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Old 28 January 2011, 02:14   #11
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As stated already rain is generally associated with low pressure systems. Air pressure can have a huge effect on tidal heights. Tide tables assume a standard air pressure at sea level of 1013mb. As a rough guide variations in pressure either side of this will result in a difference for the predicted height of roughly 1cm per 1mb higher or lower so if pressure was 993 the tide would be approx 20cm higher than predicted and 20cm higher if it was 1033mb.

Combine this with the fact that fresh water is less dense (and there for weighs less) than sea (salt) water and it gets worse. As the fresh water is lighter than the incoming salt water it will be pushed back in a bit but as it's lighter it will flow over the top of the incoming sea water and flow out to sea (downhill). This can be a chunk of water anything up to 18 inches deep depending on geography, how much it's rained etc and can there fore give a big increase in water level and also give the visual effect that the river is ebbing when in fact the level is rising as the fresh water ebbs at the top but the sea water underneath is flooding and therefore increasing in depth.

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Old 29 January 2011, 04:31   #12
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Crikey, thanks Hamster! I had no idea things were that complicated.

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Old 29 January 2011, 05:09   #13
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Great websites and excellent explanations of weather shall keep them and pass it on to students great resource info for teaching.
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Old 29 January 2011, 09:24   #14
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It's also a question of time and space, a larger volume of water either fresh or salt water needs to move through the same space in the same time the level has to get higher, it will also get faster.

I used to live ner the river wear and during heavy rain and higher tides the water level rose a lot, there was also a good deal of mixing so the ideal laminar flow mentioned earlier may not have been happening. It wasnt the sort of river to swim in with your mouth open so a proper test was not conducted.

Also went out of Whitby an a diver cx'n assessment after torential rain and the amount of water and debris coming down ws impressive, 3500rpm just stand still good experience though.

New boat is here, very happy!
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