East Coast Tides - Predictions vs Reality?

Little Rascal

Little Rascal

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I would like to better understand the non-astronomical factors affecting tides on the east coast.
I understand the general principles but what are the things to look out for that cause the real tides to vary from predictions in this area?

Thanks,
Jon
 
Little Rascal said:
I would like to better understand the non-astronomical factors affecting tides on the east coast.
I understand the general principles but what are the things to look out for that cause the real tides to vary from predictions in this area?

Thanks,
Jon
Click to expand...

Air pressure mostly. When there is high pressure it holds the water down, and VV. Also constant high winds can push the water up or down depending on the wind direction
 
Issue on surges is the air pressure NE Scotland, a rapid increase in pressure there when combined with the flood moving down the coast can have a big effect.
 
That's very interesting - thanks for the link.
That's more of a variation than I would have expected - nearly half a metre each way within a day or so!
Can anyone explain what contributes to the negative surge then?
 
A rule of thumb is that a 1 mb change in pressure away from 1013 mb means a change in Sea Level of 1cm, so (looking at the records on Walton Lifeboat weather station), highest and lowest pressures recorded are 1044 and 964mb, so for the first the sea level would have been 31cm below prediction given no other effects, and for the second, 49cm higher
 
Gladys said:
A rule of thumb is that a 1 mb change in pressure away from 1013 mb means a change in Sea Level of 1cm, so (looking at the records on Walton Lifeboat weather station), highest and lowest pressures recorded are 1044 and 964mb, so for the first the sea level would have been 31cm below prediction given no other effects, and for the second, 49cm higher
Click to expand...

I am very greatly obliged to m'learned friend - but facing a low of 964 mb, I suspect I'd have more on my mind than 49 cms of possible higher tide...........
 
Gladys said:
A rule of thumb is that a 1 mb change in pressure away from 1013 mb means a change in Sea Level of 1cm, so (looking at the records on Walton Lifeboat weather station), highest and lowest pressures recorded are 1044 and 964mb, so for the first the sea level would have been 31cm below prediction given no other effects, and for the second, 49cm higher
Click to expand...

This calculation is absolutely correct for a static situation, but as for all systems involving large masses, it's the dynamic calculation that dominates. If a series of depressions blows over the northern N sea, sucking the sea up then pushing it down, in synchronism with the flood and ebb of the tide, it's just like giving a pendulum a series of small pushes at its natural frequency. The effect of this on tidal height is far greater than the static effect of a sustained pressure difference which is very well given by Larry's method. In fact, it was precisely a dynamic situation like this which gave rise to the 1953 Big Tide.

Of course, pressure variations in other areas also have their effect and it was not until computers got big enough to run some pretty fancy mathematical models of the oceans, and weather forecasting improved (also mostly because of mathematical modelling) in the 1970's that it became possible to make any useful predictions. This is now done for the UK by the National Tidal & Sea Level Facility (NTSLF):

http://www.ntslf.org/

part of the National Oceanography Centre in Liverpool, who have provided the excellent predictions of the surges we have seen over the past couple of years. That capability in 1953 would have saved hundreds of lives.
 
In the past couple of years we have had occasional positive and negative surges of up to a metre at Sheerness for no apparent reason, not obviously related to sustained wind or unusual atmospheric pressure. Sometimes the dynamic effects, as explained above, are subtle but cumulative.

The moral is not to bother quoting/calculating tidal heights to the nearest centimetre! Secondary port corrections are mostly an exercise in futility.
 
With regard to horoscopes did anyone see Dara O'Brian's suggestion that horoscopes are replaced by racism? After all with horoscopes you only get 12 stereotypes. Racism offers many many more, so statistically is more likely to be reliable. And for all intents and purposes tidal predictions I find are usually pretty reliable - instances where I need precise indications are rare - usually marina cills and they have tide gauges. More important than precise depths is flow direction. Ten centimetres won't affect passage planning but half an hour favourable tide can.
 
The NTSLF site doesn't seem to be giving predictions or real time data at the minute:( is that just for me or can other people access it?
Are there any other sources of actual tidal heights?
 
Regardless of the cause..

It does make for interesting nav if you are calculating the time when you can get over the bar..

TCG broadcast tide hights at various times of the day so you can compare against predicted.
 
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