What Causes High Tide and Low Tide? Why Are There Two Tides Each Day?


It's low tide at Robin Hood's Bay, North Yorkshire, which is a popular scenic area of the Northeast coast of England. Photos by R A Kearton/Getty Images

Tides have caused a lot of trouble recently. During high tide periods, low-lying areas that border the ocean now flood way more often than they used to. Between 2000 and 2017, the average frequency of "high tide flooding" across the United States increased by 50 percent. Floods of this sort block roadways, harm infrastructure, and back up storm drains.

Around 40 percent of the global population lives within 62 miles (100 kilometers) of a coastline. Recent flood trends must have a lot of these people wondering about tidal science. How do tides happen? Why do some areas get more dramatic tides than others? And why can't the sea level just stay constant everywhere, all the time? Today we're going to look at the physics and idiosyncrasies of planet Earth's tides.

Pushing Water

Check out the diagram below. In the picture, you will notice that our planet is sitting inside a blob of ocean water that's kind of shaped like a rugby ball. There's a bulge in the ocean on each side of the planet. Observe that one bulge is protruding from the half of the globe that's facing the moon while the other is located on Earth's opposite end.

Why do these bulges exist? In a nutshell, they're primarily caused by the moon's gravitational pull upon the Earth. That force can have two separate components. It can pull matter "vertically," by which we mean perpendicularly to the Earth's surface. And it can also pull things "horizontally" — i.e.: in a direction that runs parallel to the face of our planet.

Now, the spot on the globe that sits right beneath the moon at any given time is called the sublunar point. Meanwhile, the spot on the other side of our planet that is directly opposite the sublunar point is known as the antipodal point. It's no coincidence that the ocean bulges are highest right over those two spots. At the sublunar point and the antipodal point, the moon's gravitational pull lacks a horizontal component — something that is also missing at the two corners of the world that are located 90 degrees away from these spots.

Those four areas are unique in that regard; every other location on Earth experiences a horizontal force that pushes water molecules in the ocean toward either the sublunar point (where the moon's gravitational force is at its strongest) or the antipodal point (where the moon's gravitational pull is at its weakest). This is why the ocean bulges up over those two areas.

Here Comes The Sun

Once every 24 hours, the Earth completes a full rotation around its axis. During this process, any given spot on the planet's surface (like, say, Long Island or Australia) will pass right through both of those ocean bulges. So — in most areas — when your home is directly under a bulge, the local tide should be high. But as it enters the space between the bulges, the tide in your area should get lower. This isn't always the case, as you'll learn next.

For now, let's discuss another factor that influences our tides. The sun also exerts a gravitational pull on the oceans, but because our solar companion is further away, its effects on the tides are less pronounced than the moon's. Still, the big ball of gas and plasma does noticeably enhance tidal bulges on a regular basis.

"The tides are [at their] largest ... when the sun and moon line up," Duncan Agnew, a geophysicist at the University of California, San Diego, says in an email. He notes that this happens during two separate lunar phases: Full moons and new moons. Astronomers and Earth scientists refer to these plus-sized tides as spring tides. (Note that the name has nothing to do with the spring season; indeed, spring tides occur throughout the year.)

During spring tides, the "high" tides are really high and the "low" tides are unusually low. Things get less extreme when the sun and moon sit at right angles to each other (relative to the Earth). Such an arrangement will produce a neap tide; a period in which the difference between high and low tides is minimal.

When Continents Interfere

Brace yourself: Things are about to get even more complex. Earth may be a "blue planet," but 29 percent of our world's surface is covered by land. Coves, cliffs and other geographic features can interfere with the tides, too, intensifying them in some locales and weakening them in others.

Most coastal areas receive two high tides per day, with a new one coming every 12 hours and 25 minutes. Yet exceptions to the rule aren't hard to find. "Ocean tides are a complicated process that involves the tidal force acting on water that is, as it were, also free to slosh around in the ocean basins," Agnew says. Many beaches on the Gulf of Mexico only receive one high tide per day, a byproduct of restricted water flow. Elsewhere, water that enters the V-shaped Bay of Fundy in Nova Scotia is pushed upward as it moves inland. This results in huge height disparities between low and high tides there called bore tides.

Last editorial update on Oct 11, 2018 05:23:22 pm.

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