A minus tide is a low tide event where the water level drops below the average low watermark. This phenomenon is indicated by a negative number on tide charts or predictions.
When tide levels are referenced, they are often measured relative to a specific vertical datum, such as Chart Datum. According to our reference, low tide can be expressed as a smaller, or possibly even a negative number. If the number is positive, the lowest water depth remains above Chart Datum. However, in case the number is negative, low tide will fall below the average low watermark. This negative value signifies a tide that is lower than the typical low tide level for that location.
Understanding Negative Tide Levels
- Reference Point: Tide heights are measured relative to a benchmark, commonly called Chart Datum or Mean Lower Low Water (MLLW).
- Positive vs. Negative: A positive number means the water level is above the reference point. A negative number signifies that the water level is below the reference point.
- Significance: A minus tide (negative value) reveals areas of the shoreline or seabed that are typically submerged, even during regular low tides.
Why Do Minus Tides Occur?
Minus tides result from the complex interplay of gravitational forces from the Moon and Sun, as well as local geography. The most extreme low tides, including minus tides, often occur during spring tides, which happen around the new and full moons when the Sun, Moon, and Earth are aligned. This alignment enhances the combined gravitational pull.
Implications and Opportunities
Minus tides offer unique opportunities and have significant implications:
- Exploring Tide Pools: Much larger areas of the intertidal zone are exposed, making it easier to observe marine life in tide pools.
- Shellfish Gathering: For recreational or commercial gatherers, minus tides provide access to areas normally underwater.
- Shoreline Activities: Walking further out on beaches or exploring areas usually inaccessible becomes possible.
- Maintenance & Construction: Provides windows for marine construction or maintenance below the normal low-tide line.
| Tide Type | Description | Tide Height Relative to Average Low Mark | Indicated By |
|---|---|---|---|
| Regular Low | Typical lowest daily tide level | At or slightly above | Small positive? |
| Minus Tide | Exceptionally low tide, below average low | Below | Negative Number |
| Regular High | Typical highest daily tide level | Significantly above | Large positive |
| Extreme High | Exceptionally high tide (e.g., King Tide) | Far above | Very large positive |
Understanding tide charts and predicting minus tides is essential for anyone planning activities in the coastal environment. Checking local tide tables provides the specific times and levels, including negative values indicating a minus tide.
