knowledge of the tideshas also been of interest torecreational beachgoers and surfers.

Navigating ships through shallowwater ports, intracoastal waterwaysand estuaries requires knowledge ofthe time and height of the tides aswell as the speed and direction of thetidal currents. This was particularlycritical to sailing ships because theyhad to take advantage of the tides

and currents to manuever correctly. Knowledge of tides and currents is stillcritical because todays vessels are much larger than the old sailing ships. Thedepths and widths of the channels in which they sail, and the increased marinetraffic leaves very little room for error. Real-time water level, water current,and weather measurement systems now are being used in many major ports toprovide mariners and port operators with the latest conditions.

Marine commerce is one area in which tideand current predictions are critical. In June2002, these four marine cranes valued at $5million cleared the Oakland Bridge in SanFrancisco Bay by approximately 6 feet. Clickthe image for a larger view.

Coastal zone engineering projects,including the construction of bridges,docks, etc., require engineers tomonitor fluctuating tide levels.Projects involving the construction,demolition or movement of largestructures must be scheduled far inadvance if an area experiences widefluctuations in water levels during itstidal cycle. Habitat restorationprojects also require accurateknowledge of tide and currentconditions.

Scientists are concerned with tides,water levels and tidal currents aswell. Ecologists may focus on the tidalmixing of near-shore waters, wherepollutants are removed and nutrientsare recirculated. Tidal currents also

This site NOAA

Tides Roadmap

Tides Lesson Plans

Welcome

What are Tides?

What Causes Tides?


Changing Angles and

Tides


Tidal Variations

Types and Causes of

Tidal Cycles



How are Tides

Measured ? Pt. I

How are Tides

Measured? Pt. II

References

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move floating animals and plants toand from breeding areas in estuariesto deeper waters. Oceanographers oratmospheric scientists may study tidal fluctuations to better understand thecirculation of the ocean and its relationship to world climatic changes.

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OAA's National Ocean Service: Cranes passing under bridge

Back

On June 14, 2002, these four industrial cranes, valued at approximately $1.25million each, arrived in San Francisco Bay from Shanghai, China. Designed torapidly hoist 40-foot-long containers from super-sized cargo ships, they had to betransported beneath the Oakland Bridge to reach their final destination, the Port of

Oakland. The tidal range of San Francisco Bay when these cranes were transportedwas 4.1 feet and the bridge had a motion of approximately 6 inches. With lightchop on the bay and winds blowing at around 10 mph, there was little room forerror. 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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OAA's National Ocean Service: Closeup of cranes passing under bridge

Back

Marine commerce is an area in which tide and current predictions are critical. InJune 2002, these four marine cranes valued at $5 million cleared the OaklandBridge in San Francisco Bay by approximately 6 feet. If you look carefully in thecenter of the image, you can see a shadowed figure between the crane and thebridge. 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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How are Tides Measured? - The Old System

This is one of the earliest mechanicalpen and ink strip recorders formeasuring tidal levels. Click the imagefor a larger view.

Since the early 1800s, NOAA and itspredecessor organizations have beenmeasuring, describing and predicting tidesalong the coasts of the United States. Thelongest continuous sea level records existsfor the Presidio, in San Francisco, California.Records for the area date back to June 30,1854. Today, the Center for OperationalOceanographic Products and Services (CO-OPS), which is part of NOAAs NationalOcean Service (NOS), is responsible forrecording and disseminating water level

data.

In the past, most water level measuringsystems used a recorder driven by a float ina stilling well. A stilling well calms thewaters around the water level sensor. Atypical stilling well consisted of a 12-inchwide pipe. Inside the stilling well, an 8-inchdiameter float was hung by wire from therecording unit above.

Special tide houses were constructed

to shelter permanent water levelrecorders, protecting them from harshenvironmental conditions. Click theimage for a larger view.

Before computers were used, water level data was recorded on a continuouslyrunning pen and ink strip chart. These records were collected by observersonce a month and mailed to headquarters for manual processing. In the 1960s,data were recorded onto mechanically punched paper tape that were read intoa computer for processing. Water levels were recorded at 6-minute intervals.Observers maintained and adjusted the clocks, and calibrated the gauges withthe 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?


Changing Angles and

Tides


Tidal Variations

Types and Causes of

Tidal Cycles



How are Tides

Measured ? Pt. I

How are Tides

Measured? Pt. II

References

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OAA's National Ocean Service: Tide strip recorder

Back

This is a close-up of one of the earliest mechanical pen-and-ink strip recorders. Inthe upper left part of the image, we can see the stylus marking water level dataonto the paper recording strip as it slowly rotates in time with an internal clock.These innovative devices required continuous monitoring and maintenance. All ofthese mechanical recorders have been replaced with electronic devices that aremuch more accurate and require less maintenance.

