Introduction to Ocean Tides

Return to Module Home


0. Objectives
1. Introduction
2. Astronomical Tides: Earth-Moon System
3. Other Astronomical Effects
4. Actual Tidal Variability
5. Tide Prediction Methods
6. Summary

0. Objectives

After completing this module, the learner should be able to do the following things:

  1. List and define terms used to describe tides
  2. List and define the forces that cause and modify tides
  3. Define tidal constituents
  4. Describe tidal datum and why it is important
  5. Describe tide prediction methods
  6. Explain when to use tide tables vs. models

1. Introduction

1.1 Why Care About Tides?

Cranes getting ready to pass under San Francisco Bay bridge

Cranes getting ready to pass under San Francisco Bay bridge

As any mariner knows, ocean tides significantly impact marine operations. In the coastal environment, successful navigation requires knowledge of the water depth and thus, water elevation levels as well. The water in a shipping channel must be sufficiently deep for ships to navigate in or out of port without running aground. In some ports, the largest ships can be less than a meter off the bottom, which requires arriving or departing at high tide. Conversely, the superstructure of a large ship must be able to pass under bridges. These photos show a cargo of large cranes passing under the San Francisco Bay Bridge at low tide. Clearly, you would not want to attempt this at high tide!

Picture of tidal flats at low tide.

Tidal fluctuations along a coastline with shallow, flat, bathymetry can significantly alter the distance to shore. As this photo shows, vast mudflats at low tide may render long stretches of coastline inaccessible for hours at a time. Such was the case when U.S. forces executed a surprise landing during high tide at Inchon, Korea in 1950. The landing would have been impossible during low tide due to extensive mud flats.

Pop-up Definition: Bathymetry
Bathymetry: The measurement of the depth of the ocean floor from sea level; the oceanic equivalent of topography.

LST unloading tanks while stranded at low tide during WWII.

Large tidal ranges can also leave ships marooned at low tide, such as during this tank unloading operation that occurred in World War II.

1.2 What is a Tide?

Marine environments affected by tides
Click to open animation in a new window.

As we just saw, changing water levels can dramatically affect coastal maritime operations. These changing water levels are caused by tides. NOAA defines tides as "the periodic rise and fall of a body of water resulting from gravitational interactions between Sun, Moon, and Earth."

Tidal currents refer to the horizontal motion of water associated with this rise and fall. We will not discuss tidal currents in this module. They will be addressed in the forthcoming module Introduction to Ocean Currents.

Pop-up Definition:
NOAA: National Oceanic and Atmospheric Administration

1.3 Important Terminology

Before we proceed to explaining tidal processes, we need to introduce some terminology.

First, an astronomical tide refers to the rise and fall of water due solely to gravitational interactions between the Earth, Moon, and Sun. These interactions are very periodic and predictable.

In comparison, a meteorological tide is the rise and fall of water due to wind and fluctuations in atmospheric pressure. Meteorological tides are only as predictable as the weather.

For the purposes of this module, the term tide alone can refer to either an astronomical tide, a meteorological tide, or to the combination of both effects (known as total tide) depending on the context. In scientific use tide usually refers to the astronomical tide, but in common usage it may refer to changes in water elevation resulting from any combination of astronomical and meteorological processes.

The astronomical forcing of tides leads to a periodicity in their rise and fall. Subsequently, we may classify tides based on their frequency as diurnal tides, semidiurnal tides, or mixed tides. A diurnal tide creates one high and one low tide per day. A semidiurnal tide results in two high and two low tides each day of nearly equal magnitude. Finally, a mixed tide is a semidiurnal tide with conspicuously unequal high and low water elevations between successive tidal cycles.

A tidal day is 24.84 hours (about 24 hours and 50 minutes). The reason the tidal day is longer than 24 hours is that the Moon orbits the Earth in the same direction that the Earth spins. In the 24 hours it takes the Earth to complete a 360-degree rotation, the Moon has moved slightly along its orbit. Thus the Earth has to spin a little farther (12 degrees) before the same location on Earth faces the Moon again.

Other key terms used to describe tides include flood, ebb, and slack water. Flood refers to the process of tidal currents entering a bay or river, while ebb is used to describe the tidal currents exiting from a bay or river. Slack water occurs between flood and ebb when the tidal current speed falls to near zero.

