This section covers what you need to know about Tides
Skills & Environment — Choose a topic
What Causes Tides
Tides are caused by the gravitational pull of the moon — and to a lesser extent, the Sun. If a PADI question asks what causes tides, the best answer is the gravitational pull of the Sun and the moon. If only one answer is listed, the moon is the primary cause.
Why there are two high tides a day
The moon pulls the ocean on the near side toward it — that's one high tide. On the far side, the Earth itself is pulled toward the moon slightly, leaving the ocean behind — that's the second high tide. The two low tides sit at the sides, 90° from the moon. As the Earth rotates, most locations pass through two high tides and two low tides roughly every 24 hours.
Spring Tides and Neap Tides
The moon determines when the tide comes in. The Sun determines how high it gets.
| Tide type | Sun & moon position | Effect | Moon phase |
|---|---|---|---|
| Spring tides | Sun and moon aligned | Gravitational pulls combine — higher highs, lower lows | Full moon or new moon |
| Neap tides | Sun and moon at 90° | Gravitational pulls partially cancel — lower highs, higher lows | Quarter moons |
Tidal Range — and Why Topography Matters
The tidal range is the difference in water level between high tide and low tide. It varies enormously from place to place — not because of the moon, and not because of proximity to the equator, but because of topography: the shape of the coastline and seabed that either allows water to flow freely or funnels and compresses it.
The funnel effect
Picture the tidal bulge as a large wave of water moving toward a coastline. If that coastline narrows into a funnel shape — like a bay or inlet — the same volume of water is being squeezed into a progressively smaller space. The result is a much greater rise in water level at the end of the funnel than would occur on an open coastline.
| Location | Tidal range | Reason |
|---|---|---|
| Bay of Fundy, Canada | Up to 16.3 m | Classic funnel-shaped inlet — one of the most extreme tidal ranges on Earth |
| Bristol Channel / Cardiff, UK | ~12 m+ | Funnel-shaped coastline compresses the Atlantic tidal bulge |
| Swanage, UK (nearby) | ~1.5 m | Open coastline — same moon, completely different topography |
| West coast of Panama | ~5.5 m | Funnel shape from Pacific coastline amplifies the tidal bulge |
| East coast of Panama | ~0.6 m | Open Caribbean coastline — very limited water movement |
| Caribbean Sea | Very small | Island chains and the American continent restrict water flow in and out |
| Mediterranean Sea | Negligible | The Strait of Gibraltar is the only water exchange point — far too small relative to the sea's size |
Beach slope also affects your experience of the tide
A 1-metre tidal range will expose a huge stretch of beach on a flat, gently sloping shore — and barely move the waterline on a steep beach. This doesn't change the tidal range itself, but it does change what you see and plan around as a diver or dive operator. When you see a PADI question about tidal range, think topography — not beach slope, and not latitude.
Skills & Environment — Choose a topic