A spring tide is a tide with an unusually large range — higher high tides and lower low tides than average — occurring roughly twice a month, around new moon and full moon.
Why It Happens
Tides are driven primarily by the Moon’s gravitational pull, with the Sun contributing a smaller additional effect. At new moon and full moon, the Sun, Earth, and Moon align (in a straight line), so the Sun’s gravitational pull reinforces the Moon’s, producing a larger combined tidal effect — this is a spring tide. The term has nothing to do with the season “spring” — it comes from the tide “springing up” higher than usual.
Practical Impact for Shipping
- Higher high water — can allow vessels to cross shallow bars or enter ports that would be inaccessible at other times
- Lower low water — reduces available depth at low tide, which matters for vessels waiting at anchor or planning departure timing in tidally-restricted ports
- Larger tidal currents — spring tides generally produce stronger tidal streams, relevant to passage planning through tidal waterways
Spring Tide vs Neap Tide
Spring tides (greatest range) alternate with neap tides (smallest range), which occur around the first and last quarter moon phases, when the Sun and Moon’s gravitational pulls partially cancel each other rather than reinforcing.
Why This Matters for Passage Planning
Ports with significant tidal range, or with a shallow bar/approach channel, often schedule vessel arrivals and departures around spring tide high water specifically to maximize available depth — tidal planning is a real, practical part of voyage planning for many routes, not an academic detail.
Exam Relevance
Spring tide formation, its relationship to lunar phases, and its practical effect on port access are core Meteorology/Tides topics in deck officer competency exams.