Last, when working with tidal information it is important to know that mean high water is defined as the average water elevation at high tide over a 19-year tidal epoch and similarly, mean low water is the average water elevation at low tide over a tidal epoch. A tidal epoch is the 19-year period required for a full cycle of the principal tide-producing forces.


1.4 Tide-Related Datums

Conceptual representation of low water tidal datums.

When we speak of tidal heights or read water depths on a nautical chart, the values are all given with respect to some particular tidal datum. The datum is the average over a tidal epoch of water height at a particular tidal phase. For example, common low water datums include mean low water (all low tides), mean lower low water (the lowest low tide in a given tidal day), and mean low water springs (low water during spring tides, discussed later in the module). Corresponding high water datums also exist.

Datums are referenced to surveyed points on land known as benchmarks, which you may have seen staked in the ground. Because tides vary so much depending on local conditions, all tidal datums are local and should not be extended very far. For example, the tidal range at the mouth of a bay and the tidal range at the head of the same bay may differ by several feet. As a result, the respective mean lower low water datums will differ significantly when referenced to the same benchmark.

Portion of nautical navigation chart in vicinity of Baltimore harbor.

The depths on a navigational chart are given with respect to a chart datum. Depending on the locality, the chart datum may be mean low water, mean lower low water, mean low water springs, lowest astronomical tide, or another tidal phase. Knowing whether the datum used on your navigation chart matches the datum of your tide prediction could mean the difference between clear sailing and running aground!

In the U.S. and related territories the standard chart datum is mean lower low water. For example, the numbers scattered across this nautical chart indicate the water depth at that location at mean lower low water. Clearly it is very important to know which datum is used for any given locale, along with the units for the depth. This information can be found in the chart legend.

1.5 Quantifying Tides

conceptual graphic depicting Spring-Neap range between MLWS and MHWS.

So just how big are tides? Tidal ranges vary from negligible to greater than 10 meters and depend upon the location.

We define tidal range in several ways: Most simply, the tidal range is the difference in height between consecutive high and low waters. The mean range is the difference in height between mean high water and mean low water. The diurnal range is the difference in height between mean higher high water and mean lower low water.

Tidal ranges depend on many factors including the phase of the Moon, the time of year, and local bathymetry.

Global map centered on the Pacific Ocean with some global tide range information.

This map shows the range of astronomical tides around the world.

How would you characterize tidal ranges in the open ocean?
(Choose the best answer.)
a) Typically less than 1 meter
b) Typically less than 2 meters
c) Typically greater than 2 meters
d) Typically greater than 4 meters

The correct answer is b). Tidal ranges in the open ocean are typically less than 2 meters.

Close to shore, tidal ranges are much more variable. Which areas show the smallest tidal ranges?
(Choose all that apply)
a) High latitudes
b) Low latitudes
c) East coasts of continents
d) West coasts of continents
e) Coasts exposed to the open ocean
f) Enclosed basin or sea

The correct answer is f). The smallest tidal ranges (less than 0.5 m) tend to occur in enclosed basins like the Mediterranean Sea, Sea of Japan, and Caribbean Sea, or in the Arctic Ocean. There is no clear pattern with regard to high or low latitude or east or west coast.

Global map centered on the Pacific Ocean with some global tide range information.

For the largest tidal ranges, the pattern is even more variable. On the global map, the largest tides occur close to shore, but no clear correlation emerges with respect to latitude, coastal orientation, or bathymetry.

1.6 Tidal Datum Exercise

Choose True or False for each of the following statements.

A tidal datum is referenced to fixed benchmarks on land. [True]

A tidal datum is a local datum and should not be extended to areas with different bathymetry. [True]

A tidal datum known as mean lower low water is used throughout the world as the standard chart datum. [False]

A chart datum may be mean low water, mean low water springs, or lowest astronomical tide depending on the chart. [True]

Answer: Tidal datums are referenced to fixed points on land, should only be used in the immediate vicinity of where they were established, and have a variety of different reference points depending on where you are on the globe. Unfortunately, there is NO global standard for chart datum so users must be careful to know which tidal datum is used as the reference for any given chart.

Return to Top

2. Astronomical Tides: Earth-Moon System

2.1 Primary Forcing by Earth-Moon System

Tidal Forcing by Earth Moon System
Click to open animation in a new window.

