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Understanding Tidal Predictions

Although most people have an understanding of the effects the sun and the moon have on the oceans of our planet, the finer details are not always apparent. The tidal curves produced by prediction software are instantly recognisable. They produce a graphical representation of the expected tidal behaviour at a specific port or location.

In the example shown below, the tides are predicted for a specific day in Cardiff on one of the biggest Spring tides of the year.
Cardiff Spring Tide
The graph shows "High Water" reaching a maximum height of 14 metres at around 8.00am, with "Low Water" occurring just over 6 hours later, at around 2.15pm. On this day low water drops below the predicted L.A.T.(Lowest Astronomical Tide). The huge tidal range of more than 14 metres, means an enormous amount of water has to flow in or out of the Bristol Channel in just over 6 hours.

About a week later the tides have diminished to small "Neap" tides.
Cardiff Neap Tide
Again the example is for a specific day in Cardiff but, the graph looks very different. The first "High Water" of the day is about 1.00am, reaching just over 9 metres, with "Low Water" arriving a little over 6 hours later at about 7.15am with about 4 metres of water left above the L.A.T. The Tidal Range this time is only 5 metres from High Water to Low. This is a huge difference from the 14 metre range of the previous Spring Tide, with 65% less water trying to leave the Bristol Channel.

To produce the monthly Spring and Neap tide graphs, the daily data is compressed.
Compressed Spring Tide Compressed Neap Tide
By placing a month of data in to one graph, a picture emerges of the Spring and Neap tides changing over time.
Typical Tidal Curves
Because Spring and Neap tides are a global phenomenon, created by the relevant position of the Sun and Moon, then local or port differences are no longer an issue: A neap tide in Scotland will be on the same day as a neap tide in the Channel Islands or Norway or Portugal, as we're all in the same area of the Atlantic.

When you compare the previous graph for Cardiff with the next few graphs, you can see that the Cardiff data, being the second highest tidal area in the world, produces more defined curves.

In the following examples for October 2020, Spring tides occur around the 4th and 18th of the month, Neaps occur around the 11th and 25th.
Aberdeen, Scotland October 2020
Plymouth, England October 2020
Dublin, Ireland October 2020
Local conditions cause differences in tide heights for a specific port or location, but the dates of Spring & Neap tides will be the same for anywhere in this part of the world.

Graphs of all the spring and neap tides for Cardiff until the end of 2024, can be found following the link below.
[Tidal Curves For 2023 - 2024]
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