Minor Moon Times glitch: BoM 2021 RIP predictions

Packo's prediction for the slack water times at Port Phillip Heads.

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Minor Moon Times glitch: BoM 2021 RIP predictions

Postby packo » Sun, 03 Jan 2021 2:35 pm

Only barely worth a mention but the 2021 RIP predictions on the BoM website (and the linked *.pdf file) have Moon Phase dates mostly 1 day too early. The relevant BoM folk are now aware of this minor glitch. Probably they won't be fixing it since this (mis)information has been in the public domain since around April 2020, and nobody else seems to have noticed or commented on it.

It is a curious one since the "Moon Info" for all other tide locations is correct, as are the "Rip Streams" pages in the "Vic Tides 2021" published by the VRCA (but taken from BoM info). How did this glitch wind up in only one place?

My contention is that when collating "Moon Data" for the BoM's "Rip Predictions", somebody either forgot to specify the longitude of the Rip as "East", or perhaps incorrectly entered the time zone for displaying the results as UTC-10 hours instead of the correct Melbourne time zone of UTC+10 hours. That incorrect time zone translates to the local time zone of the Hawaiian islands, which is on the other side of the international dateline.

This error would have resulted in the Moon Phase times and dates (which occur at the same instant worldwide) being correctly reported for observers in Hawaii, rather than in Melbourne. The -20 hour time difference means any astronomical event occurring before 8pm Melbourne time would be reported as some time in the previous day for Hawaii. This would explain why about 80% of the Moon Phase dates in the BoM "Rip Predictions" are reported as one day too early.

No real biggie, but if you did rely on these predictions to see the first sliver of a New Moon in the Western sky, then you will not see it. Looking to the Eastern sky 14 days later for a rising Full Moon, you would find it already up, and not quite full. (Come back the next evening!)

Update 11/02/2021: Who would have thought the above piece of trivia would be in any way relevant to scuba diving? Well we are only in the 2nd month of the year and I saw today RedBoats had advertised an early evening night dive with a (wrong) "NEW MOON" date:-
I've sent a note to Luke but I'm sure nobody else but me really cares about these minor issues. Maybe I shall nudge the BoM guys again to fix it because you just never know who may be innocently using this incorrect moon information.

Update 23/03/2021: The saga continued on:- A few days ago I noted that the BoM Rip Predictions for 2022 are now available. Naturally I had a peek, only to discover the same Moon Info problem in those files! Been in touch with the BoM boffins about this. They have come back to say yes, somebody did get the timezone sign mixed up leading to a 20 hour error. All is now fixed for both 2021 & 2022 which is a nice outcome.

****** Enough on the Moon Glitch, What About the BoM/(Cardno) Slack Water Times? ******

The 2021 slack water predictions seem to follow the usual pattern of being on average about 5 minutes later than Packo times for Flood Slacks, and about 11 minutes later for Ebb Slacks. However there are times when particularly the Ebb slack times run around 20 minutes later with the flood tide already running and gaining strength at that time (according to me.)

Of course weather effects can easily shift times around by similar amounts, and all divers should be making these allowances. Nevertheless I have a concern that on average the BoM tables might lead to many folk "missing the bus" in getting into the water at the safest time. If instead, the BoM times had been earlier than the Packo predictions, then I probably wouldn't be pursuing this issue. No one is harmed by arriving too early at a southern PPB dive site, but turning up late is a different matter entirely.

Those who have followed my "tides journey" will know that I believe the later BoM times are due to a defective prediction algorithm and represent a potential risk for divers. It is proving a devilishly hard claim to prove because weather effects are also pulling the slack times around by similar amounts of time. I am making some progress with observations from a free drifting, GPS logging, "current tracker buoy". Some new things are being discovered that could be useful in the future but much work remains to be done first.

Nevertheless the algorithm used for the official predictions can still be attacked on theoretical grounds in that for a small number of days per year it produces predictions that are "clearly impossible". For 2021 some of these are around the very weak tides on Feb 25th & 26th and also 5th Sept. One of the defects in the algorithm is that its behaviour is particularly suspect for small and unusual tidal patterns.

The 8:56am time for the Ebb slack on the 26th Feb is 25 minutes behind the Packo time of 8:31am. The later time for the reversal from outgoing to incoming flow at the Heads is an "impossible time" given that at 08:56 the tide predictions for all Bay tide gauges from Geelong to the Heads are:-

Code: Select all

Geelong: Has been rising for 7 minutes.
Point Richards: Has been rising for 15 minutes.
Williamstown: Has been rising for 23 minutes.
Hovell Pile: Has been rising for 12 minutes.
West Channel Pile: Has been rising for 21 minutes.
Queenscliff: Has been rising for 224 minutes.
Point Lonsdale: Has been rising for 243 minutes.