Astronomical tides are forced by gravitational attractions among the Earth, Moon, and Sun, and are thus affected by the Earth's rotation and orbit and the Moon's orbit.

To begin, let's simplify things and assume that the Earth has no continents and does not rotate. As the Moon orbits the Earth, it exerts a gravitational pull, shown here in red. Because gravity decreases with distance, this gravitational pull is stronger on the side of the Earth closest to the Moon and weaker on the side of the Earth farthest from the Moon.

As the Moon orbits the Earth, the whole Earth-Moon system rotates. The axis of this rotation lies at the collective center of mass of the Earth-Moon system, which is offset from the Earth's center toward the Moon. As the Earth swings around this axis, a centrifugal force develops that is opposite and equal to the Moon's gravitational pull. Because the Earth is a solid mass, the centrifugal force is the same everywhere on the Earth, as shown by the yellow arrows in this animation. The centrifugal force is less than the gravitational force on the side facing the Moon, but is greater on the side facing away from the Moon. When we add the centrifugal and gravitational forces together on each side of the Earth, a net force results. This net force causes two water bulges: one toward the Moon and one away from the Moon.

2.2 The Earth-Moon-Sun System

Tidal Forcing by Earth-Moon-Sun System
Click to open animation in a new window.

Now, let's add the effects of the Sun. Although the Sun is much further away than the Moon, it is extremely large and it, too, exerts a gravitational pull on the Earth-Moon system and contributes to a tidal bulge. The same arguments that applied to lunar gravitational and centrifugal forces apply to the Sun. Consequently, we see tidal bulges oriented toward and away from the Sun just as we saw tidal bulges oriented toward and away from the Moon. However, as this animation shows, the solar forces that contribute to the tidal bulges are only about half as strong as the lunar forces

2.3 The Lunar Cycle

Tidal Forcing During a Complete Lunar Cycle
Click to open animation in a new window.

Now let's examine how lunar forces and solar forces interact as the Moon orbits the Earth. This animation shows the Earth traveling along its orbit around the Sun as the Moon orbits the Earth. We see that that twice per lunar orbit the Earth, Moon, and Sun line up and the gravitational and centrifugal forces add together to create larger tidal bulges. Also twice per lunar orbit, the solar and lunar forces act at right angles to each other, which reduces the size of the tidal bulge and increases the height of the minima. As we'll see later, these regular patterns lead to what we call spring and neap tides.

2.4 Visualizing Water Motion from Tides (Tractive Forces)

Visualizing Water Motion from Tides
Click to open animation in a new window.

The gravitational and centrifugal forces of the Moon and the Sun acting on the water do not actually lift the water off the Earth. The vertical component of these forces is miniscule compared to the inward pull of the Earth's gravity. However, the horizontal component of these forces is strong enough to slide water across the Earth's surface. This creates deeper and shallower water that we refer to as high tide and low tide, respectively. We refer to the horizontal components of the total force as tractive forces.

2.5 Effect of Earth's Rotation

Earth's Rotation Under Tidal Bulges
Click to open animation in a new window.

Now that we have tidal bulges resulting in deeper and shallower water, let's examine what happens when we introduce the rotation of the Earth. This animation looks straight down on the North Pole. As point A, located on the equator, passes under tidal bulges, what tidal pattern results? (Choose the best answer.)
a) Diurnal
b) Semidiurnal
c) Mixed

The correct answer is b) Semidiurnal. As the Earth completes a single rotation, we see a pattern of two high and two low tides of equal magnitudes.

Modeled and observed semidiurnal tide at Cutler Naval Base, ME

This ideal scenario creates a semidiurnal tide that might look like this on a tide chart, with two high peaks and two low peaks each tidal day.

2.6 Time Series Interpretation Exercise

Three-day tidal time series

Which kind of tide is shown in this time series of changing water elevation?
(Chose the best answer.)
a) Semidiurnal tide
b) Diurnal tide
c) Mixed tide*

Answer: The correct answer is c) Mixed tide. This three-day time series shows two peaks and two minima per day, which as first glance seems like a semidiurnal tidal pattern. However, because the two high tides peak at different heights and the two low tides reach different low elevation points, this pattern is best described as a mixed tide.

