The most familiar evidence of the tides along our seashores is the observed recurrence of high and low water - usually, but not always, twice daily. The term tide correctly refers only to such a relatively short-period, astronomically induced vertical change in the height of the sea surface (exclusive of wind-actuated waves and swell); the expression tidal current relates to accompanying periodic horizontal movement of the ocean water, both near the coast and offshore (but as distinct from the continuous, stream-flow type of ocean current).
Knowledge of the times, heights and extent of inflow and outflow of tidal waters is of importance in a wide range of practical applications such as the following: navigation through intracoastal waterways, and within estuaries, bays and harbors; work on harbor engineering projects, such as the construction of bridges, docks, breakwaters, and deep-water channels; the establishment of standard chart datums for hydrography and for demarcation of a base line or "legal coastline" for fixing offshore territorial limits—both on the sea surface and on the submerged lands of the Continental Shelf; provision of information necessary for underwater demolition activities and other military engineering uses; and the furnishing of data indispensable to fishing, boating, surfing, and a considerable variety of related water sport activities.
Tides Online is offered freely over the Internet to ensure that, across the nation, users have easy access to these important data. For the majority of the tide gauges, water level values are updated hourly. However, during severe weather conditions, such as hurricanes, specific gauges located along the projected path of a storm will be "triggered" to transmit data every 20 minutes. Tides Online detects these stations and lists them on the main page for easy access to the data.
When gauges have not been "triggered," users may navigate to their station of interest by clicking on the "State Maps" or "Regional List" options on the main menu. Click here to see the "State Maps" option with the states that have tide gauges highlighted in yellow. Clicking on the state of interest zooms the user down to a level where a gauge may be selected. For example, click here to see the tide gauges in Virginia. Now click on the tide gauge station listed as "Chesapeake Bay Bridge Tunnel, VA" to see current water level, wind, air pressure and temperature, and water temperature information for this location. It probably looks quite different from the data gathered from the same tide gauges during the height of Hurricane Floyd in September 1999 (See graphic above right).
Tides Online has a wide variety of users, including recreational boaters, commercial fisherman and television weather broadcasters.
Just like Tides Online, Internet users may choose their water level station of interest by selecting the "State Maps" or "Stations List" option on the main menu, or selecting one of the stations specifically identified during a severe storm. To access this information via phone, dial (301) 713-9596 and follow the directions below.
Many people like these products because they're easy to use and are updated continuously. Why not try them on your next fishing trip or visit to the beach!
Uses for water level data range from hydrographic purposes for making nautical charts to absolute global sea level monitoring for better understanding the circulation of the ocean and its role in world climate. NOAA Ocean Service's modern water level measurement system allows for a variety of real-time, near real-time and long-term applications. Real-time applications include hydrography, nautical charting, maritime navigation and tsunami warnings. Near real-time applications include circulation surveys as well as hydrography and storm surge warnings. Long-term applications include marine boundary determinations, tide predictions, monitoring long-term sea level trends, coastal construction, oceanographic research, climate research and absolute sea level monitoring.
Relevant Web Sites
National Water Level Observation Network (NWLON) gauges
Center for Operational Oceanographic Products and Services (CO-OPS)
Media Contact:
July 3, 2002 — The word "tides" is a generic term used to define the alternating rise and fall in sea level with respect to the land—produced by the gravitational attraction of the moon and the sun. To a much smaller extent, tides also occur in large lakes, the atmosphere and within the solid crust of the earth, acted upon by these same gravitational forces of the moon and sun. Additional non-astronomical factors such as configuration of the coastline, local depth of the water, ocean-floor topography, and other hydrographic and meteorological influences may play an important role in altering the range, interval between high and low water, and times of arrival of the tides.
NOAA's Tides Online is a Web-based product, which provides users with the latest graphical and tabular water level observations, predictions and meteorological data in real-time or near real-time from 175 National Water Level Observation Network gauges in the United States, including Alaska, Hawaii and other U.S. territories. NWLON, run by NOAA Ocean Service's Center for Operational Oceanographic Products and Services (CO-OPS), serves as one of the "backbones" of NOAA's coastal ocean observing system by supplying long-term, sustained measurements of tides and water levels. Some of the records contain data spanning more than 1.5 centuries, making them some of the longest geophysical records in the United States. The NWLON system serves as one of the fundamental observing system platforms for the NOAA tsunami warning system and it provides NOAA Weather Service with water level information needed for storm surge monitoring. Furthermore, it provides reference datums for all marine boundary applications; national shoreline and nautical chart products; and coastal construction, dredging, surveying and mapping. Lastly, it also serves as the infrastructure for implementation of NOAA's Physical Oceanographic Real-Time Program® (PORTS®), a decision-support tool that allows the marine community to immediately access water levels, oceanographic and meteorological data needed for safe navigation.
Great Lakes Online is also offered by NOAA to provide users with the latest information from NOAA's water level observing network in the Great Lakes. Real-time water level and meteorological data are provided to users via the Internet or an interactive telephone response system.
Click here to see the "State Maps" option with the states that have water level stations. Selecting the state of interest presents the user with a detailed view of station locations. For example, click here to see the tide gauges Michigan. Now click on the tide gauge station listed as "S.W. Pier, MI" to see current water level, wind, air pressure and temperature, and water temperature information for this location. To access NOAA's Great Lakes Online Interactive Voice Response System, dial (301) 713-9596, enter the # key and the 4 digit code to select a specific station (See graphic to the right).
WHY MEASURE WATER LEVELS?
NOAA's Tides Online