Frequently Asked Questions - Tide Predictions and Data

There are any number of web sites that have tide predictions on them. Due to the rapid growth of the World Wide Web, we do not attempt to keep up with the other sites that have tide predictions on them; nor do we verify the accuracy of their predictions. Caution should be exercised when using tide predictions from other websites. Some obtain their tide predictions from our office; others obtain their predictions from sources that may or may not be accurate or up to date.

Q: What is the difference between "Tide Charts" and "Tide Tables"?

"Tide Charts" and "Tide Tables" are actually two different products.

"Tide Charts" refer to a series of maps that show the water levels throughout a bay or estuary at a particular point in time. These maps normally show the water levels on an hourly basis after high tide. "Tide Charts" are only available for a few locations around the USA.

"Tide Tables" refer to tidal predictions tables that show the daily high and low tide predictions for a particular location. "Tide Tables" can be generated for more than 3,000 locations around the USA. For more information on obtaining "Tide Table" predictions, please read this Frequently Asked Question.

Q: What is the relationship between "Tides" and "Tidal Currents"?

The vertical rise and fall of the tides, created by the gravitational force of the Moon and Sun acting on the oceans water, also creates a horizontal motion of the water in the bays, harbors and estuaries. These are tidal currents. In general, as the tides rise there will be a current flowing from the oceans into the bays, harbors and estuaries; this is termed a "flood current". As the tides fall there will be a current flowing towards the oceans; this is termed an "ebb current". There are also periods when there is little or no horizontal motion of the water; this is called "slack water".

Many professional and recreational users of tide and tidal current information have a "rule of thumb" to assume a relationship between the times of high/low tides and the times of the currents. That the times of slack water will be at the same time as the high and low tides, and that the flood and ebb current will occur between the high and low tides. Unfortunately, this assumed "rule of thumb" does not hold for most locations.

The relationship between the times of high/low tide and the times of slack water or maximum current is not a simple one. There are three "base case" conditions. The first is a "standing wave" type of current. In a standing wave the times of slack water will be nearly the same time as the high and low tides, with the maximum flood and ebb current occurring mid way between the high and low tides. The second is a "progressive wave" current. In a progressive wave, the maximum flood and ebb will occur around the times of the high and low tides, with the slack water occurring between the times of high and low tide. The third case is a "hydraulic current". In a hydraulic current, the current is created by the difference in height of the tides at two locations joined by a waterway. The current will be at its maximum flood or ebb when the difference in the two heights are the greatest. The slack water will occur when the height of the tide at the two locations in nearly the same.

Hydraulic currents occur at a limited number of locations. Some examples would be:

Progressive currents are most common at the oceanic entrance to many bays and harbor. Standing wave conditions are most common at the head (most inland point) of larger bays and harbors. Most areas of the coast will fall somewhere in between a progressive and standing wave current. [Example (jpg)] The exact relationship between the times of high and low tides and the maximum current or slack water is unique to each location and cannot be determined from a generic "rule of thumb".

Because the tidal currents are created by the same forces which cause the tides, the currents can be predicted in much the same way as the tides. Observational data on the currents at a location can be analyzed using the same methods employed to analyze tides, and the results of that analysis can be used to generate predictions of tidal currents. However, because the relationship between tides and tidal currents is unique to each location, tide predictions and tidal current predictions are generated separately.

It is up to the user to insure that they are using the correct type of predictions for their activities.

Q: What are the "Perigean Spring Tides"? Do they cause coastal flooding?

The question of "Perigean Spring Tides" has been one that our office has been required to answer on numerous occasions over the years. The whole issue of coastal flooding during the "perigean spring tides" stems from a misunderstanding of the information presented in a book by Fergus J. Wood, published in 1978.

