Tides are the longest of all waves, with a wavelength that can equal half of the Earth’s circumference. As surfers, we wake up every morning thinking about the tides and how they may affect the quality of the overall surf. (Will the tide be drained, shallow, causing surfable waves at our reef or beach break to be heaving fast down the line (rideable face of the wave), possibly throwing steep barrels? Or…will the tide be high, swampy, with too much water over the reef or sandbars so the waves barely break, making for a slow, mushy, frustrating surf session?) Tides are periodic, short-term changes in the height of the ocean surface at a particular place. The California coast has a mixed tidal pattern: often a higher high tide, followed by a lower low tide, followed by a lower high tide, followed by a higher low tide each lunar day. A lunar day is 24 hours and 50 mins (the amount of time it takes for the moon to be in the sky at the exact same position as it was the night before), which means that the high and low tides arrive 50 minutes later each day. The reason for this is that the moon is orbiting Earth in the same direction as Earth’s axial rotation (the Earth rotates eastward). In California, the zero tide level is the average level of the lower of the two daily low tides.
The main cause of the tides is the gravity of the moon and the sun acting on the ocean. The forces that actually generate the tides varies inversely with the cube of the distance from the Earth’s center to the center of the tide-generating object (the moon or the sun). Distance is the most important in this relationship. The sun is 27 million times more massive than the moon (so it has a much larger gravitational force) but it is 387 times farther away than the moon so the sun’s influence on the tides is only about half that of the moon’s.
The moon does not revolve around the center of the Earth. Instead, Earth and Moon together—the Earth-Moon system—rotate around a common center of mass (1023 miles) beneath the Earth’s surface. As it revolves around Earth, the moon (gravity) attracts the ocean creating a tidal-bulge. The motion of the Earth (inertia) creates a second tidal-bulge on Earth, opposite side of the moon. Tides occur as Earth rotates (at nearly 1000 miles per hour given no interference like land masses, deep water, etc.) beneath these bulges. The bulges are the crests of the planet-sized waves that cause high tides. Low tides correspond to the troughs of the tidal waves.
The Ocean responds simultaneously to inertia and the gravitational force of both the Sun and the Moon. If the earth, moon, and sun are all in a line the lunar and solar tides will combine resulting in higher high tides…and lower low tides—this is called Spring Tides. If the earth, moon, and the sun form a right angle, the solar tide will pull opposite and decrease the lunar tide—this is called Neap Tides. Neap Tides occur at two-week intervals, a week after the spring tide.
Everything mentioned above is known as the equilibrium theory of tides (developed by Isaac Newton) and is this surfer’s (and writer’s) method of approximately predicting the tides. There are at least 140 tide-generating/tide-altering forces and factors. Seven of the most important of those must be considered in order to mathematically predict tides. Or simply check your local tide calendar because it is the study of past records that allows tide tables to be projected into the future with an accuracy of about 1.2 inches for years in advance. Even still, tides are difficult to predict as other things like storm surges, tsunami, onshore or offshore winds can all affect tidal height or the arrival time of the tidal crest (the high tide wave spanning half the earth’s circumference).
Remember that tides are a kind of wave. The crests of these waves—tidal bulges—are separated by a distance of half of Earth’s circumference. In the equilibrium theory, the crests of these waves would always point toward (or away from) either the moon or the sun as Earth turned beneath them. The dynamic theory of tides takes into account the speed of the long-wavelength tide wave in water of varying depth and how the moving water (tide wave) reacts with coastlines around the world. Due to the Coriolis effect, the tide waves move counterclockwise in the Northern Hemisphere (as the high tide moves in then withdraws to low tide in a particular place), and clockwise in the Southern Hemisphere. This can drastically affect the tidal range (height difference between high-water and low-water level). In California, the tidal range is a little more than 7ft. In the Bay of Fundy in New Brunswick (Canada) the tidal range is nearly 50 feet, leaving once docked boats lying in the sand at low-tide. As water (tidal current) rushes in our harbors, bays, and surf breaks it can actually temporarily improve the condition of the surf giving the wave an extra push (this is called a flood current). Often at high tides or low tides, the currents go completely still—slack tides—as the tides change direction. As the water rushes away from our coasts/surf breaks it causes an ebb current, which can create choppy, nearly unsurfable waves—as the tidal currents rush back out to sea like a river ripping through the waves. These (planet sized) tide waves can create tidal bores at narrow river mouths and inlets. A steep wave is created as the tide crest moves upstream, and the narrow river mouth forces the wave to move at a speed that exceeds the theoretical shallow water speed for that depth. The forced wave breaks forming a spilling wave that bores upriver. The Silver Dragon, the name for the bore on China’s Qiantang River may be up to 26 feet high and travel 25 miles per hour.
Check out this amazing footage of tidal bore surfing in Indonesia. Watch how the tidal crest forces its way upriver uprooting trees (1:05 minute mark):
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