A tide chart serves as a predictive tool, outlining the rhythmic rise and fall of ocean water levels at specific coastal locations. These charts provide a forecast of water depths and currents, which are influenced primarily by the gravitational forces of the moon and sun. Interpreting this information can significantly improve safety and success for various marine and shore-based pursuits.
Essential Data Points on a Tide Chart
A typical tide chart presents several key pieces of information, organized to provide a clear picture of future tidal conditions. Each chart specifies the date for which the predictions are valid. It also indicates the specific geographical location, often a particular harbor or coastal station, as tidal patterns vary significantly even over short distances.
The core data points include the time of each high tide and low tide event within a 24-hour period. Alongside these times, the chart lists the height of the water level, usually expressed in feet or meters, relative to a specific reference point. Some charts may also include predicted current speeds or directions, especially for areas with strong tidal flows, which are often presented in tabular format or as graphical curves illustrating water level changes over time.
Interpreting Tide Chart Data
Interpreting a tide chart involves understanding the relationship between the predicted times and heights. To determine the water level at a specific moment, locate the nearest high and low tide entries on the chart. If your desired time falls between a low and a high tide, the water level will be rising; conversely, if it’s between a high and a low, the water will be falling. Many charts include a curve that visually represents the tide’s progression, allowing for approximate visual interpolation of the water level.
The period around the peak of high tide or the trough of low tide is known as slack tide. During these brief intervals, the horizontal movement of water (currents) slows before reversing direction. Identifying slack tide is useful for activities like diving or navigating narrow channels, as current strength is minimal during these times. Tidal movements follow a pattern: there are two high tides and two low tides each lunar day, which is slightly longer than a solar day, causing the times of high and low tide to shift forward by approximately 50 minutes each day.
To calculate the approximate tide height at a time between high and low tides, use the “Rule of Twelfths.” This rule states that the tide rises or falls by 1/12 of its total range during the first hour after slack water, 2/12 in the second hour, 3/12 in the third, 3/12 in the fourth, 2/12 in the fifth, and 1/12 in the sixth. While an approximation, this method provides a useful estimation for planning. For example, if the total tidal range is 6 feet, the water level would change by about 0.5 feet in the first hour after slack water.
Practical Uses and Safety Considerations
Reading a tide chart offers benefits for various coastal activities. For fishing, understanding tidal movements helps identify optimal feeding times for many species, as moving water can concentrate baitfish and nutrients. Boaters rely on tide charts to navigate safely, ensuring sufficient water depth to avoid grounding in shallow channels or to clear bridges and overhead obstructions at high tide. Kayakers and paddleboarders often consult charts to plan trips with favorable currents, making paddling easier and safer.
Beachcombers and shell collectors can use low tide predictions to access areas of the beach that are typically submerged, revealing new finds. Surfers monitor tide charts in conjunction with swell forecasts, as specific tide levels can create better wave breaks at different locations. Being aware of tidal changes prevents individuals from becoming stranded by a rising tide on sandbars or isolated beaches. Strong tidal currents, especially in inlets and around headlands, can pose a hazard to swimmers and small craft, making knowledge of the current’s strength and direction important.
Finding Tide Charts and Advanced Considerations
Tide charts are widely available. Online platforms and dedicated mobile applications provide real-time tidal predictions for thousands of locations worldwide. Many local newspapers, fishing supply stores, and marinas also publish printed tide tables, particularly for their immediate area.
Some advanced tide charts may include predictions for tidal currents. These current predictions specify the speed and direction of the flow, which can be important for navigation in areas with strong tidal streams. While moon phases influence tidal range, some charts might also indicate them, as full and new moons typically result in larger tidal ranges (spring tides), while quarter moons lead to smaller ranges (neap tides).