The moon is the primary source of the gravitational forces which cause the tides. The proximity of the moon in relation to the earth does have an effect on the range of the tides at any given time. In each of its 28-day elliptical orbits, the moon reaches a "perigee," its closest point of approach to the earth. During these periods, there will be a slight increase in the average range of tides. The increases in the range of the tides is seen by a slightly higher than average high tide, as well as a slightly lower than average low tide. Additionally, twice each month, around the times of the new moon and full moon, when the earth, sun, and moon are nearly in line, there is an increase in the average range of the tides. These are called "spring tides." Three or four times a year, the occurrence of a new or full moon will coincide with the "perigee" of the moon, which Mr. Wood has termed the "perigean spring tides".

The difference between the "perigean spring tides" and the normal tidal ranges for all areas of the coast is small. In most cases the difference is only a couple of inches. The largest difference occurs in certain areas of the Alaska coast where the range of the tide was increased by approximately 6 inches. But considering that these areas have an average tidal range of more than 30 feet, the increase is but a small percentage of the whole (less than a 2% increase).

Mr. Woods' book examines the occurrences of coastal flooding though history. What he discovered is that coastal flooding did occur when there was a strong onshore wind, such as a hurricane or nor'easter, which occasionally occurred at the same time as a "perigean spring tide."

The problem has been that a number of people have misinterpreted the information presented in this book to mean that coastal flooding would occur whenever the "perigean spring tides" occur. This has led to articles published in various media sources that incorrectly predict widespread coastal flooding at the times of the "perigean spring tides," causing needless concern.

Most people who live along the coastline know that coastal flooding can occur whenever there are strong onshore winds, whether there is a "perigean spring tide" or not. Additionally, this flooding will be worse if the storm strikes around the time of high tide rather than around the time of low tide.

But in ALL cases, it is the storm winds which cause the coastal flooding, not the tides. Coastal flooding is the result of meteorology (the weather) not astronomy (normal tidal fluctuations). All astronomical considerations are accounted for in the NOS tide and tidal current predictions.

Q: Are there tides in the Great Lakes?

The water levels of the Great Lakes have long-term, annual, and short-term variations. Long-term variations depend on the precipitation and water storage over many years. Annual variations occur with the changing seasons. There is an annual high in the late spring and low in the winter. These changes occur at a rate that can be measured in feet per month.

True tides, changes in water level caused by the gravitational forces of the sun and moon, do occur in a semi-diurnal pattern on the Great Lakes. The investigations of the U.S. Coast and Geodetic Survey indicate that the spring tide, the largest tides caused by the combined forces of the sun and moon, is less than 2 inches (5cm) in height. These minor variations are masked by the greater fluctuations in lake levels produced by wind and barometric pressure changes. Consequently, the Great Lakes are considered to be essentially non-tidal.

Other factors may create quite large fluctuations of lake levels, but for only short periods of time lasting from minutes to several days. The two most dramatic are storm surge and seiche.

Storm surge is a sudden, and often unexpected, rise in lake level associated with a rapid change in atmospheric pressure, a pressure jump, or a storm wind over a short period of time. If high winds are steady for a long period ,(e.g.,several hours), a change in water level will occur known as wind set-up. Once the winds cease or shift direction, a standing wave called a seiche may occur. This is very similar to the wave created by sloshing water in a bathtub. The seiche will cause lake surface levels to oscillate from higher to lower with decreasing amplitude over a period of several days. It is this oscillating water level that is most often confused with a tide because the period of the oscillation in some of the Great Lakes is very similar to the period of the tides.

In summary:
Water levels in the Great Lakes change not because of the tides, but because of meteorological effects imposed on the longer-term changes in the amount of water in the lakes.

For present and historic levels in the Great Lakes, see the Great Lakes Water Level Data or Historic Great Lakes Water Level Data. You can also visit Great Lakes Online, a separate section of our website dedicated to the Great Lakes.

For additional information on lake level changes, see the US Army Corp of Engineers Great Lakes Hydraulics and Hydrology webpage.

Q: Where can I find information on the 100-year, 50-year, 10-year tide levels?

Although these values are often referred to as "tide levels" for coastal areas that experience tides, they are more appropriately associated with flood heights and not the tides. Observational data from our office is often used to determine these values, but they are derived values that are developed and published by other agencies.

