Tide tables are schedules that tell you when the water levels will rise and fall, a crucial resource for anyone who works or plays near the water.

Time waits for no one, and neither do the tides. Whether you're a child racing to finish your sandcastle or an experienced sailor, the Earth's oceans follow a rhythmic rise and fall every day. This phenomenon, caused by the gravitational pull of the moon and the sun on the Earth's surface, is known as tides. Understanding when these changes will happen is crucial for many, even the young beach architect.

Around the globe, government organizations monitor tides daily to predict the timing and intensity of the highest and lowest tides in different areas. This collected data is compiled into tide tables. Tides have minimal effect in the open ocean or lakes, so these tables are primarily used for coastal regions, including inlets, bays, and estuaries.

Boats, not surprisingly, depend on tidal data. Navigating through shallow ports, narrow intracoastal waterways, or under low bridges can be dangerous without knowledge of the tides. Depending on the location, the difference between high and low tides (also known as the tidal range) can exceed 49 feet (15 meters) [source: NOAA: Tides and Water Levels]. Underestimating this range could be disastrous for a sea captain guiding a ship under a low bridge.

Fishermen often rely on tidal patterns to boost their catches, as some fish species gather during high or low tides. Additionally, strong tidal currents can push fish into concentrated areas, and knowing when these currents will occur gives fishermen the opportunity for a successful haul.

Tide tables are also valuable for planning coastal construction, environmental restoration, and disaster recovery efforts. They're commonly used by people engaging in recreational activities, like surfers seeking the perfect wave.

On the next page, discover how a branch of NOAA creates tide tables and learn why their forecasts aren't always entirely precise.

Tide Table Predictions

Tidal heights result from a complex interplay of factors involving the positions of the Earth, moon, and sun. As the moon orbits Earth, Earth orbits the sun, and Earth rotates on its axis, the distances between these bodies and their relative positions are constantly changing. This variation in distance and alignment leads to fluctuations in tidal heights and strength of tidal currents.

For instance, when the moon and sun align with the Earth, their combined gravitational pull strengthens the tides. Conversely, when they form a right angle with Earth, their forces partially cancel out, resulting in weaker tides. The moon's gravitational influence is more powerful than the sun's, as the moon is 390 times closer to Earth, making its pull twice as strong as that of the sun [source: NOAA: Tides and Water Levels].

Although tidal patterns are mainly shaped by predictable astronomical forces, creating accurate tide tables isn't entirely straightforward. Various factors, such as ocean currents, wind patterns, the shape of the ocean floor, and land formations, also come into play. Ocean currents and wind direction can amplify or diminish a tide's effect, while topography can either accelerate or decelerate tides and even alter their height.

The most reliable tidal predictions are typically for coastal regions, where tides are less influenced by winds and other weather phenomena. Areas with shallow waters or situated in bays are more affected by a range of other factors. For example, an estuary fed by a river may experience tides much higher than expected if heavy rainfall adds river water to the mix. Similarly, in an inland harbor, strong winds can reduce tidal levels if they push against the tide's incoming force.

In the United States, the Center for Operational Oceanographic Products and Services (CO-OPS), part of the National Ocean Survey, is the official body providing tidal predictions for about 3,000 locations nationwide. CO-OPS also offers tidal forecasts for other countries and territories.

Since the available astronomical data alone are insufficient to predict tides, actual tidal measurements over an extended period are also used in many regions. The National Ocean Survey oversees a network of 140 tide stations that collect various data every six minutes, including the following measurements:

The gathered data is transmitted every hour and is promptly accessible online through sources such as the links provided on the following page.

Stations that generate full daily tide predictions are referred to as reference stations. The remaining stations, known as subordinate stations, generate their predictions by applying a specific formula based on the data from reference stations. This formula is derived by analyzing the historical correlation between the tides at the two stations.