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OAA'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 seehow the analog data recorder (ADR) is situated inside the house with the float, andthe stilling well located directly beneath it. Attached to one of the piers pilings is atidal staff. Essentially a giant measuring stick, this device would allow scientists tomanually observe the tidal level and then compare it to the readings taken by theanalog recorder.

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OAA's National Ocean Service: Digital data recorder

Back

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OAA's National Ocean Service: Digital data recorder

This is a close-up view of the early analog to digital punch data recorders thatreplaced the earlier pen-and-ink strip recorders. These devices would literallypunch a hole into a specially marked strip of paper every six minutes, recordingthe tidal level at that time. At regular intervals, the paper strips would be removedfrom the devices and fed into electronic computers. The punches from the stripswould be analyzed and graphed. These devices were the precursers to today'sadvanced electronic monitoring systems. Although more accurate than the olderpen-and-ink recorders, they still required frequent maintenance and adjustments.

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How are Tides Measured? - The New System

Tide houses continue to be built andused to protect equipment from the

elements. Click the image for a largerview.

Advances in technology have helped solvemany of the problems associated with theold tidal recording systems. Microprocessor-based technologies allow for customizeddata collection and have improvedmeasurement accuracy. While older tidalmeasuring stations used mechanical floatsand recorders, a new generation ofmonitoring stations uses advancedacoustics and electronics. Today's recorderssend an audio signal down a half-inch-widesounding tube and measure the time it

takes for the reflected signal to travel backfrom the water's surface. The soundingtube is mounted inside a 6-inch diameterprotective well, which is similar to the oldstilling well.

A monitoring station attached directlyto a pier. Click the image for a largerview.

In addition to measuring tidal heights moreaccurately, the new system also records 11different oceanographic and meteorologicalparameters. These include wind speed anddirection, water current speed anddirection, air and water temperature, andbarometric pressure.

Like the old recorders, the new measuringstations collect data every six minutes.However, whereas the old recordingstations used mechanical timers to tellthem when to take a reading, timing iscontrolled on the new stations by aGeostationary Operational EnvironmentalSatellite (GOES). The stations also usethese satellites to transmit their data hourlyto NOAA headquarters. In the event of astorm, the stations can be programmed to

transmit their data every six minutes. Fieldteams can quickly check and maintain thesystems using laptop computers. Inaddition, all of the raw and processed dataare available over the Internet.

This site NOAA

Tides Roadmap

Tides Lesson Plans

Welcome

What are Tides?

What Causes Tides?


Changing Angles and

Tides


Tidal Variations

Types and Causes of

Tidal Cycles



How are Tides

Measured ? Pt. I

How are Tides

Measured? Pt. II

References

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Today, tide monitoring stations arevery accurate, require littlemaintenance, and are part of a largernationwide network. Click the imagefor a larger view.

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OAA's National Ocean Service: Modern tide house diagram

Back

While similar in design to older tide houses, these newer enclosures are designedto protect sensitive electronics, transmitting equipment, and backup power anddata storage devices. The older stilling well has been replaced with an acousticsounding tube and the tidal staff with a pressure sensor. The new field equipmentis designed to operate with the highest level of accuracy with a minimum ofmaintenance, transmitting data directly back to NOAA headquarters for analysisand distribution.

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OAA's National Ocean Service: Tide station on pier

Back

Not all monitoring stations are housed in protective enclosures. This water leveland meteorological recorder is attached directly to a pier. On the far left is theacoustic sounding tube and sensor. Rising up from the piling is a solar cell, andabove that, a satellite transmitter. The remainder of the recording electronics arehoused in a small weatherproof box (open).

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OAA's National Ocean Service: Satellite tide network

Back

The earliest tidal monitoring stations were small and self-contained, but theyrequired frequent visits for maintenance and adjustment. Today, stations are stillself contained but are very accurate, require little maintenance, and are part of alarger nationwide network. Today, data are transmitted to NOAA headquarters viasatellite shortly after they are collected. After rapid computer analysis, the dataare immediately posted to one of several Web sites where they can be universallyaccessed. With these systems in place, scientists can run diagnostic checks on the

equipment without needing to travel into the field. This saves both time andmoney.

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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.

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This site NOAA

Tides Roadmap

Tides Lesson Plans

Welcome

What are Tides?

What Causes Tides?


Bulges

Changing Angles and

Tides


Tidal Variations

Types and Causes of

Tidal Cycles

What Else Affects 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 SurveyNOAAs National Ocean Service| National Oceanic and Atmospheric Administration| U.S. Department of Commercehttp://oceanservice.noaa.gov/education/kits/tides/tides12_references.html


Documents

tides tutorial.pdf