2.7 Lunar/Solar Forcing Exercise

Select the choice that best completes the sentence.

On average, the astronomical tide-producing forces of the Sun (the component of the gravitational force that affects Earth's ocean tides) is about _____________ that of the Moon.
a) double
b) equal to
c) ½
d) ¼


Tidal Forcing by the Earth-Sun-Moon System

The correct answer is c) ½. As represented in this figure, the Sun exerts about half the astronomical tide-producing forces on the Earth that the Moon does.

Return to Top

3. Other Astronomical Effects

3.1 Planetary Alignment: Spring Tides

Alignment of Earth, Moon, and Sun During Spring Tides

While the Earth's rotation under gravitationally induced water bulges creates daily tidal fluctuations, other astronomical effects create tidal variations on time scales of weeks, months, and even years. In this section, we'll discuss the three most important of those effects.

One significant contributor to non-daily variations in tide height results from the alignment of the Earth, Moon, and Sun. When the Earth, Moon, and Sun are aligned, on Earth we see either a full Moon or a new Moon. In this situation, the gravitational / centrifugal forces of the Sun and the Moon are aligned and add together. This creates a large tidal range; higher high tides and lower low tides than normal. We refer to tides occurring in this planetary alignment as spring tides.

3.2 Planetary Alignment: Neap Tides

Alignment of Earth, Moon, and Sun During Neap Tides

When the Moon rotates 90 degrees out of alignment with the Earth and Sun, the effects of the Moon and Sun no longer add together. Consequently, tidal range reaches a minimum; the high tides are lower than normal and the low tides higher than normal. We refer to tides occurring in this configuration as neap tides.

In the 29 days it takes for the Moon to complete a lunar cycle (full Moon to full Moon), how many spring and neap tides do we experience?
[1, 2, 3, 4] spring tides
[1, 2, 3, 4] neap tides.

During one lunar cycle, the Earth, Moon, and Sun are aligned twice: once at the new Moon and once at the full Moon. Thus, spring tides occur twice every 29 days. Similarly, neap tides occur twice in every lunar cycle, occurring when we see a quarter moon.

In Depth: Origin of the terms "spring tide" and "neap tide":
The "spring" in spring tide has nothing to do with the season spring.
Spring: from the Anglo-Saxon Springan, a verb meaning to spring, rise, swell, or burst, often applied to wells, springs, and streams.
Neap: from the Anglo-Saxon Nep, an adjective meaning "lacking" or "scanty."

A quarter Moon appears as a half disc, or half of the full Moon, and is perhaps more commonly know as a half Moon. The term quarter Moon comes from the fact that it appears about 7 days after the new moon, or a quarter of the way through a lunar cycle.

3.3 Spring and Neap Tides Example

Tide Time Series showing Elevation Changes as a Result of Neap and Spring Tides in the Philippines.

This one-month time series of water elevation shows the effect of the Moon's orbit around the Earth for a location in the Philippines during July. Click the graph where we see neap tides.

Neap tides occur during the first quarter Moon in the middle of the month and again during the last quarter Moon late in the month and have a range of less than half a meter. The spring tides occur during the new Moon and full Moon when the Earth, Moon, and Sun are aligned. Their tidal range exceeds a meter. Recall that the higher high tide and lower low tide associated with spring tides establish the MHWS and MLWS datums we introduced earlier.

Tide Time Series showing Elevation Changes as a Result of Neap and Spring Tides in the Philippines.

3.4 Lunar Declination

Monthly Cycle of Lunar Declination Changes
Click to open animation in a new window.

Another astronomical impact on tidal variability is caused by the shifting location of the Moon relative to the Earth's equator. This effect occurs because the Earth's spin axis is inclined to the plane of the Moon's orbit. In this animation, we have stopped the Earth's rotation. As the Moon orbits the Earth, its position in the sky oscillates from north of the equator to south of the equator. When viewed from Earth, the height of the Moon in the sky changes throughout each month. We refer to this apparent height of the Moon in the sky as lunar declination. One full lunar declination cycle requires 27 days: the time it takes the Moon to orbit the Earth. In that time, the Moon shifts from a position over the equator to as far as 28.5 degrees north of the equator and back again over approximately two weeks. Then the Moon moves south of the equator and back to complete the 27-day cycle.

