What causes tides, how do tides work and what are sea levels?

Last Updated on November 29, 2023 by Matt Hapgood

10 Minute Read

From a human standpoint, we recognize tides as the regular rise and fall of the sea surface. This rise and fall, known as the tide, is caused by the gravitational pull of the moon and sun.

Because of this, tides are extremely reliable and predictable.

They regularly rise and fall and we always know exactly when to expect these changes to occur. Tides are basically long-period waves that move through the ocean based on this gravitational pull.

They start out in the deep sea and move their way toward the coastline where we see the change take effect. When the highest part of this wave (the crest) reaches a location, high tide occurs.

When the lowest part of this wave (the trough) reaches a location, low tide occurs. The difference in height between the two is called the tidal range.

As a surfer who grew up at a spot on Earth with a tidal change of 10 meters, yes meters, I feel suitably knowledgeable on the subject of tides.

So, in this blog post, we’ll look at everything you need to know about tides, and how the tide can affect the surf and waves at the beach.

LUNAR TIDES

The moon is much closer to the Earth than the sun, so the moon’s attraction is much greater. The sun is 360 times further from the earth giving the moon a more significant role in producing tides than the sun. The side of the moon closest to the Earth has a bulge.

This bulge is more noticeable in the ocean than on land because the ocean is liquid. The rise and fall of the ocean level is a result of the moon’s gravitational field pulling as it orbits the earth.

The moon takes 27.3 days to orbit the earth so the tidal cycle is 27.3 days long. There are two high tides and two low tides every 24 hours and 50 minutes.

There are six hours and 12.5 minutes between high and low tides and 12 hours and 25 minutes between high tide and the next high tide. Because of the angle of the moon in relation to Earth, the two high tides (or low tides) each day are not always of equal height.

The height of these tides actually differs on a daily basis due to the changing distance between the Earth and the moon.

SOLAR TIDES | SPRINGS, NEAPS, AND OTHER VARIATIONS

Although the moon is the most prominent factor in tides, the sun does play a role too. Solar tides are about half the size of lunar tides. Rather than being seen as separate tides, solar tides are simply a variation of lunar tides. Whenever there is a full or new moon (the sun, the earth, and the moon are all in alignment) the solar tide adds to the lunar tide creating the highest of high tides and the lowest of low tides.

These are called Spring tides. The sun and moon will be at right angles from each other one week later. In this position, the solar tide partially cancels out the lunar tide creating moderate tides known as Neap tides.

During each lunar month (the duration between new moons which is 29.5 days) there are two sets of Spring tides and two sets of Neap tides.

Similar to how the angles of the moon, sun, and earth affect the tides, so do their distances from one another. The moon follows an elliptical path around the Earth, so its distance varies significantly throughout the lunar month.

Once each month when the moon is closest to the earth (at pedigree) its tide-generating forces are stronger, producing larger ranges in the tides. When the moon is farthest from the earth (about two weeks later), the tide-generating forces are weaker and the tidal range is less than average. The same goes for the sun.

When the sun is farthest from the earth (around July 2), the tidal ranges are smaller. Whereas when the sun is closest to the earth (around January 2), the tidal ranges are greater.

HIGH TIDES

The side of the earth facing the moon will have a bulge called the directional tide because this is where the gravitational attraction between the moon and the earth is strongest.

The bulge is caused by the moon’s gravitational attraction, pulling the water towards the moon. While this is happening, inertia attempts to keep the water in place but is weaker in this location, thus why the bulge occurs.

Meanwhile, the opposite side of the earth will experience a similar bulge called the opposite tide. This bulge occurs because the inertial force of the earth is greater than the gravitational force of the moon at this location because it’s further from the moon.

So the inertial force keeps the water moving away from the moon, also causing a bulge but in the opposite direction.

This is why high tides occur simultaneously on opposite sides of the earth.

LOW TIDES

Low tides are simply the receding waters between high tides. They also occur twice per day. However, because there are two high tides and two low tides every 24 hours and 50 minutes (because the moon rises 50 minutes later each day), the tide cycles differ by 50 minutes each day too.

So you can expect the low tide to occur 50 minutes later than it did the previous day.

AMPHIDROMIC POINTS

Amphidromic points, also called tidal nodes, are geographical locations that have no tidal amplitude for one harmonic constituent of the tide. At these points, there is almost no movement in sea level from tidal action. The tidal range increases with distance from this point. This is why some locations have large tidal ranges and others have smaller tidal ranges.

Wind Direction Image — *A typical wind map showing the direction and strength/speed of the wind. Pale blues and greens are light winds, and yellow and orange are more moderate wind speeds.*

OTHER THINGS THAT AFFECT TIDES

Although tides are timely and predictable, there are other factors that can affect them here on Earth. For example, the shape of a coastal line can distort the tidal pattern. Areas that are fairly close together can experience high tide hours apart whereas other areas that are a similar proximity can experience high tide just minutes apart.

It’s impossible to predict these differences without detailed geographic information. Geography also affects tidal range. The height of the tides can vary greatly depending on geographic factors. These geographic effects are why you should use local tide tables whenever possible to get the most accurate information for your particular location. Weather can also have an impact on the tide.

Strong winds and abnormal atmospheric pressure can alter tides. As an example, a strong wind blowing from the ocean toward land can pile up the water creating a tide that’s higher than predicted.

