How Tides Affect Surf
Tides are probably the most often used yet least understood reference to explain why the surf is breaking the way it is breaking. “The tide push will help.” “There’s just too much water – tide’s drowning everything out.” This all sounds very guru, but what does it all really mean? More importantly, how can we use tide information to help us find better waves? The good news is that tides are predictable. You can open up any almanac and know that Huntington Beach is looking at a 0.65 feet high tide at 9:32 a.m. … ten years from today. The bad news is that the movement of the Earth, moon, and sun which are the extremely predictable drivers of tides are only half the story. There are typographical (shape of the ocean’s surface) as well as meteorological (changes within the Earth’s atmosphere) factors which are less easier to predict and interpret. This is why what you read on Surfline may not always jive with what you end up experiencing once you’re out in the lineup.
There are two primary forces which influence the rise and fall of the ocean. The gravitational pull from the moon (a lunar tide) is the primary force. Although the gravitational pull from the sun (a solar tide) certainly comes into play, the Earth’s own rotation is actually the secondary force. Contrary to popular belief, the moon does not rotate solely around the Earth. Rather, both the Earth and its moon rotate around an axis towards the edge of the Earth. This rotation is not dissimilar to how an Olympic hammer thrower rotates off his axis.
Consider further that the Earth’s oceans cover over 70% of its surface. As such, a bulge emerges on the side of the Earth facing its moon and a separate bulge on the opposite side created by the centripetal force of the Earth’s rotation. Think of a bucket of water tied to a rope swung around your head. Centripetal force is what keeps the water pushed into the bucket and keeping it from dropping to the ground.
Amid this Earth – moon dosey doe, the Earth itself is rotating on its own axis making full rotations every 24 hours. As the Earth rotates “under” these bulges, all else equal, we witness the seeming rise and fall of tides on the shorelines. Since it takes 24 hours for the Earth to make a full rotation, we generally see two sets of low and high tides each day.
All the while the Earth and its moon are together rotating around that giant scorching ball of fire we refer to as the sun. Although the sun is 400 times larger than the moon (creating gravitational forces 180 times greater), it is also 400 times the distance between the Earth and its moon. As such, the gravitational pull from the sun (45%) is slightly less than the moon (55%) but still plays a large factor in determining tides.
When the sun, Earth, and moon are aligned, we witness the highest high tides and lowest low tides. This is otherwise known as “spring tides” which has less to do with the season rather than the characteristics of the tide which “springs” from drastic lows to drastic highs.
When the Earth is centered between the sun and the moon (a full moon phase), the gravitational force of the moon is unimpeded on one side of the Earth while its own rotational force coupled by the gravitational force of the sun is unimpeded on the other side.
When the moon is centered between the Earth and the sun (a new moon phase), the gravitational forces of the moon and the sun both combine to create tides even higher than during full moon spring tides. Although we generally witness a second high tide caused by the rotational force of the Earth, that second high tide is not as drastic relative to a full moon spring tide.
When the moon is in its quarter phases, its gravitational pull is at a right angle (perpendicular) to that of the sun. During these phases, their gravitational pulls along with the Earth’s own rotational force cancel each other out causing minimal variations between high and low tides.
The difference between spring and neap tides typically vary by about 25%.
Mother of All Tides
Remember the Earth and moon’s ring-around-a-rosey? In actuality, this circular duet is uneven (elliptical) with the gap between them every so often closing to a point known as the moon’s “perigee.” Although this gap varies by a mere 8%, the gravitational influence can vary by as much as 25%. When the moon is in perigee during a full moon phase, we see unusually low tides. When the moon is in perigee during a new moon phase, we see unusually (sometimes hazardous) high tides. These so called Perigean spring tides occur approximately every 1.5 years.
This cosmic tango between the Earth, moon, and sun occur with clockwork precision. It’s this predictability that allows us to determine the degree and timing of high and low tides well into the future. And what if these celestial bodies go out of sync? Well, washed out surf will probably be the least of our concerns.
What Lies Beneath (& Above)
Since waves are created when their depth is less than half their wavelength, the height of tide is only half the story. The other half is told through the ocean typography or more specifically the shape of the sea floor along your local break. Simply put, the steepness of the sea floor shapes the waves breaking over it. For now, forgetting the effects of tide, a gradually increasing sea floor will create mushy, slow waves. By contrast, a steeper bottom forces the wave to break faster creating faster waves with more push. At the extreme end are bottoms that rise up so fast, that waves don’t even have a chance to break and simply surge up on shore.
Now let’s take the effect of tides into consideration. Assume a spot where the sea floor starts out gradual but then ramps up to a very steep incline. During lower tides, you can expect to see mushy, longboard waves. As the tide creeps up, you might notice these same mushy waves gain a bit of oomph and push behind them. At extreme high tides, you might be completely bummed to find no waves breaking at all!
Let’s look at a spot that has the exact opposite typography. Further out, the sea floor comes up rapidly as it gradually flattens out to meet the shore. A spot like this would be completely shut down during extreme low tides. As the tide comes up however, you’ll see fast shortboard waves giving way to mushy crumblers during peak high tide.
Of course, these are merely two dimensional examples accounting for three distinct changes in ocean typography. In the real world, the sea floor is a hodgepodge of sandbars, channels, rocks, and reef all constantly shifting with the ebbs and flows of tides, storms, river drainage, and a slew of other natural and man-made factors.
What lies beneath, although tricky to determine, is far less complicated to ascertain than the forces that lie above the ocean. Namely, atmospheric factors including tidal surges caused by winds and pressure changes. These are not as easily predicted and often require sophisticated and ample computing power to model with any degree of accuracy. Put another way, it’s why we’ll always rely on people like Sean Collins and his venerable oracle of Huntington Beach otherwise known as Surfline.
Centripetal forces and Perigean spring tides all sound pretty fancy but how can we apply this to what matters – surf? Let’s take my home break as an example. Barring sophisticated sonar equipment, I need to back into what I think the typography must look like.
I’ve observed that the place tends to break rather mushy on mostly any tide offering gusto only in N/NW swell directions. From this I can deduce that in summer months, when we get predominantly southern hemi’s, the ocean floor in that direction tends to slant fairly gradually. But throw in a northern hemi combo and the place can light up into an often overlooked shortboarder heaven. That sea floor must gain some grade when looked at from that direction. However, the otherwise fiscally inept local authorities found the cost to plow back the burm too high. All the sand they had pushed back in the winter in anticipation of storm swells are still there. So closer to shore, the sea floor suddenly rises very rapidly causing severe backwash in anything resembling a high tide.
This is a simple example of how you can use tide information, even the predictable low and high tide indicators, to help to decide how the surf will break at your local spot. As you might have guessed, it takes a fare amount of time and experience to really have a spot “wired” and know how it will react to certain tides. Factors beyond nature can also have a significant effect.
As for my break? Yes, it takes quite the rare confluence of positive factors for the place to really go off. These factors include (in order of predictability) tide levels, swell magnitude and direction, and local government mental capacity. Although I still make a point to check tides and swell predictions, I still go to this spot because it’s a place where everyone knows each other. Sure we complain about there being too much water – the tide drowning everything out. Or swearing that the tide push will help – even though it rarely does.
We don’t really care. We’re just there to have fun.
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