Understanding Tides: An Introduction

What Tides Are

Tides are the regular rise and fall of the sea surface caused mainly by the gravitational pull of the Moon and the Sun, combined with the rotation of the Earth. Although these astronomical forces are global and predictable, the actual tide you experience at any point around the UK coastline is shaped just as much by local geography — the shape of the coast, the depth of the seabed, and the way tidal waves move around our continental shelf.

The Forces Behind Tides

Lunar & Solar Gravity

• The Moon’s gravity is the main driver of tides.
• The Sun adds a smaller but important influence.
• The alignment of the Sun and Moon creates spring and neap cycles.

Earth’s Rotation

• As the Earth rotates, the oceans respond to these gravitational pulls, creating bulges of water. Whilst these appear to move around the planet, in reality the planet is moving under the bulges of water, whilst facing the moon (hence the spring and neap cycles as the moon phase changes).
• In most places around the UK, this results in two high tides and two low tides each day.

Why Local Conditions Matter

The UK coastline is full of headlands, bays, estuaries, and underwater features, which shape the tide in powerful ways:

Faster currents when:

• Water is forced through narrow gaps
• The seabed shoals quickly (gets shallow fast)
• Flow bends around headlands or islands
• There’s a deep channel next to a shallow bank

Slower currents when:

• The seabed is broad and shallow
• The coastline widens
• The seabed deepens gradually
• Flow spreads out into open bays

This is why tide tables are location specific: the underlying physics is global, but the outcome is local.

High Water, Low Water, and Slack Water

• High Water (HW) is the peak of the tide — the highest sea level before the tide begins to fall.
• Low Water (LW) is the lowest sea level before the tide begins to rise again.
• Slack Water is the period when the tidal current slows and changes direction.

A key point: Slack water does not necessarily occur at HW or LW.

This is because water level and water movement are related but not identical. The tide may reach its highest level while the current is still running, especially in areas with strong tidal streams or constricted channels. The water level can “top out” before the flow stops, or the flow may reverse before the level reaches its minimum.

For Divers: Slack Water Isn’t Always “No Current”

Divers often make plans around slack water, but it’s important to understand what slack really means:

• Slack water is the turning point of the current, not the turning point of the tide height.
• In many UK locations, especially around headlands, islands, and narrow channels, slack may occur well before or after HW/LW.
• Some sites experience “dynamic slack”, where the current slows but never fully stops.

The timing and duration of slack can change depending on the spring–neap cycle:

• Spring tides (stronger tides) often have shorter, sharper slack periods.
• Neap tides (weaker tides) may have longer, gentler slack periods.

For this reason, divers should always use local tidal stream data, not just tide tables, when planning a dive.

Further Reading

1. National Tidal & Sea Level Facility (NTSLF). “All About Tides.” Authoritative UK government resource explaining tidal mechanics, lunar influence, and why Britain experiences two highs and two lows per day.
2. Met Office – “Tides” A clear UK‑focused explanation of what causes tides, including spring and neap cycles.
3. NOAA National Ocean Service – “What Are Tides?” A foundational introduction to tidal forces, flood/ebb currents, and the relationship between vertical and horizontal tidal components.
4. Tide & Time (UK) – “Tidal Science Explained” A UK‑centric overview of tidal waves, their origins, and why understanding tides matters for navigation and safety.
5. Les Marées – “Tides: Simple Explanation” A straightforward description of how tides work, the role of the Moon and Sun, and the predictability of the tidal cycle.
6. Maritime College – “Tide Basics” A concise explanation of tidal forces, spring/neap tides, and how tides appear as long‑period waves approaching coastlines.