Some sources for this information include:
Federal Emergency Management Agency (FEMA)
US Army Corp of Engineers

Q: I am trying to set my clock/watch to predict the tides. How can I find information to help me?

NOAA National Ocean Service does not produce any types of tide clocks or watches. The manufacturers of such products often refer their customers to NOAA for the information needed to properly set their clocks or watches. There are two type of information which have been requested in setting tide clocks and watches; Lunitidal Interval and Highest Tide.

The Lunitidal Interval is the average time difference between when the moon passes over a time meridian and the following high or low tide. If the time meridian referenced is the local time meridian, then the lunitidal intervals are called the "Local Intervals." If the time meridian referenced is the Greenwich meridian, the lunitidal intervals are called the "Greenwich Intervals." Generally, the manufacturers of such items use the Local Intervals for their calculations. Our office does not track or maintain the Local Intervals; we calculate and track the Greenwich Intervals for various locations. This information can be found in the Datums section of our website.

The Greenwich Intervals we provide can be converted to the Local Interval using the following formula:

Local Interval = GI - (0.069 * L)

The Highest Tide normally refers to the highest tide for a particular year. For practical purposes, there is no single "highest tide" for a year. Depending on the location, there will be between 2 and 8 days during the year when the high tide is at it highest predicted height for that year (with a precision of 0.1 foot). We do not attempt to track which day(s) this will occur on as the date and time may be different for each location. You may determine the date of the "highest tide" for your uses from the High/Low Tide Predictions section of our wesbite. After selecting the station you are interested in, you will be provied the times and heights of the high and low tides for that location through the entire year. In general, the highest high tides and lowest low tides occur around the dates of the New Moon and Full Moon; these are known as the "spring tides". Dates of the New Moon and Full Moon can be found in the Astronomical Data section of our website.

Q: How can I find information on the range of the tides?

Within the "Water Level Observations" section of our website you will find a heading labeled Accepted Datums. In this area you will be able to obtain datum values for water level stations. The two commonly used tidal ranges are listed for each station. These ranges are the Mean Tidal Range (MN) and the Diurnal Tidal Range (GT).

Q: Where are the highest tides?

Below is a listing of 50 locations from around the world where the largest range of tides that have been observed.

Q: It appears that the range of the tides gets larger the further the location from the equator. What causes this?

If you only consider a few locations, this may seem to be true; but it is not the case. Literally hundreds of locations in the extreme north and south latitudes have small tidal ranges, and a number of stations closer to the equator have large tidal ranges. For example, Korea is at roughly the same latitude as most of the U.S. coast. Numerous stations in Korea have a tidal range of more than 20 feet, most of the U.S. coast has a tidal range of 3-6 feet. A number of locations very near the equator which have a tidal range of 15-17 feet.

The tidal range of a particular location is dependent less on it position north/south of the equator than on other physcial factors in the area; topography, water depth, shoreline configuration, size of the ocean basin, and others. For example, let's consider the southern coast of Alaska and British Columbia. The configuration of this coastline is very similar to a funnel, with the narrow end at Cook Inlet. The tides travel as a "wave" across the oceans, and in many other respects act as a "wave"; this type of configuration tends to accentuate the "wave" at the narrow end of the funnel. This is part of the reason for the large tidal ranges, 30+ feet, in the area of Cook Inlet. If you look at the tidal ranges for stations on the Bering Sea, outside this funnel but at the same latitude, you will find a tidal range of 5-7 feet.

Another factor is the size of the ocean basin in which the tides are located. The Pacific Ocean is by far the largest of the world's oceans. This means that the moon's gravity has a large area of water to work on; therefore, its effect tends to be greater. Compare this to stations in the Baltic Sea. Many stations in the Baltic are as far north as any station in Alaska. However, due to the small size and shallow depth of the ocean basin, the tidal ranges in this area are relatively small.

Q: Are the stations presented on your website the only ones that tide predictions are available for?

The list of stations available from the High/Low Tide Predictions section of our website includes all of the stations that we can provide predictions for along the U.S. coast. This list include presently operating and historically observed tidal stations. This list of stations is updated annually to include any newly observed or revised stations.

