Tidal Forces - The Gravy And Gravity

The Science Behind Tides

Why would we want to know the phase of the moon and how does it have anything to do with surfing?

Tides are the result of gravitational interactions between the Earth, Moon, and Sun. The Moon, being much closer to Earth, exerts a stronger gravitational pull on our planet than the Sun does. This pull creates a 'bulge of water' on the side of Earth facing the Moon, as well as on the opposite side due to the centrifugal force of Earth's rotation. These bulges result in high tides, while the areas between them experience low tides.

The Sun also plays a role. Although its gravitational pull on Earth is weaker than the Moon's, its influence becomes significant when combined with the Moon's pull. Depending on the alignment of the Sun, Moon, and Earth, tidal forces can either amplify or diminish, leading to two key tidal phenomena: spring tides and neap tides.

When these celestial bodies align, their combined gravitational forces create spring tides, characterized by higher high tides and lower low tides. Conversely, when the Sun and Moon are at right angles relative to Earth, their gravitational forces partially cancel each other out, resulting in neap tides, which have a smaller tidal range and milder water movement.

Spring Tides: Bigger Swells and Stronger Currents

During spring tides, which occur during the New Moon and Full Moon, the amplified tidal forces lead to more extreme high and low tides. For surfers, this often means larger swells and faster-moving water, which can create powerful, barreling waves. However, the stronger currents can also make conditions more challenging and unpredictable.

Neap Tides: Calmer and More Predictable Waves

Neap tides occur during the First Quarter and Third Quarter Moon phases, when the gravitational forces of the Sun and Moon are perpendicular (at an angle of 90° to to each other). This alignment reduces the overall tidal range, leading to:

• Less extreme high tides: The high tide doesn't reach as high as during spring tides.

• Less extreme low tides: The low tide doesn't drop as low as during spring tides.

• More moderate water levels: The tide stays closer to a "middle" range, with smaller fluctuations between high and low tide.

If a spot that favours mid-tide will be more surfable for a longer period.
However, if a spot requires low-tide, like a reef break, it may not work at all during the First and Third Quarter moon phases.

Surfing Example: Jeffrey's Bay, South Africa

Jeffrey's Bay, or "J-Bay," is one of the world's most famous surf breaks, known for its long, fast, and perfectly shaped right-hand point break. The tidal conditions at J-Bay can significantly influence the quality of the waves, making it a great example of how lunar phases and tides affect surfing.

During spring tides, the larger tidal range can enhance the wave quality at J-Bay. The increased water movement often leads to bigger, more powerful swells, which can create epic, hollow barrels—perfect for advanced surfers looking for a thrilling ride. However, the stronger currents can make paddling out more challenging and require greater skill to navigate.

During neap tides, the smaller tidal range at J-Bay results in more consistent and manageable waves. The slower currents make it easier to paddle out, and the waves tend to break more predictably, offering longer, smoother rides. This makes neap tides an excellent time for intermediate surfers or those looking to practice their technique without the added challenge of strong currents.

Tide Psyience

Newton's law of universal gravitation laid the foundation for understanding how the Moon and Sun's gravitational forces create tides on Earth.
"The ocean's tides are caused by the gravitational pull of the Moon and Sun on the Earth's oceans."

Laplace expanded on Newton's work by developing the dynamic theory of tides, which explains how tidal forces interact with Earth's rotation and ocean basins. Newton's equilibrium theory assumed a static, water-covered Earth, but Laplace accounted for Earth's rotation. He showed that the Coriolis effect (a result of Earth's rotation) influences tidal currents and the distribution of tidal bulges. This means tides are not just simple bulges moving with the Moon but are also affected by the spinning motion of the Earth.

Laplace recognized that the shape, size, and depth of ocean basins play a critical role in how tides behave. For example, narrow bays and shallow continental shelves can amplify tides, while deep, open oceans may have smaller tidal ranges. This explains why some locations, like the Bay of Fundy in Canada, experience extreme tidal ranges, while others, like the Mediterranean Sea, have minimal tides.

