If you were to sit on the beach for twenty four hours in a row, you would notice the level of the water getting higher and then getting lower again. And if you did this every day for a while, you’d see this happening every day. This is an ocean tide: the regular rise and fall of the sea level. And here’s why it happens…
The moon pulls the ocean to it creating high tide at that point. Inertia makes a tidal bulge (high tide) on the other side of the Earth. The rest of the globe has low tide. The Earth rotates once a day, resulting in two high tides per day. The Sun’s gravity influences the magnitude of the moon’s effect.
An ocean tide is an extremely powerful force of nature. As a result of this, if you want to live on the seaside or simply just enjoy a coastal vacation, understanding the tides is paramount to your safety. So, let’s look at ocean tides; what causes them, and how they work…
What Causes Ocean Tides?
Most people know that the moon has something to do with ocean tides, even if they are not sure exactly how a celestial body can influence a massive, Earth-bound body of water. It is, in fact, the gravitational pull of the moon on the Earth that causes ocean tides.
Additionally, the ocean tides are also influenced by the gravitational pull of the Sun, the Earth’s rotation, and sea basin geomorphology.
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How Does The Moon Affect Earth’s Ocean Tides?
The moon has the most significant influence on the ocean’s tides because of its proximity to the Earth and the fact that it has its own gravitational field.
As the moon orbits the Earth, it exerts a gravitational pull. The Earth’s crust is too rigid to respond in any significant way to this pull, but the fluid oceans are free to move to a certain extent.
The Earth’s atmosphere and its own gravitational field stop the ocean waters from being pulled off of the surface of the planet altogether.
The gravitational pull of the moon is larger on the side of the Earth that is nearest to the moon, so the seas are pulled further out at this point, creating what is known as a tidal bulge.
In physics, which is one of the sciences that govern nature, there exists a principle known as inertia. Inertia is a resistance to change.
So, on the far side of the Earth, which in this case is the side furthest from the moon, inertia is trying to stop the pull of the ocean towards the moon by pulling it in the opposite direction.
This does not prevent the formation of a tidal bulge on the near side of the Earth (the side closest to the moon). Instead, it creates a complimentary tidal bulge on the far side of the Earth.
High Tide And Low Tide
The moon is responsible for the existence of high and low tides.
The points on the Earth at which the tidal bulges form are experiencing high tide.
Everywhere else on the Earth, the effects of inertia and lunar gravitational pull are relatively equal. These points on the Earth are experiencing low tide.
High tide and low tide are points in time where the ocean tide is the highest and lowest it gets at that point, respectively. The difference between high tide and low tide is known as the tidal range.
The moon does not orbit the Earth in a circle. Instead, it follows an elliptical path. This means that it is closer to the Earth at some points along its orbit than at other points.
The orbital point where the moon is closest to the Earth is known as perigee, and it occurs every twenty eight or so days. During perigee, the proximity of the moon to the Earth results in greater gravitational pull on the oceans, increasing the average tidal range.
The lowest tidal range will coincide with the moon’s furthest point of orbit, called apogee, which is reached approximately fourteen days after perigee.
Ocean Tides Affect The Moon As Well
Even though the moon controls the ocean tides, the ocean tides tidal bulge actually exerts a return gravitational effect on the moon.
This reciprocal effect is gradually influencing the lunar gravitational field’s impact on the ocean tides.
The Earth spins on its axis faster than the moon orbits the Earth, so the tidal bulge ends up pulling against the moon as it spins away.
Inertia once again comes into play, and the moon resists the tidal bulge’s gravitational pull and pulls back against the Earth to the extent that it even slows the Earth’s rotation down.
This pulling of the moon and then pulling of the Earth creates friction, and the moon is pushed further away from the Earth, increasing the size of the orbit and marginally decreasing the strength of the lunar gravitational pull on the oceans.
What Would Happen If The Moon’s Orbit Shrank?
If something were to happen and the moon was to come closer to Earth, we, as creatures on this planet, would have some problems.