The list of stations available from the Tides Online section of our website includes all of the tidal stations that are presently being observed.

Q: Last year I was able to get tide predictions for a specific station, but this year it is not listed. What happened?

We are in the process of a comprehensive review of all tide predictions stations. This process started several years ago in early 2003 and will be continuing for the next few years. We started the review in Maine and will be proceeding down the Atlantic Coast to Florida, across the Gulf Coast to Texas, then up the Pacific Coast from California to Alaska, and finally to Hawaii. This comprehensive review of all the listed stations is attempting to insure that the predictions provided are the best available. Annually, there will be numerous changes to the stations listed in the High/Low Tide Predictions section of our website as a result of this review. Each year there are a number of stations which are updated with new information, some new stations are being added, and some historic stations are being removed. Between 2007 and 2008, more than 400 stations were updated, added or removed.

Stations are updated when there is new data collected at the location. Stations are added when data is available for a previously unlisted location. Stations are removed when the data available does not meet todays standards for prediction. If a station you are familiar with is removed from our listing, we are no longer supporting tide predictions at that location. Please review the list for nearby stations for which predictions are available that may also meet your needs.

Q: How can I get tide predictions for next year or any year?

Daily tide predictions for the present calendar year are available for more than 3,000 locations around the USA in the High/Low Tide Predictions section of our website. The predictions presented in this section of our website are updated in late October or November to cover the next calendar year. For example, the predictions for 2009 will be added to the website in October or November of 2008. There are portions of the year when we have both the "old year" (2008) and the "new year" (2009) of predictions; each year of predicitons will have a different URL address. If a bookmark or website link is used to access predictions for a specific location, that bookmark/link will need to be updated to access the new year of predictions once they are available.

Daily tide prediction for a single calendar month, for dates outside the one year presented on our website, can be obtained via e-mail from: Tide.Predictions@noaa.gov. Subsequent requests for additional months of predictions will be subject to the fee for custom predictions described below.

Hourly tide predictions for any station that has accepted harmonic constituents, about 200 of the 3,000 listed in the "Predictions" section of our website, can be generated directly from the Tide Predictions page of our website. These predictions consist of either hourly heights or 6-minute interval heights of the tides and can be generated for virtually any date after January 1, 1800. The data can be provided graphically or in a spreadsheet format.

Daily tide predictions of more than one month in length (other than the annual predictions described above) are not available through the web or via e-mail response from our office. You can obtain long-term tide predictions on a calendar-year basis by calling our office at (301) 713-2815 between 7AM-3PM Eastern Time. You may also obtain tide predictions by submitting a request to our e-mail address at: Tide.Predictions@noaa.gov. If you elect to submit your request via e-mail, include the following information:

**** PLEASE NOTE ****
There is a fee for long-term tide predictions. This fee is normally $40 for a calendar year of predictions at one location; each additional year or location is $5. We are only able to make tide predictions for specific locations where NOS tide data has been collected. Refer to the High/Low Tide Predictions section of our website for a list of the available stations.
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Your Full Name
Your Full Mailing Address (NOT e-mail)
Your Phone Number (Including Area Code)
The type of predictions you want (Tides or Tidal Currents)
The location(s) for which you want predictions
The dates for which you want predictions
The format and options you want included (see next question below)
- AM/PM or Military Time (24 hour)
- With or Without Daylight Savings Time
- Standard, International, or Manuscript Format

Since Standard and Manuscript formats can be provided on hard-copy (paper), diskette or CD; when requesting either of these formats, please indicate which media you want provided. If no preference is listed, a paper copy will be provided.

The tide predictions will be sent to the mailing address you provide.

Q: What formats are tide predictions available in?

Tide predictions are available in several formats.

International Format - This format is an electronic ASCII file that provides time and height of tide information for a single location in a column-delimited format. This format is most suitable for importing into a database, plotting, or other computer program. The following example for EASTPORT, ME, 2005 shows the predictions in 24-hour clock and in Eastern Standard Time. This information can be modified to show AM/PM Time and Daylight Savings Time.

