What Causes Tides? High and Low Tides Explained

By: Mark Mancini  | 
low high tide
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 over the first quarter of this millennium. 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 37 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. What causes tides? Why do some areas get more dramatic tides than others? And why can't the sea level just stay constant everywhere, all the time? Here, we look at the physics and idiosyncrasies of planet Earth's tides.


The Moon's Gravitational Pull

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 protrudes 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.


The Sun and the Tides

Once every 24 hours, Earth completes a full rotation around its axis. As the Earth rotates, 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 in a moment.


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, resulting in plus-sized tides called spring tides. (Note that the name has nothing to do with the spring season; indeed, spring tides occur throughout the year.)

During spring tides, high and low tide are more extreme: The "high" tides are really high and the "low" tides are unusually low.

There are less extreme tides when the sun and moon sit at right angles to each other (relative to Earth). Such an arrangement will produces what's called neap tides. A neap tide is 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. Over in Nova Scotia, water that enters the V-shaped Bay of Fundy is pushed upward as it moves inland. This results in huge height disparities between the low and high tides there, called bore tides.

High Tide FAQ

What are high and low tides?
High and low tides refer to the regular rise and fall of the ocean's waters. High tide occurs when water covers much of the shore after rising to its highest level. Low tide is when the water retreats to its lowest level, moving away from the shore.
What happens during high tide?
During high tide, the ocean’s waters creep up the shore, deepening the water. This happens as a body of water gets closer to one of the two bulges created by the moon’s gravitational force.
What causes low tide?
The moon's gravitational pull or tidal force causes two bulges on Earth (and its water) — one at the point closest to the moon and the other on the direct opposite side of the planet. As the Earth turns, a region gets closer to or further from the bulges. The further it is from one, the lower the tide.
What objects affect the tides?
In addition to the moon, the sun and coves, cliffs and other geographic features may affect the tides. The sun exerts its own gravitational pull on the oceans while geographical features can interfere with tides, intensifying them in some parts and weakening them in others.