In Depth: The Length of Lunar Cycles (aka Lunar Months)

How long is a lunar month? That depends on what you measure. There are several different lunar cycles and they all impact tides.

If we look at the Moon in relation to a distant star (as opposed to the Sun), we find that it takes the moon 27.32 days to orbit the Earth. Astronomers refer to this period as a sidereal month. This time almost exactly equals the time required for a full lunar declination cycle (known as a tropical month) and thus determines the cycle for tropical or equatorial tides.

However, when we look at the moon in relation to the Sun, we get a longer period: new Moon to new Moon requires 29.53 days. Astronomers refer to this as a synodic month. A synodic month is longer than a sidereal month because the Earth-Moon system is orbiting the Sun in the same direction as the Moon is orbiting the Earth. Therefore, it takes about 2.2 extra days for the Moon to return to its position between the Earth and Sun where the new Moon occurs. The synodic month determines the spring/neap tide cycle.

Last, when we examine the distance of the Earth to the Moon, we find a period of 27.55 days from perigee to perigee. Astronomers refer to this period as an anomalistic month and its length determines the cycle of perigean tides (discussed later in the module).

3.5 Lunar Declination: Equatorial Tides

Equatorial Tides that Occur with Lunar Declination over the Equator
Click to open animation in a new window.

Changes in lunar declination affect both the frequency and amplitude of tides. To illustrate this point, let's first start with the case of zero lunar declination with the Moon over the equator, and then look at what happens when we move the Moon to a position south of the equator.

As we see in this animation, when the Moon is over the equator, the tidal bulges are also centered over the equator. Locations at both high and low latitudes rotate "under" two high tides of equal height during the tidal day, resulting in a semidiurnal tide. In fact, in this idealized configuration, every location on Earth experiences a semidiurnal tide, though points at higher latitudes experience a smaller tidal range.

This geometric configuration of the Earth and Moon creates global tidal patterns known as equatorial tides. During equatorial tides, tides everywhere tend to have smaller diurnal inequalities. Diurnal inequality is the difference in height between successive high waters or successive low waters (or both).

3.6 Lunar Declination: Tropic Tides

Tropic Tides that Occur with Maximum Lunar Declination
Click to open animation in a new window.

However, when the Moon is at its furthest extent off the equator to either the north or the south, the Moon's gravitational pull creates tidal bulges that are also off the equator. We refer to tides that occur during this alignment as tropic tides.

When we compare tropic tides to equatorial tides, how do they compare?
Please complete the following statement, then click Done.

Under tropic tides, diurnal inequality [increases, decreases, remains the same] and
tidal range [increases, decreases, remains the same] relative to equatorial tides at the same location.

During tropic tides, diurnal inequalities tend to increase (the extreme being one high/one low for a tidal day). This effect is more pronounced at mid and high latitudes, and minimal at the equator. Also note that the range between the higher high and the lower low of the tidal day tends to increase. While the animation here ignores bathymetric and Coriolis effects, the general relationship still holds.

3.7 Impact of Earth-Moon Distance

Changing distance between the Earth and the Moon
Click to open animation in a new window.

The third primary astronomical effect that creates non-daily tidal variations is the changing distance between the Earth and the Moon. The Moon has a slightly elliptical orbit. At its farthest (apogee) the Moon is about 406,000 km to the Earth. At its nearest (perigee) the Moon is about 363,000 km from the Earth, or about 10% closer.

Will tidal range be greater at apogee or perigee?
a) Apogee
b) Perigee

The correct answer is b) Perigee. The smaller distance at perigee increases the gravitational pull, which leads to larger perigean tidal ranges relative to the smaller apogean tidal ranges.

Interestingly, the orbit of the Earth around the Sun is also elliptical, which causes a similar, but smaller, tidal range effect known as aphelion and perihelion tides. Greater tidal ranges are observed during perihelion when the Earth is closest to the Sun (generally during the first week of January).

Return to Top

4. Actual Tidal Variability

4.1 View of Constituents

Total Astronomical Tide and its Primary Constituents
Click to open animation in a new window.