The increased lunar gravitational pull would increase the size of the tidal bulges and ultimately increase the tidal range. The land along most coastlines would become uninhabitable as it would flood twice a day.
Smaller land masses, like islands, may become completely uninhabitable, disappearing under the ocean’s surface for portions of the day. Areas like the heavily populated UK would suffer as the people would be forced into a smaller land area.
What Would Happen If The Moon Disappeared?
This is actually a fascinating concept, and it shows clearly just how important the moon and tides are to life as we know it.
If the moon disappeared, there would still be tides because of the Sun’s gravitation pull. However, these tides would be approximately one third of the size that they are with the moon present.
Additionally, because the effects that ocean tides have on the moon cause the Earth’s rotation speed to decrease, if the moon were lost, the Earth would spin faster. A faster spinning Earth means a shorter day, and you may end up with closer to one thousand days in each year.
So, for all the busy people out there who are struggling to fit everything into a twenty four hour day, thank the moon that it exists!
Obviously, if the moon disappeared, there would also be other, non-tide related effects like extremely dark nights and even variations in the angle of the Earth’s rotation.
How Does The Earth’s Rotation Affect Ocean Tides?
As the Earth rotates and its moon orbits around it, the part of the ocean that is closest to the moon changes. This is why the tides move in and out, and coastal regions experience both low and high tides.
One rotation of the Earth marks the course of a single day for us. So, the same point on the Earth’s surface is going to be closest to the moon and then furthest from the moon within that rotational cycle.
What does this mean for tides?
Well, it means that almost every coast on the Earth will experience two high tides each day.
You should note that there are really twelve hours and twenty five minutes between two high tides, so it is not quite two per day, but it is close enough that no one quibbles over it in general discussions.
Flood Tide And Ebb Tide
The time leading up to high tide from the low tide point is called the flood tide.
The time leading to low tide from the high tide point is called the ebb tide.
How Does The Sun Affect The Earth’s Ocean Tides?
The Sun also exerts a gravitational pull against the oceans, but to a much lesser extent than does the moon.
Because of the exponentially greater mass of the Sun when compared to the moon, you might think that the solar gravitational pull would be larger.
However, the distance of the Sun from the Earth is so great that the lunar gravitational pull exceeds solar gravitation.
The gravitational pull of the Sun can either add to that of the moon or diminish it slightly. It depends on the relative positions of the Sun, moon, and Earth.
Spring Tide And Neap Tide
When the moon and the Sun are in alignment with the Earth, their gravitational forces are combined to produce even higher high tides. These are called spring tides.
Spring tides occur with the new moon (when the moon is directly between the Earth and the Sun) and with the full moon (when the Earth is directly between the moon and the Sun).
Spring tides occur once every fourteen or fifteen days, which means that there are two spring tides per lunar month.
As the moon and the Sun move out of alignment, they reach a point where they form a right angle with the Earth as the apex. This is the halfway point between two spring tides, and it is known as the neap tide.
There are also two neap tides per lunar month.
Perigean Spring Tide
Approximately six to eight times each year, the spring tide coincides with the moon’s perigee to produce even bigger tidal ranges—up to about forty percent bigger. This results in what is officially known as the perigean spring tide, but many people call it the king tide.
How Does The Sea Basin Geomorphology Affect The Earth’s Ocean Tides?
The tidal model that has been presented thus far is partially theoretical because it does not take into account the effects of land on the tides.
High tide, low tide, spring tide, and the neap tide still exist. However, they are not the same at every point on Earth. In fact, the first high tide at one point can be lower or higher than the second high tide at that same point and on the same day.
As the moon’s gravitational field pulls the ocean water towards it, some of this movement is blocked by big land masses. The result is that the tides move in complex and unique patterns around the different continents and islands as opposed to in straight lines.
Semi-Diurnal Tides, Mixed Semi-Diurnal Tides, And Diurnal Tides
The sea basin geomorphology gives us a whole different set of tide classifications.