With the predicted tides rising all over the Bay at 08:56 we must conclude there is inflow through the Heads and the Ebb slack is already over. ie. The official Ebb slack prediction time of 08:56 is "impossibly late".

If we take the Packo Slack Time of 08:31 those tide predictions become:-

Code: Select all

Geelong: Will fall for another 18 minutes.
Point Richards: Will fall for another 10 minutes.
Williamstown: Will fall for another 2 minutes.
Hovell Pile: Will fall for another 13 minutes.
West Channel Pile: Will fall for another 4 minutes.
Queenscliff: Has been rising for 199 minutes.
Point Lonsdale: Has been rising for 218 minutes.

This illustrates the "Packo volume balance" concept where at Heads slack water time with no inflow or outflow, we must have slowly falling waters over the vast northern areas of the Bay to exactly balance the rapidly rising waters over the six times smaller southern area of the Bay nearer the entrance. This balanced water transfer from north to south at Heads slack water time is simply due to the residual southward momentum in the central Bay region that is yet to experience slack water.

For Flood slacks the "balance point" is where the rapidly falling southern waters need to be balanced by slow rises all across the vast northern areas. Again this flow is the result of residual momentum in the central basin of the Bay where slack water is another 10 or so minutes away. Note that in this argument we can neglect any river inflow volumes, and any evaporation volume losses, because both are at a tiny scale compared to tidal volume changes.

We can visualise this concept as a squiggly "balance line" drawn right across the Bay at the instant slack water occurs at the Heads. That line marks the boundary between rising tides on one side of the line, and falling tides on the other side. For Ebb slacks the line connects all points having their low tide at the same time as slack water at the Heads. For Flood slacks the line joins all points having their high tide moment at that time.

Under the Packo modelling for most tide cycles this balance line wiggles its way across the Bay passing a little north of the West Channel Pile and a little south of Hovell Pile. The line generally shifts a few km to the south for weak tide cycles and a little further to the north for very strong tide cycles. In all cases the surface area north of the line is approximately six times the surface area to the south.

This is in accord with the fact that at slack water time at the Heads, the average rise or fall rate in that southern area is around six times faster than the average fall or rise rate in the six times larger northern area. Thus the net rate of water volume change is balanced across the two regions at close to zero, giving no net inflow or outflow through the Heads. To me this seems a robust and very common sense situation that must exist at Heads slack water time.

Under the BoM/(Cardno) predictions, the "balance line" is seen to wander around far more widely. Some of their predictions put it well south of both the West Channel and Hovell Piles, while other predictions (like 08:56 26/2/21) place the "balance line" several miles up the Yarra River! There is no possibility of "volume balance" in this last case.

Even slack time predictions on or after the Williamstown Hi/Lo tide times would seem to provide little scope for the necessary "volume balance". The official predictions have 176 slack predictions (or about 12%) that are in this category. The Packo predictions have zero slacks at or after the Williamstown Hi/Lo times.

****** What's All This "Packo Post" Stuff About Anyway? ******

For new readers to these posts, a summary of the aims of the various "Packo Rants" for scuba divers is:-

1) To stamp out an old "very delayed slacks" myth for places well inside the Heads. This is a dangerous (but persistent) myth.

2) To stamp out some official misinformation on how and why tidal streams reverse at the Heads.

3) To push for tidal stream "reversal rates" to be published alongside "reversal times". The present situation is a little like publishing a boating forecast that gives wind direction but not strength!

4) To push for weaknesses in the official slack water prediction algorithm to be addressed.

The score card so far is roughly:-

Item 1): Some success but some very stubborn pockets remain! Hopefully nobody dies.

Item 2): Success probable at some future time but changing personnel in the Port Authorities is very frustrating.

Item 3): Not so sure about achieving this despite it being an obviously worthwhile thing to do. Some without a science background find it confusing and perhaps cannot see any merit in it. Will probably have to wait until Item 2) is achieved. It then would be a more obvious and natural follow-on.

Item 4): This might be the toughest one because commercial interests are involved, and no one will want to admit to any deficiencies despite possible safety improvements. I'll really need some pretty good "proof in the field" to make any headway with this one.

cheers and safe diving in 2021.

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