The different astronomical forcing mechanisms discussed above combine to create the total astronomical tide. If we apply a mathematical technique called harmonic analysis to water levels observed over a long period, we can isolate the contribution from each mechanism. We refer to each isolated contribution as a tidal constituent. M2 accounts for the gravitational and centrifugal forces caused by rotation of the Earth-Moon system. The S2 represents the contribution from the Sun's gravitational pull. The N2 accounts for the changing distance to the Moon. The K1, O1, and P1 account for declination changes in the Earth-Moon-Sun plane. There are many additional, progressively weaker, astronomical constituents that contribute to the total astronomical tide that are not shown. When we add all the constituents, we arrive at the total astronomical tide.

This interactive graph shows an example of the total astronomical tide along with the individual contribution by some of the main constituents. To see a constituent, click its name. Note that this analysis only applies to one specific location. The relative contribution of each constituent varies by location.

In this example, which constituent exerts the greatest control on tides?
a) M2
b) S2
c) N2
d) K1
e) O1
f) P1

The largest constituent by far is the M2, which is fairly typical. However, the relative magnitude of the remaining constituents varies tremendously from location to location.

4.2 Other Sources of Variability

Western Pacific Eastern Asia Topographic Map

Based on the discussion to this point it is clear that tide characteristics vary with time and location, but the forcing mechanisms are regular and predictable. In addition to the primary astronomical forcing of tides, a great deal of variability in tides is caused by bathymetric effects and the Coriolis force. To further understand these effects, we must consider the wave-like nature of tides.

4.3 Viewing Tides as Waves

Tidal Wave in Chesapeake Bay
Click to open animation in a new window.

Tides are essentially very long waves with wavelengths on the order of hundreds or thousands of kilometers in the open ocean. Viewed over large areas for several hours, they can look similar to ordinary ocean waves that roll up on the beach every ten seconds or so. The crest of the tidal wave is actually high tide, and the trough of the wave is low tide. In estuaries and bays a tidal wavelength is still long—tens to hundreds of kilometers—but shorter than typical open-ocean wavelengths. This animation shows a tidal wave in Chesapeake Bay. The waving blue image shows water levels, while the static image on the bottom depicts bathymetry. Note the wave-like propagation of the tide up the bay over a period of several hours.

4.4 Tidal Waves in the Open Ocean

Position of High Tide and Low Tide Trough During One Tidal Day
Click to open animation in a new window.

Viewing these waves of high and low tide over an ocean basin, we see a very complicated pattern. This animation shows cotidal lines for the principal lunar tidal constituent (M2) in the North Atlantic. The red line connects the crests of the tidal wave, while the blue line connects the troughs of the tidal wave. Intermediate lines represent tidal stages that fall between high and low tide. The location where the lines meet in the center is known as an amphidromic point—a location where the tidal range is zero.

In this animation, T0 occurs when the Moon crosses over the Greenwich Meridian. As the clock advances in 2-hour intervals, the locations of the high and low tide sweep around the northern Atlantic basin in a counterclockwise motion. In some parts of the world, the tidal wave rotates around the ocean basin, as shown here. In other locations, such as in the South Atlantic and equatorial region, the tidal wave propagates northward with little rotation.

4.5 Tide Wave Interaction Near Coastlines

Animation showing the slowed progression of a tide wave as it encounters shallow waters.

Several important things occur when tidal waves move from deep water in ocean basins to shallow water on continental shelves. Local effects induced by wave propagation in shallow water tend to overwhelm the astronomical forces. Consequently, the tidal wave progression on the continental shelf results mainly from the landward propagation of the open ocean wave. This type of wave is known as a "free wave," whereas in deep water, the wave is more of a "forced wave." This process is akin to the continued propagation of a wind-forced wave after the wind dies.

Looking at this conceptual graphic, what happens to the speed of the tidal wave as it encounters shallower water? (Choose the best answer.)
a) It increases
b) It decreases
c) It stays the same

The correct answer is b) It decreases. As the tidal wave encounters shallower water depths, the propagation speed slows down and tide crests move closer together. This is why the times of high (or low) tide can be tens of minutes or even hours apart at different locations in the same bay. As the tidal wave propagates landward and slows, the tidal range often, but not always, increases.

4.6 Meteorological Effects on Tides

Astronomical and Meteorological Contributions to Total Tide
Click to open animation in a new window.