Standard Format - This format is available in hard copy and an electronic ASCII file that provides time and height of tide information for a single location in a page-readable format. The following example for EASTPORT, ME, 2005 shows the predictions in AM/PM Time and has been adjusted for Daylight Savings Time. The information can be modified to provide 24-Hour Clock (military time) and without adjustments for Daylight Savings Time.

Manuscript Format - This format is available in hard copy and an electronic PostScript file that provides time and height of tide information for a single location in a 3-month per-page format. This format includes additional information such as the day of week and phases of the moon. The following example for EASTPORT, ME, 2005. shows information in 24-Hour Clock and Local Standard Time. The information can be modified to show AM/PM Time and Daylight Savings Time.

Q: How accurate are the predictions?

The accuracy of the tide predictions is different for each location. Periodically, we do a comparison of the predicted tides versus the observed tides for a calendar year. The information generated is compiled in a Tide Prediction Accuracy Table. We work to insure that the predictions are as accurate as possible. However, we can only predict the astronomical tides; we cannot predict the effect that wind, rain, freshwater runoff, and other short-term meteorological events will have on the tides.

In general, predictions for stations along the outer coast are more accurate than those for stations farther inland; such as, along a river, or in a bay or other estuary. Inland stations tend to have a stronger nontidal influence; that is, they are more susceptible to the effects of wind and other meteorological effects than stations along the outer coast. An example of an inland station that is difficult to predict is Baltimore, Maryland. This station is located at the northern end of Chesapeake Bay. Winds that blow along the length of the bay have been known to cause water levels to be 1-2 feet above or below the predicted tides.

Stations in relatively shallow water, or with a small tidal range, are also highly susceptible to meteorological effects, and thus, difficult to accurately predict. At these stations, short-term weather events can completely mask the astronomical tides. Many of the stations along the western Gulf of Mexico fall into this category. An example is Galveston, Texas. This station is in a bay that is relatively shallow and has a small opening to the sea. At this station it is possible for meteorological events to delay or accelerate the arrival of the predicted tides by an hour or more.

Q: Where can I get tide predictions for locations outside the United States?

Due to an agreement through the International Hydrographic Organization, we receive annual tide predictions for countries outside the United States every year. Therefore, you can obtain tide predictions for many locations around the world for the present calendar year by contacting our office. There are some few locations outside the United States that we can make tide predictions for beyond the current calendar year, but in most cases you will need to contact the Hydrographic or Oceanographic Agency in that country.

NOTE: For countries that have a website, a link is also provided.

You can contact our office at (301) 713-2815 between 7AM-3PM Eastern Time. You may also request tide predictions by submitting a request to our e-mail address at Tide.Predictions@noaa.gov. If you elect to submit your request via e-mail, include the following information:

The tide predictions will be sent to the mailing address you provide.

**** PLEASE NOTE ****
There is a fee for long-term tide predictions. This fee is normally $40 for a year of predictions at one location; each additional year or location is $5.
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Q: How can I get a copy of the program that you use to make tide predictions?

Our office offers copies of the program that we use to make tide predictions for the NOS Tidal Reference Stations. Our office is not in the business of software development and distribution. The program we use was developed for our use and is not user-friendly. The program is written in FORTRAN and requires the use of a separate control file for each location that predictions are being made for. This control file must be altered each time you want to make tide predictions for different time periods. A copy of the tide prediction program is available from our office by calling (301) 713-2815 between 7AM-3PM Eastern Time. You can also submit a request for a copy of the program by sending e-mail to our address: Tide.Predictions@noaa.gov. If you elect to submit your request via e-mail, include the following information:

Your Full Name
Your Full Mailing Address (NOT e-mail)
Your Phone Number (Including Area Code)
Note in your message that you want the tide prediction program
The Location(s) you want control files for

**** PLEASE NOTE ****
There is a $54 fee for a copy of this program. The program comes with the control file for one NOS Tidal Reference Station of your choice and copies of the "Manual of Harmonic Analysis.." and "Computer Applications to Tides..." listed below. Harmonic constituents for additional stations are available at a cost of $3 each.
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Q: I want to develop my own program to predict the tides. What information can you provide to help me do this?