Resonance and Tidal Oscillations

Laplace introduced the concept of resonance, where the natural oscillation periods of ocean basins interact with the tidal forces. If the frequency of the tidal force matches the natural frequency of an ocean basin, the tides can be amplified.

This is why certain regions experience much larger tides than others, even though the gravitational forces are the same globally.

Tidal Waves and Propagation

Laplace's theory describes tides as long-wavelength waves that propagate across ocean basins. These waves are influenced by the Earth's rotation, coastline geometry, and underwater topography.

For example, tidal waves can reflect off coastlines, creating complex interference patterns that affect local tidal heights and currents.

Harmonic Analysis

Laplace developed mathematical tools to break down tidal patterns into a series of harmonic components (sine waves), each representing the influence of different astronomical forces (e.g., the Moon, Sun, and their varying distances and alignments).

This approach allows scientists to predict tides with great accuracy by analyzing the contributions of each component.

How Laplace's Work Relates to Surfing

Laplace's dynamic theory of tides helps explain why tidal behavior varies so much from one surf spot to another. For example:

• Reef Breaks: At shallow reef breaks, the interaction of tidal waves with the seafloor can create powerful, hollow waves during low tide. Laplace's theory helps explain how the shape of the reef and the surrounding ocean basin influence this process.

• Point Breaks: At point breaks like Jeffrey's Bay, the consistency of waves during neap tides can be attributed to the smaller tidal range and the way tidal waves interact with the coastline.

• Beach Breaks: The shifting sandbars at beach breaks are influenced by tidal currents, which Laplace's theory helps explain through the interaction of tidal waves with coastal topography.

Practical Example: Tides at Teahupo'o, Tahiti

Teahupo'o is a famous reef break known for its heavy, barreling waves. The tides here are critical to wave quality:

• Low Tide: During low tide, the waves break over the shallow reef, creating the iconic thick, hollow barrels that Teahupo'o is famous for.

• High Tide: At high tide, the waves lose their intensity because the water is too deep for the reef to shape them effectively.

Laplace's dynamic theory helps explain why Teahupo'o's waves are so sensitive to tidal changes. The shallow reef and the shape of the ocean basin around Tahiti create a resonant effect, amplifying the tidal forces and making the break highly dependent on specific tidal conditions.

Practical Example: Skeleton Coast, Namibia

Tidal Range and Wave Shape:

• The Skeleton Coast experiences a moderate to large tidal range, depending on the alignment of the Moon and Sun (spring vs. neap tides).

• During low tide, waves break over shallow sandbars or reefs, creating hollow, powerful waves that are ideal for experienced surfers.

• During high tide, the waves may lose some of their intensity as the water becomes deeper, causing them to break more softly or farther offshore.

Tidal Currents:

• Strong tidal currents are common along the Skeleton Coast due to the interaction of tidal waves with the coastline and underwater topography.

• These currents can make paddling out to the lineup more challenging, especially during spring tides when the tidal range is larger and currents are stronger.

Wave Consistency:

• The consistency of waves on the Skeleton Coast is influenced by the tides. During neap tides (First and Third Quarter Moon phases), the smaller tidal range can create more stable and predictable wave conditions.

• During spring tides (New and Full Moon phases), the larger tidal range can lead to more dramatic changes in wave quality throughout the day, with the best conditions often occurring around mid-tide.

Conclusion

Laplace's dynamic theory of tides revolutionized our understanding of tidal behavior by incorporating Earth's rotation, ocean basin geometry, and wave dynamics. For surfers, this theory provides a scientific basis for understanding why tides vary so much between different breaks and how to predict optimal surfing conditions. By applying Laplace's insights, surfers can better appreciate the complex interplay of forces that shape the waves they ride.

In summary, tracking lunar phases helps predict tides: spring tides (New/Full Moon) bring bigger waves; neap tides (Quarter Moons) offer calmer surf.

Whether you're looking for the bigger waves of spring tides or the calmer conditions of neap tides, the Moon plays a key role in shaping waves.