In addition to astronomical tides, wind and fluctuations in atmospheric pressure also cause changes in water elevation. We refer to these changes as meteorological tides, or wind tides. We determine the meteorological tide by simply subtracting the predicted astronomical tide from the observed tide. The range of meteorological tides is typically less than a meter. However, in regions of low astronomical tides, the meteorological tides can dominate the total tide.

This interactive graph shows one example of total, astronomical, and meteorological tides. Click the buttons to reveal each tide type. In this case, the meteorological and astronomical tides have nearly equal ranges. Note that some of the highest high and lowest low total tides occur just after 12/17/00. This is a period of neap tides, but strong meteorological tides also contribute to the total tide.

4.7 Wind Characteristics and Meteorological Tides

Short Duration Wind Events
Click to open animation in a new window.

Short-term fluctuations in the wind on the order of hours do not usually impact sea elevation. Typically, winds must blow in the same general direction for a substantial time period—a day or more—to cause significant elevation changes.

In mid and high latitudes, meteorological tides are more pronounced in winter when the storm systems have stronger winds and larger pressure fluctuations. However, tropical storms will produce the greatest effect.

When discussing meteorological tides we refer to water rising at the coastline as "set-up." Likewise, we refer to falling water levels as "set-down."

In Depth: Seiche

4.8 Meteorological Tides in Bays: Local Versus Open Ocean Winds

Non-Local Wind Effects on Water Level in Bays
Click to open animation in a new window.

Sea level in a bay often responds more to winds over the continental shelf than it does to wind over the bay. The limited fetch in a bay restricts the amount of set-up or set-down. Meanwhile, wind over the continental shelf can cause a set-up or set-down at the entrance to the bay or estuary. The water elevation in the bay then rises or falls accordingly. Thus, local winds are not necessarily good predictors of the wind tide.

Operationally, if you expect winds over the continental shelf to create "set-up," then you should expect water levels to be higher than predicted. Conversely, if you expect winds over the continental shelf to generate "set-down" conditions, the water levels may be lower than predicted.

We will discuss this issue in a forthcoming module, Introduction to Ocean Currents.

Additionally, winds blowing parallel to a coastline can cause set-up or set-down due to Coriolis effects turning the water toward or away from the coast.

[1] We discuss this issue in the module, Introduction to Ocean Currents.

[2] We will discuss this issue in a forthcoming module, Introduction to Ocean Currents.

4.9 Impact on Tides Exercise

Which of these phenomena has the largest impact on tide elevations?
(Choose the best answer.)
a) Winds over the continental shelf
b) The Moon and Sun
c) Local bathymetry
d) It depends

The best answer is d) It depends. For any given time and location meteorological events such as the extreme winds with a cyclone, astronomical forcing by the Moon and Sun, or the local bathymetry could be the dominant factor in controlling tidal elevations.

Return to Top

5. Tide Prediction Methods

5.1 Historical Measurements from Tide Gauges

San Francisco tide house with gauge

There are two methods for predicting tides: working from historical tide gauge measurements and using dynamic circulation models.

Thousands of tide gauge stations around the world have measured sea level for durations of weeks to years. This photo shows a tide gauge station near San Francisco that began observations in 1854. It has produced the longest continuous tidal record in the Americas. Once the tide level has been monitored more or less continuously for a tidal epoch, the contributions of the different astronomical tidal constituents can be determined mathematically. The astronomical sea level can then be predicted at any time in the future at that location. Of course, only the astronomical tide, not the meteorological tide, can be predicted using this technique. Note that tide predictions obtained this way are only valid near the location of the tide gauge. Bathymetric effects can result in significantly different tides even relatively close by.

5.2 Tide Tables

Tide Tables
Click to open animation in a new window.

Reference tide elevation tables based on observations (commonly referred to as tide tables) have been published for decades, even centuries, for many locations with tide gauges. These publications have traditionally been available to mariners in hard copy format. Several government agencies produce tide tables, including NOAA and the British Admiralty, as well as private industry.

Many NOAA and NGA navigation charts also contain tidal range information for the local area of the chart.