As mentioned previously, most coasts have two high tides and, consequently, two low tides every day.
When these high tides are of relatively equal heights, and the low tides are also equal, then they are known as semi-diurnal tides.
When the high tides are of different heights, and the low tides are also different, then they are known as mixed semi-diurnal tides.
Almost every coast experiences two high tides and two low tides, but there are a few exceptions. These exceptions only have one high and one low tide each day. These are known as diurnal tides.
Storm Surges
Tides can also be temporarily affected by massive storms like hurricanes, cyclones, and typhoons.
Storms like involve such high volumes of precipitation that they are capable of raising the water levels enough to increase the tidal swells.
Places that are particularly vulnerable to storm surges are those where the sea basins are narrow and shallow (such as the Bay of Bengal) or low-lying areas over which the storms can pass (such as Florida).
Storm surges can cause severe damage, as you have no doubt seen, and they can last for hours or even a few days.
What Is A Tidal Wave?
Most people think that a tidal wave is another word for a tsunami, but this is actually a misconception because tsunamis are not caused by tides.
Tidal waves are the actual waves produced by the gravitational effects of the Sun and moon and the Earth’s rotational effects.
Tidal waves are shallow water waves, which means that their wavelengths are greater than the depth of the water.
While tsunamis are devastating natural disasters, tidal waves are only as dangerous as their tidal range. So, in areas where there is a large tidal range, tidal waves can be hazardous because of the vast difference in water levels, but they are not considered natural disasters.
Where Is The Highest Ocean Tide On Earth?
The relative height of high tide is entirely dependent on the height of the low tide. So, the highest ocean tide is the one with the largest tidal range.
In Canada, between New Brunswick and Nova Scotia, lies the Bay of Fundy. This bay holds the title of having the largest tidal range on the planet.
The average ocean tidal range is three feet. The Bay of Fundy has a range of fifty three feet (just over sixteen meters).
How Does Global Warming Affect The Earth’s Ocean Tides?
Global warming has the potential to impact our oceans and continents significantly and in many different ways. One of these ways is by altering the tides.
Firstly, you need to know that the ocean absorbs about ninety percent of the heat that is in the atmosphere. This means that if the atmosphere is hotter due to global warming, the ocean will absorb more heat.
When the temperature of water increases at a constant pressure, then the volume will also increase. This means that the hotter the ocean is, the higher the tide will rise.
Furthermore, by raising the temperature of the oceans, the rate at which glaciers and other large masses of ice melt consequently increases. Melting water then also increases the volume of the oceans, causing the tides to rise higher.
While the effects of global warming increase the height of the tides, they do not necessarily alter the tidal range. This means that the rising sea levels are permanent, and more and more land mass is covered by the ocean.
The rising tides have the potential to completely swallow up islands as well as cities and towns built on coastlines. The beginnings of this are already seen across the world, as evidenced by an increase in nuisance flooding.
Can You Swim When The Tide Is Going Out?
You can swim while the tide is going out, but you have to be careful. The whole ocean line is moving backward, and it can pull you out with it if you are not strong enough or aware of what is happening.
Another really dangerous aspect of an ebbing ocean tide is the increased risk of rip currents forming. Rip currents are narrow channels of ocean water that flow away from the beach oceanward and at speeds as great as eight feet per second.
If you get caught in a rip current, do not panic. Attempting to swim back to the shore against a rip current will just exhaust you and increase your chances of drowning. Instead, you should swim in a parallel line to the shore until you out of the flow of the current.
Can You Swim When The Tide Is Coming In?
You can also swim when the tide is coming in, but one of the risks is getting cut off from land. While you may have only swum out a few feet from shore, the incoming tide can quickly cover the nearby land.
If you have already been swimming for a while, or if you are a weak swimmer, this can spell big trouble for you.
Even if you are not swimming, some rocks and land forms get covered during high tide, which can trap you on temporary islands.
When Is The Safest Time To Swim In The Ocean?
The best time for swimming in the ocean is when the tidal currents are at their weakest and water movement is at a minimum.
