Module 11 of 17
Tides & Tidal Streams
Tidal heights, tidal curves, tidal diamonds, stream atlases, and secondary port corrections.
What Causes Tides?
Tides are caused primarily by the gravitational pull of the Moon (and to a lesser extent the Sun) on the Earth's oceans. As the Earth rotates, bulges of water follow the Moon, creating the rise and fall of sea level we call tides.
Most places around the UK experience semi-diurnal tides — two high waters and two low waters in roughly 24 hours and 50 minutes. The extra 50 minutes is because the Moon orbits the Earth, so high water occurs about 50 minutes later each day.
Key Points
- Tides are caused by the gravitational pull of the Moon and Sun
- UK has semi-diurnal tides — two highs and two lows per ~25 hours
- High water occurs ~50 minutes later each day
Springs and Neaps
When the Sun and Moon align (new moon and full moon), their gravitational pulls combine, producing spring tides — the highest highs and lowest lows. When the Sun and Moon are at right angles (first and third quarter), their pulls partially cancel, producing neap tides — smaller range between high and low.
The spring/neap cycle takes about 14 days (half a lunar month). Spring tides occur a day or two after the new and full moons. Tidal range at springs can be double or more compared to neaps.
Key Points
- Spring tides — Sun and Moon aligned (new/full moon). Largest range.
- Neap tides — Sun and Moon at 90° (quarter moon). Smallest range.
- Spring/neap cycle ≈ 14 days
- Springs occur 1–2 days AFTER new/full moon
Tip: Spring tides are NOT related to the season of spring. They 'spring' up to their highest.
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