Tides are the regular rising and falling of the ocean surface, caused by the gravitational pull of the moon and the sun. The phenomenon of tides is a result of the gravitational interaction between the earth, moon, and sun, and is also influenced by the shape of the earth's surface and the rotation of the planet. Tides play a crucial role in shaping the coastal environment. In this post, we will explore how tides are caused around the world, the effects of the moon, sun, and geographical features on tidal flow.
"I have always been fascinated by the ocean, to dip a limb beneath its surface and know that I'm touching eternity, that it goes on forever until it begins here again." - Lauren DeStefano
The fundamental cause of tides is the gravitational force between the earth, moon, and sun. The science bit says that the gravitational force between two objects is directly proportional to the product of their masses and inversely proportional to the square of the distance between them. The moon's gravitational force is 2.2 times stronger than the sun's, even though the sun's mass is much greater. This is because the moon is much closer to the earth than the sun.
The gravitational force of the moon attracts the earth, and the earth's gravity attracts the moon. This attraction causes the water on the earth's surface to bulge in the direction of the moon, creating a high tide. At the same time, the water on the opposite side of the earth is pulled away from the moon, creating a second high tide. It's a bit more complicated than that and also has to do with pressure from a gravitational squeeze, but we'll leave that for another time.
The gravitational force of the sun also affects tides, although its effect is less significant than that of the moon. When the sun and moon are aligned, their combined gravitational pull creates the highest high tides and the lowest low tides, known as spring tides. When the sun and moon are at right angles to each other, the gravitational forces are weaker, and the difference between high and low tides is smaller, known as neap tides.
We can buy tidal almanacs each year which will give us times of high and low water, together with the corresponding tidal heights. Most will also identify when springs and neaps occur, based on lunar cycles.
"In every outthrust headland, in every curving beach, in every grain of sand, there is the story of the earth." - Rachel Carson
Tidal patterns are not just influenced by the moon and the sun but also by the shape of the earth's surface. The more observant of you, especially those that live on South Island, New Zealand or the Scottish Highlands, will have noticed that the earth's surface is not uniform; it has variations in the shape of the continents, ocean basins, and underwater ridges. These variations in the shape of the earth's surface can influence tidal patterns in specific areas. For instance, when the tide reaches a shallow area or a narrow strait, it can create a higher tidal range. Similarly, when the tide reaches a broad, shallow bay, it can create a lower tidal range.
Geographical features such as coastlines, estuaries, and bays also influence tidal patterns. In the same way, bays tend to create clockwise and anti-clockwise flows along the coast, as the tidal flow changes.
Headlands, jutting out into a tidal stream may cause back eddies and violent overfalls, especially where the headland just into the stream by way of a shallower ridge, underwater. This forces the fast flowing water upwards at speed in order to flow over the shallower water.
The shape of the coastline can influence how high the tide gets, where it reaches its maximum, and how fast it recedes. For instance, in a concave coastline, the water is funnelled into a narrower area, sometimes creating higher tides. In contrast, in a convex coastline, the water is dispersed over a broader area, resulting in lower tides. Estuaries and bays act as amplifiers of tidal currents. The tide enters an estuary and gradually narrows, forcing the water to rise and creating a higher tidal range.
In addition to the gravitational pull of the moon, sun, and the earth's shape, other factors can also influence tidal patterns. The wind is one of the significant factors that can influence the tidal flow. Strong winds can cause the water to pile up in one area, creating a temporary rise in the water level. This rise in water level is known as a wind tide. Additionally, when the air pressure is particularly high, or low, it can have a significant effect on the predicted tidal height at a specific location.
It is not uncommon for coastal towns to experience floods when a particularly high equinoctial spring tide coincides with a strong onshore wind and an unusually low atmospheric pressure. In the South West of England, I regularly experienced this event in Salcombe, Devon.
If you'd like to know more about how to estimate tidal heights and flows for passage & pilotage planning and calculating an estimated position or course to steer, check out our multi media posts, below. If you want to know how to calculate tidal heights or flows check out our videos, here.
A Poll For Our Readers | Your Views?
Is this kind of content of interest?
Yes
Yes, but I prefer videos or multimedia content
No
Don't know
Understanding tidal patterns in sailing, How to predict tides while sailing, Tidal currents: What are they and how do they impact sailing, Tides vs. Waves: What’s the difference?, The gravitational pull: Understanding what causes tides, How moon phases affect tidal patterns in sailing, Tidal charts: How to read and use them while sailing, Sailing safety tips for dealing with tides, Exploring the science behind tidal patterns and sailing, Tidal height: How it can affect your sailing experience.
