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what the tides are doing... (and the UK South coast double tides - explained

skip the rant & go straight to the story of our double tides...


What the tides are doing...

...not an easy question to answer since the hydrographic office forced us to remove the details from the site: Apparently, they don't just own copyright of the raw data (observations); they claim copyright on the results of any calculations made from their data.

That means we're not allowed to publish the results obtained from the tidal prediction software we bought specially for the purpose, unless we pay huge sums of money in royalties to the hydrographic office. But get this - the hydrographic office is a government department (part of the MoD). The data is clearly not confidential since they sell/license it, so the only conclusion I can draw is that the government has gone into business and commerce.

Since the taxpayer has already paid for the MoD to do the observations and calculations, it does seem "a little unfair" to have to then pay once again to use what I've already paid for.

Thankfully, there are a number of websites who do publish tidal predictions - including one belonging to the hydrographic office - though most of them will only predict for today and/or the next few days, rather than the one year's worth previously found on this page.


EasyTide - hydographic office in disguise

WXTide32 - free prediction software to download

BBC - The BBC's ever-expanding website has a comprehensive set of tidal predictions for the UK

If you want to buy your own quality tidal prediction software, I can personally recommend both of:

Neptune Tides on 0118-988-5309, and

Tide Plotter

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The story of the South Coast double tides:

Some background facts:

  • The tides are governed by the relative positions of the earth & the moon & to a lesser extent, the sun.
  • When earth, sun & moon are all in line (cosmologically speaking), we have spring tides as a result of the additive combination of the gravitational pulls
  • When sun, moon & earth form a right angle, we have neap tides - sun and moon are out of alignment leading to reduced gravitational pull.

Still with me? Ok, let's move on a little:

  • Spring tides result in large tidal range (Ie: high highs & low lows)
  • Neap tides result in smaller tidal range (ie: lower highs & higher lows)
  • As the tides are governed by the moon, so the frequency of the tides is governed by the lunar cycle (new->waxing->full->waning->new) of approx 29.5 days.

During the lunar cycle, there are two periods of alignment, and two periods of misalignment, so the time between consecutive spring tides is 29.5/2 = 14.75 days. ie: just over 2 weeks. Likewise the time between consecutive neaps. Subsequently, it takes just over one week to go from springs to neaps, or from neaps to springs. Got it?

If the world were completely flat & featureless, and the oceans all of uniform depth, the tidal range would be more or less the same everywhere (land masses taken out of the equation). But there's the rub - for those that haven't noticed, about 3 tenths of the world's surface is not watery at all. And neither are the sea & ocean beds at an entirely consistent depth.

These situations cause the tidal flows to throw a bit of wobbly in some places. One of the wobbliest places on the planet (tide-wise) is the UK's south coast. Along a stretch of coast between Portland in the West, and Gosport to the East (although it starts to go wobbly around Lyme Regis), there is a double tide observed around spring tides. In Portland, it's a double low tide, at Poole a double high tide. These effects are easily observable in these places.

Now the big question: What causes this double tide along this part of the S coast?

Various theories have abounded for many years - many of them being to do with the presence of the Isle of Wight. I did a little research on the web to see what theories were presented there: The consensus seems to be that Wight is only a contributory factor, there being some four to 32 contributory factors, all playing a part at different states of the tide - one factor being the presence of the Cherbourg peninsula.

Whilst it doesn't give the whole story, one of the better descriptions I found is located here. Rather than reproduce it on this page and further expand this already inflated note, I shall leave those of you with sufficient remaining interest to look into this yourselves.

And here's another link to a scientific paper delivered in 1882: It's pretty long but does provide a valuable insight.

I'm indebted to Neil Millward for providing the following info (Oct, 2011):

Just for interest, you may like to note that the double tide phenomenon is observable, on occasions, in the River Dart [Dartmouth, Devon].  I have a trace from an electronic tide gauge in Dartmouth at 1 minute intervals showing a second, and slightly lower, HW 15 mins after the primary HW.  The Admiralty tide tables, according to the Head of Tides at UKHO, contain a note saying the phenomenon is observable between Newhaven and Start Point (a few miles S of Dartmouth).

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