Tides are the regular, predictable movement of large bodies of water caused primarily by the gravitational pull of the moon and, to a lesser extent, the sun. This constant rise and fall of the sea surface governs the rhythm of coastal environments worldwide. Understanding the timing of this cyclical process is important for many ocean activities. The duration of the low phase is not a long, static period, but rather a brief moment within a much larger, continuous process.
The Basic Tidal Cycle
The ocean’s water level is constantly changing, following a predictable rhythm based on the moon’s orbit. The full tidal cycle, from a high tide back to the next high tide, typically takes 12 hours and 25 minutes in most coastal areas. This timing is determined by the lunar day, which is 24 hours and 50 minutes long, meaning that each day the tides occur about 50 minutes later than the day before.
Most regions experience a semi-diurnal pattern, featuring two high tides and two low tides of roughly equal height each lunar day. The water level transitions through four stages: the high tide peak, the falling water known as the ebb, the low tide trough, and the rising water called the flood. A simple rule of thumb suggests that the water spends approximately six hours and 13 minutes moving from a high tide to the succeeding low tide.
Duration of the Low Phase
Low tide is defined as the precise instant the water reaches its lowest vertical point before reversing direction. This moment is often followed by a brief period of minimal horizontal water movement known as “slack water.” Slack water is the short pause when the tidal current ceases before the flow switches from the outgoing ebb tide to the incoming flood tide.
The actual duration of the low phase, when the water level is barely changing, is very short, sometimes lasting only a few minutes. While the water takes about six hours to fall from high to low, the lowest point is momentary. The transition to the rising flood tide is usually quick, correcting the misconception that the water stays low for an extended period.
Factors Influencing Tidal Timing
The predictable six-hour interval between high and low tide is an average, and the exact timing is rarely uniform across all locations. Geographic features, such as the shape of the coastline, the depth of the ocean floor, and the presence of inlets, significantly alter the speed and arrival of the tidal wave. These local characteristics can cause the time between high and low water to be longer or shorter than the standard approximation.
Astronomical alignments also introduce variability in the timing and range of the tides. When the sun, moon, and Earth align during new and full moons, their combined gravitational pull creates a greater tidal range known as a spring tide. Conversely, when the celestial bodies form a right angle during the quarter moons, the diminished force results in smaller neap tides. Furthermore, not all coasts follow the standard semi-diurnal pattern; some areas, like the Gulf of Mexico, experience a diurnal cycle with only one high and one low tide daily, while others have a mixed pattern with two unequal tides.
Interpreting Tide Charts
To determine the precise moment the low phase ends for any specific location, one must consult a tide chart or tide table. These charts are the result of complex calculations that account for local geography and astronomical forces, providing a prediction of future tidal events. The tables list the exact time of the lowest point, known as the low water time, and the height of the water, which is measured against a specific vertical reference point called the datum.
By checking the predicted low water time on the chart, a person can identify the precise instant the low tide cycle is complete. This point is immediately followed by the start of the flood tide, which marks the beginning of the water level rising toward the next high tide. Tide charts are the most accurate tool to understand the exact end point of the low tide for coastal activities.