Everything you need to know about tides

Why do we have tides? How many tides are there a day? How are tide predictions calculated? All of these questions and more, answered. In this article, I'll many of the factors which influence the tides.

Ste Wright | 4 min read

What causes the tides?

The tides are caused by the gravitational pull of both the sun and the moon. As the earth spins on its axis, the side facing the sun has gravitational influence. The moon is the biggest controller of our tides, so as it orbits the earth, the side of the earth closest to the moon has its high tide.

The amount by which the sun or the moon influence the height of the oceans around the world is known as the tidal bulge. What happens if both the sun and the moon are both pulling from the same direction? The answer to this lies in our next question.

A full moon
The moon has the biggest influence on our tides

What is a spring tide?

A spring tide occurs when the sun and moon align to combine their gravitational force on the sea, causing the seas to rise by the greatest amount. This happens during a new or full moon. They aren't anything to do with the season 'spring', instead are named by the action of the tides "springing forth".

Seven days after a spring tide, the sun and moon are at right-angles to each other, meaning the tidal bulge is at its lowest. This is when the tidal range is at its least and is known as a neap tide. Read more about Spring and Neap tides.

How often are spring tides?

Spring tides occur twice per lunar month. A lunar month is ther period between two full moons, occurring every 29.5 days. As mentioned before, spring tides have nothing to do with the season of Spring.

How many tides are there a day?

High tides occur every 12 hours and 25 minutes. As a result, there are two high tides and two low tides every day for a vast majority of areas, known as a semidiurnal tidal cycle. Some areas in unique geographical areas such as the gulf of Mexico only have one high and low tide per day. This is known as a diurnal tidal cycle.

Gulf Breeze Florida
The Gulf of Mexico experiences just one high and low tide a day known as a diurnal tidal cycle

How are tide times and height predictions calculated?

Tide times are chiefly calculated based on astronomical patterns such as the earth's orbit around the sun and the moon's orbit around the earth. That said, predicting the tide heights and times isn't as easy as you might imagine.

There are other factors to consider aside from the position of the earth in relation to the sun, the moon in relation to the earth. Across the world, there is a network of tide stations which measure additional factors which aid in the calculation of tide times and heights. These include:

  • Tide levels
  • Wind speed and direction
  • Atmospheric and water temperatures
  • Barometric pressure
  • Current speed and direction

Topologial features such as land masses and peninsulas can limit or enhance the flow of water which also have an impact on tide times and height predictions. This is why tidal predictions are made for pre-defined locations across the world where the impact of these is known and tide stations are present.

Clevedon pier
The unique topological features of the Bristol channel mean it has the second highest tidal range in the world

What's the difference between a tide table and a tide chart?

These two terms are quite often used interchangeably, but are by definition, different to one-another. A tide table details the times and heights of high and low tides. A tide chart details the values of the heights and times between high and low water in addition to the extremes.

That's it for this article. Hopefully you will have learned something. I'll follow it up with more question and answer articles.

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