In terms of current strength and water movement, the build-up to high tide starts slow, increases to a peak, and then slows again until the high tide has been reached.
In the same way, the breakdown to low tide starts slowly, reaches a peak, and then slows again until the low tide is reached.
This means that the best time for swimming in the ocean is probably the hour just before and after high tide or just before and after low tide. These times are often referred to as slack tide.
Even though it is safer to swim during slack tide, the ocean is never without danger, so you always have to be aware of what is going on. For example, even though they are more likely to occur in an ebbing tide, rip currents can still form during slack tide.
Always consult your local tide table before swimming in the ocean, and make sure that the area you are swimming in is monitored by a lifeguard.
Do Tides Shape A Coastal Landscape?
Tides are one of the major influences on the formation of a coastal landscape. In fact, coasts can be classified based on how high the tide rises there.
There are three tidal classes of coast:
- Micro-tidal coasts, where the tidal range is less than six and a half feet (two meters)
- Meso-tidal coasts, where the tidal range is between six and a half and thirteen feet (two to four meters)
- Macro-tidal coasts, where the tidal range is over thirteen feet (four meters)
As you can imagine, tides will cause erosion and deposition, affecting barrier formation and morphology, including estuaries and tide pools.
Estuaries And Tides
All rivers eventually lead to the ocean. Estuaries are known as partially enclosed bodies of water where freshwater rivers meet the saltwater seas. They are also called the tidal mouths of rivers.
Estuaries form as a result of permanent tidal changes such as those that occur with global warming. They are also affected by the normal tidal processes—they can go from being filled with water as the tide flows in to being predominantly muddy ground as the tide flows out.
Estuaries can be classified according to their tidal processes very similar to the coasts., i.e., they are
- Micro-tidal estuaries, where the tidal range is less than six and a half feet (two meters). These estuaries are long, wide, and shallow.
- Meso -tidal estuaries, where the tidal range is between six and a half and thirteen feet (two to four meters). These estuaries are stubby with tidal meanders.
- Macro-tidal estuaries, where the tidal range is over thirteen feet (four meters). These estuaries are shaped similarly to a trumpet and extend far inland.
Tide Pools
Where tides flow over rocky outcroppings, they cause erosion of soft spots in the mineral and cause small or large depressions. Once these depressions start to form, more and more tidal water starts to collect in them, remaining behind when the tide ebbs.
Eventually, not just water is left behind. The tides also wash marine creatures and plant life into the tide pools, making them seem like miniature oceans themselves.
Summary Of The Different Tides
| Tide Name | Controlled By | |
| 1 | High Tide | Moon’s gravitational field |
| 2 | Low Tide | Moon’s gravitational field |
| 3 | Flood Tide | Earth’s rotation |
| 4 | Ebb Tide | Earth’s rotation |
| 5 | Spring Tide | Sun’s gravitational field |
| 6 | Neap Tide | Sun’s gravitational field |
| 7 | Perigean Spring Tide | Moon’s perigee + Sun’s gravitational field |
| 8 | Diurnal Tide | Sea Basin Geomorphology |
| 9 | Semi-Diurnal Tide | Sea Basin Geomorphology |
| 10 | Mixed Semi-Diurnal Tide | Sea Basin Geomorphology |
| 11 | Slack Tide | Combination of the moon’s gravitational field, the Sun’s gravitational field, and the Earth’s rotation |
Conclusion
Ocean tides are a fascinating subject to explore because they so clearly show how intricate is the design of the universe. The fact that a celestial satellite can determine the flow of our planet’s ocean is incredible.
The moon exerts the most influence on the ocean tides, but they are also influenced by a number of factors. These include the Sun’s gravitational pull, the Earth’s rotation, sea basin geomorphology, storms, and climate change.
While the tides are not devastating like tsunamis, they can be dangerous if they are no understood. If you live on the coast or if you vacation at the coast, it is always wise to become familiar with the tides to make your interactions with the ocean as safe as possible.