We can offer you a number of products/services:

(1) Our Restless Tides - This booklet describes the astronomical forces that cause the tides in terms that can be understood by the average student. (cost $5)

(2) Manual of Harmonic Analysis and Prediction of Tides (Special Pub 98) - This book describes the process of tidal analysis and the mathematics behind tidal predictions. The material covered in the book is fairly detailed, with illustrations, graphs, and tables to provide additional information. (cost $22)

(3) Computer Applications to Tides in the National Ocean Service (Supplement to Special Pub 98) - This booklet describes the methods that NOS uses to predict tides. The booklet contains sample algorithms for tidal data analysis and tidal predictions. (cost $15)

(4) Tide Prediction Package - The predictions package includes a copy of items 2 and 3 listed above. The package also includes a limited copy of the tide prediction program (NTP4) used by NOS to generate tide predictions. The program is a FORTRAN program and is not very user-friendly. The program requires the use of a control file containing the harmonic constants for each location that tides are to be generated for. This control file has to be altered each time that tides are being predicted for a different time period. Copies of the compiled and source code for the program are provided. The program only works for locations for which harmonic constants are available, These are the NOS tidal reference stations (those listed on our website). The package comes with a control file (harmonic constants) for one Tidal Reference Station of your choice. Please include your choice of station with your request. (cost $54)

(5) Tidal Harmonic Constants - Copies of the tidal harmonic constants are available for any of the stations for which these values have been calculated. These values can be obtained through the Harmonic Constants section of our website. These values can be provided in the format required by the NTP4 program listed above at a cost of $3 each.

(6) Table 2 Files - These tables list NOS secondary stations. Their tide predictions at these stations are generated by applying time and height adjustments to tide predictions. There are two Table 2 files for the USA:

- Tide Tables for the East Coast
- Tide Tables for the West Coast
(cost $74 each)

**** PLEASE NOTE ****
As bays and estuaries change due to natural and artificial processes (shoaling, erosion, dredging, construction), the tides in the area can be affected. The tidal harmonic constants and Table 2 corrections are subject to change as more recent data are received and analyzed.
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You can discuss your request or obtain any of the products listed above by calling our office at (301) 713-2815 between 7AM-3PM Eastern Time. You may also request these products by submitting a request to our e-mail address at Tide.Predictions@noaa.gov

If you elect to submit your request via e-mail, include the following information:

Your Full Name
Your Full Mailing Address (NOT e-mail)
Your Phone Number (Including Area Code)
The information you want

The information requested will be sent to the mailing address you provide, along with an invoice for payment.

Q: A few years ago I was able to obtain books with the Tide and Tidal Current Predictions for the entire U.S. coast from the Government Printing Office. I do not seem to be able to find them now. Are they still being produced by the Federal Government? How can I get one?

Due to reduced funding, NOAA's National Ocean Service (NOS) ceased printing and distributing the annual Tide and Tidal Current Tables. The role of NOS with regard to the publication of these Tables was redefined to be that of maintaining and updating the tidal predictions database from domestic and international sources and generating the annual predictions and associated information. Beginning with the 1996 edition, the actual printing, distribution, and sale of these Tables has been assumed by private-sector printers.

The titles of the NOS publications affected are:

Tidal Current Tables:

Many local publishers and printers around the country publish regional and localized tide and tidal current predictions in booklet, calendar, and other formats. The data printed in these local and regional publications is, in many cases, obtained directly from NOS. For availability of localized prediction tables, consult marinas and marine supply companies in your area. Since literally hundreds of such publications exist, we cannot list them.

As of this date, two publishing firms are printing complete East and West Coast tide and tidal current predictions for U.S. ports and coastal areas - ProStar Publications and North Wind Design & Production. As additional companies advise NOS of their publication of complete NOS tide and tidal current data for either the East and/or West Coast, they will be added here.