Pop-up Definitions:
NOAA = National Oceanographic and Atmospheric Administration (
NGA = National Geospatial-Intelligence Agency (

5.3 Digital Tide Tables

Example of TotalTide software

Sample products from XTide software

There are also many digital versions of tide tables based on gauge observations. These include the NAVOCEANO Geophysical Fleet Mission Program Library (GFMPL), Admiralty TotalTide, XTide, and many others, not all of which are publicly available. In addition, many Websites provide predictions of tides for ports and harbors.

5.4 Dynamic Circulation Models

Dynamic Circulation Models
Click to open animation in a new window.

Dynamic circulation models can also predict tides. These numerical models can offer several advantages to predictions derived from tide gauge observations:
1. Models provide tide predictions for locations far from tide gauges,
2. Models can include meteorological tides caused by wind and other atmospheric effects, and
3. Models also calculate currents.

5.5 Model Availability

Forecast Water Levels for Chesapeake Bay
Click to open animation in a new window.

Circulation models can be used to generate many custom and highly detailed products for both tides and currents. In the U.S., NOAA runs several operational circulation models in estuaries and bays. For example, this animation depicts predicted water elevations from NOAA's Chesapeake Bay Operational Forecast System (CBOFS). NOAA runs similar models for San Francisco Bay, New York Harbor, and Galveston Bay. The Navy also runs operational models for tides, but these are generally not available to the public.

However, predictions from historical measurements are usually considered "ground truth" for models. In fact, at the location of an actual tide gauge, the historical analysis approach provides a better prediction of astronomical tidal behavior than physics-based circulation models. The model predictions depend on input from regional or global circulation models, and bathymetry. These errors in the input data degrade the accuracy of the circulation models.

Circulation models will be discussed further in a forthcoming module, Introduction to Ocean Currents.

5.6 Tide Prediction Products

3-day tidal time series prediction for St Thomas-de-Kent

Whether from tide gauge data, or a model, the most common tide prediction product is a time series chart for a given location that shows changes in water elevation with time.

High and low tide table for Saint-Thomas-de-Kent

Tide tables frequently present only the elevations and times for high and low tide. For other times, the user must interpolate between the highs and lows. This is especially true when using gauge data as the source.

Hourly Predicted Tide Heights for Saint-Thomas-de-Kent

Other products may present tide predictions in table form with hourly or finer time increments. Model-derived products typically use time increments of an hour or less.

5.7 Choosing the Best Tide Prediction Product

Time series graphs showing Forecast and Observed Winds and Tides at Chesapeake Bay Bridge, 29 Apr - 1 May, 2006

If tide products are available from both a circulation model and a tide gauge for a particular location, the tide gauge product will provide a more accurate prediction for the astronomical tide. However, if the weather forecast leads you to expect a significant meteorological tide and your circulation model includes meteorological forcing, then the model may provide a more accurate prediction. This graph shows the observed and forecast winds and tides at the mouth of Chesapeake Bay. Sustained winds from the north-northeast apparently resulted in set-up, with water elevations about a foot higher than the level of the astronomical tides. The CBOFS model nicely simulated this feature.

Unfortunately, circulation models in coastal regions are typically limited to the forecast duration of the meteorological model, usually two to five days. If you need a longer-range forecast, astronomical predictions from tide gauges can be extended to any time in the future. Finally, if you require a prediction for a location far from a tide gauge, then you should use a model. Determining how far away from a gauge one can safely extrapolate tide predictions requires considerable local experience.

5.8 Truth About Tide Predictions Exercise

Why are numerical models used to predict tide elevations?
(Choose all that apply.)
a) Predictions from gauge measurements cannot be accurately extrapolated to other locations*
b) Historical observations from a gauge cannot be used to predict the tidal components that result from astronomical forcing
c) Numerical models can include the impact of wind and other meteorological effects*

The correct answers are a) and c). To get accurate tide predictions for locations far from a tide gauge it is necessary to use a model. Forecasts based on gauge observations cannot be extended very far from the gauge location. Also, circulation models, when coupled with atmospheric models, are able to predict the meteorological impact on tide elevations. Answer b) is not true. When harmonic analysis is applied to tide gauge records of sufficient length, astronomical tides can be predicted well into the future.

Return to Top

6. Summary

Key points to remember about ocean tides are:


Tidal Datums

Classification of Tides

Astronomical Tides

Secondary Astronomical Effects

Meteorological Tides

Predicting Tides

Module Quiz
User Survey
Return to Top
Return to Module Home