TIDES IN RIVERS, LAKES, AND OTHER BODIES OF WATER

Although all bodies of water are technically subject to tidal effects, the tides are so small that even in the case of large lakes their effects are masked by river inflows, wind, etc. all of which have a greater impact on differences in water height than tides in these particular bodies of water.

Because the earth’s oceans are all interconnected and so expansive, they’re the only body of water in which we can really see and experience the tide’s impact.

TIDES AND SURFING

So how does all of this affect surfing conditions? Is there a “best” tide for surfing? The tide influences many decisions that surfers make like where to surf and what board to use. Generally high tide has slower, fatter wave breaking whereas low tide has faster, more hollow wave breaking.

As we travel up and down the coastline, some areas are more drastically affected by tides like reefs for example.

Avid surfers are very in tune with the movements of the tides because we see and experience the small changes in the way the waves break. Understanding tides is powerful knowledge that can positively impact our surfing.

HEIGHT OF TIDE vs TYPE OF TIDE

As a surfer, heading to the beach at the wrong time of day could leave you with disappointing conditions. Some may say that there’s a best tide for your particular location (high tide or low tide) but because of all the variations and factors that can affect tides, it’s more complicated than that.

On some Spring tides a reef may be too shallow whereas on some Neap tides, the water may not go out far enough to make a wave.

Because there’s so much variation of high and low tides throughout the year, it’s better to look at them as absolute heights.

We often blame poor conditions on the swell or the wind, which are contributing factors. However, if we watched and predicted the tide more carefully, we might have better luck.

When you look at a tide table for your local surf spot, pay attention to the column next to the times of high and low tide which measures the height of the tide.

TIDAL PUSH

Every break has its favored tides. Although as a general rule, most spots favor a low tide going toward a high tide, when the tide is on the rise. This is because there is a sort of tidal push where the tide is pushing water in at the same time as the waves are coming in, so they have a common direction.

Whereas an outgoing tide has momentum opposing the momentum of the waves coming in. However, this is still seen as speculation and has yet to be fully proven.

Researchers from the University of Plymouth decided to test the theory using data from offshore buoys in an area with a large tidal range of 7.5 meters. Their research found the theory to be true, wave energy was higher during incoming tides, the peak being just one hour before high tide.

This goes to show how in tune surfers can be with the ocean, the waves, and the tides just from their own lived experience.

What is a Tidal Bore?

A tidal bore is a type of tidal phenomenon that occurs in some rivers and estuaries that have a high tidal range. It is a surge of water that travels upstream against the river’s current.

Here are some key facts about tidal bores:

Caused by the incoming tide forming a wave that propagates against the river current. The shape and size of the river mouth influences the formation.
Occurs in rivers where there is a large tidal range (difference between high and low tide) such as those flowing into the Bristol Channel in the UK (video below).
The leading edge or wave front is called the bore while the tall wavy back is the head.
Speed depends on the topography but bores generally travel between 10-15 mph upstream. Some of the largest can reach speeds over 20 mph.
Creates a roaring sound as the bore approaches. Wave heights vary from just inches over 3 feet up to 16 feet tall for the world’s largest bores.
Popular with surfers and whitewater paddlers. Requires skill to ride as the wave is turbulent with strong undertows.
Also called “eagre” or “egret” in parts of the world. Examples include the Severn Bore in the UK and the Pororoca on the Amazon.
May have ecological importance in breathing life into stagnant waters by mixing oxygen and churning up sediments.

In summary, a tidal bore is a surging tidal wave that forms in coastal rivers and streams with the right conditions, providing an opportunity for experienced surfers to ride unique natural waves upstream.

OTHER WAYS TIDES AFFECT SURF

Waves break in water that is around 1.25 times deeper than the waves are tall. So if the bottom topography of a location has an abrupt change, like a flat shelf of rock that intersects a cliff, waves can even disappear during certain tides.

During high tides the waves would hit the cliff without breaking and during low tides they might break right onto the edge of the rock or the tides could leave the rock shelf exposed with no water above it.

However, in most places, the bottom topography will slope toward the shore allowing waves to break regardless of the tide. But the location along the slope will still vary based on the height of the wave and tide. Strong tidal currents can affect waves in other ways, too.

They can create boils and whirlpools. Or if the tidal flow is moving in an opposing direction to the waves, it could cause them to pile up and increase in size. Large incoming tides can even create surfable waves in rivers and bays called tidal bores. And you can even surf these tidal bores too!

HOW TO READ AND INTERPRET TIDAL CHARTS

If you’ve learned a lot from this blog and want to begin studying the tides at your local surf spot, you’re going to need to understand how to read and interpret tidal charts. A tide chart will tell you the local times of high tides and low tides as well as their heights.

The National Oceanic and Atmospheric Administration (NOAA) provides a free online tool for accessing past and future tidal information.

There are also many online resources and apps that can help you find local tide information for the station nearest your location. Once you’ve found your location, you’ll want to find the days/times you plan to go surfing.

Make note of the high tide and low tide times as well as their heights and change in height.

As you begin tracking this information and surfing in the same location, you’ll start to notice correlations between the waves you’re surfing and where the tide is and you’ll figure out the best tidal conditions for your local surf spot.

FINAL THOUGHTS

So there you go! Now you’re an expert in all things tidal.

I hope you enjoyed all of the information and hopefully, your brain is not too frazzled!