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Rising water temps

Connected, but not relevant to temperatures...

I was once able to watch a group of barbel sitting effortlessly on the bottom, just before a significant drop-off. Because of the surface tension being 'bent' for want of a better phrase, is there an increase in the water pressure at such a point and does this enable the fish to hold their places more easily (a bit like the down-force created by a rear wing on an F1 car)? If so, would these not be excellent places to place a bait even on rivers where you can't actually observe the fish?
 
Linear pressure will be higher in shallower water because water needs to flow faster for the same volume flow rate. Downward pressure wouldn't increase/decrease much for "steady state" flow, for a given depth.
 
Sam Lloyd said:
Pretty sure you've just told me a formula I've already stated.... If you want to know how much of a change it can be ill tell you. The difference between 950mbar and a 1050mbar day will be a meter in pressure level. At 2 meters abour 85 percent of the pressure will be due to air pressure, not the water.
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What is a 'mbar day' Sam?

Anyway....according to google :D (all figures approximate)

Your quoted nominal 950 mbar will exert 13.8 psi of pressure on any surface (in our case, water)

Your quoted nominal 1050 mbar will exert 15.2 psi of pressure

Every foot of water exerts .43 psi pressure (so 10ft = 4.3 psi, and so on)

So, at 2m (6.56 feet) the water pressure would be 6.56 x .43 psi = 2.82 psi

at 1m (3.28 feet) the water pressure would be 3.28 x .43 psi = 1.41 psi

(Actually that Im depth information isn't strictly relevant to your maths, as you ended up stating only what the difference would be between 950 mbar of air pressure and 1050 mbar, given a depth of 2m )

So,

with air pressure at 950 mbar, with a 2m depth, the total pressure = 2.82 psi +13.8 psi = 16.62 psi. So, the air pressure is (13.8 over 16.62) x 100 = 83% of that total pressure.

with air pressure 1050 mbar, and a 2m depth, the total pressure is = 2.82 psi + 15.2 psi = 18.02 psi. So, the air pressure is (15.2 over 18.02) x 100 = 84.4% of that total pressure. (as you say, about 85%)

However, you were saying 'How much of a difference that will be'. The actual pressure difference felt by the fish at 1050 mbar air pressure, compared to that at 950 mbar, with a depth of 2m....is an increase of only 1.4% of the total....not exactly the huge difference you were implying methinks?

Oh...and water is effectively NOT compressible, as Jim stated.

All good fun though Sam, lol.

Cheers, Dave.
 
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elementary physics BFW style!!! ;)

Can I throw into the mix, changing water density and viscosity?

oh and Liquids are most certainly compressible!
 
Sam Lloyd said:
Pretty sure you've just told me a formula I've already stated.... If you want to know how much of a change it can be ill tell you. The difference between 950mbar and a 1050mbar day will be a meter in pressure level.

For 950 mbar, pressure at depth "x" on day 1 = 950mbar + water pressure. pressure at depth "x" on day 2 = 1050 + water pressure. For the same depth water pressure will be the same, as is depth x. so stick them together and you get day 1 +100mb = day 2. 100mb= density x gravity x depth. we know gravity is 9.81 (we will say 10 to simplify the maths for you Dave) and density is 1 so depth change for the same pressure level is 1 meter. That doesn't matter if your at 2 metres or 2 kilometres.


At 2 meters abour 85 percent of the pressure will be due to air pressure, not the water.

atm/atm+water= 83%, ie about 85%.
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Dave, have you studied thermodynamics or fluid mechanics??

And water is compressible but had very little to do with any of this. Which confuses me as Jim stated he didn't understand the influence of air pressure on water pressure, then went on to try and lecture me on hydraulic systems :rolleyes:
Someone also said solids are incompressible...... Lectures on electron degeneracy pressures by PM only because I'm bored now....:D
 
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Sam Lloyd said:
Dave, have you studied thermodynamics or fluid mechanics??

And water is compressible but had very little to do with any of this. Which confuses me as Jim stated he didn't understand the influence of air pressure on water pressure, then went on to try and lecture me on hydraulic systems :rolleyes:
Someone also said solids are incompressible...... Lectures on electron degeneracy pressures by PM only because I'm bored now....:D
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Me...studied thermodynamics or fluid mechanics :eek: Certainly not Sam :D that is why my original post started with the words 'according to google'...and a smilie. However, it struck me afterwards that that may have been too subtle for some ......

If you really wish to 'simplify the maths for me', as you so condescendingly put it, then stop playing with words Sam, and tell me, expressed in psi, what the total pressure would be at a depth of 2m of water, when air pressure was at 950 mbar....and secondly, what it would be at that same depth, if the air pressure was at 1050 mbar...just that Sam, nothing more. I don't want to know about day one/day 2...just the figures, the pressure in pounds per square inch, for each of those two examples.

Ok, water IS compressible if we wish to be pedantic, but the amount it is compressible by in normal circumstances is considered marginal, so as I said, EFFECTIVELY (that is the word I used in my first post Sam), it is reckoned to be incompressible. It requires HUGE pressure to compress water, at normal temperatures, as you well know. To demonstrate that, just for simple souls like myself, to sort of put things into perspective.....perhaps you would care to enlighten us by explaining the amount a given volume of water decreases by (the degree of it's compression) in the deepest areas of the ocean, where the pressure can be 8 tons per sq. inch.

The answer is not a lot...but you tell us how much Sam, just so we know.

Cheers, Dave
 
If the atmosphere is 950mbar it will be 1150 mbar at 2 meters. If the atmospohere is 1050 is will be 1250 at 2 metres. So its a difference of 1.45 PSI :)

As for how much mater is compressed, someone has already quoted wiki as saying its 1.8 percent. I've never said it would affect shallow depths, I just conrrected a common misunderstanding ;)

Merry xmas Dave
 
Sam Lloyd said:
If the atmosphere is 950mbar it will be 1150 mbar at 2 meters. If the atmospohere is 1050 is will be 1250 at 2 metres. So its a difference of 1.45 PSI :)

As for how much mater is compressed, someone has already quoted wiki as saying its 1.8 percent. I've never said it would affect shallow depths, I just conrrected a common misunderstanding ;)

Merry xmas Dave
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I was about to say...:cool:
 
Well there has been a plethora of information posed on temperature, pressure and goodness knows what else and the original poster is probably none the wiser.

It is not possible (thank goodness) to predict the outcome of a day’s fishing from the prevailing conditions. I have caught well on days that looked hopeless and struggled on days that seemed perfect and that will be true of all of us. However what you can do is adapt the way you are fishing on any given day to maximise your chances of catching well. Successful match men do this week in and week out but many of the anglers I see just plough the same old furrow all day long and then blame anything but their ability to adapt to the day as to why they are not catching.
 
David Gauntlett said:
What is a 'mbar day' Sam?

Anyway....according to google :D (all figures approximate)

Your quoted nominal 950 mbar will exert 13.8 psi of pressure on any surface (in our case, water)

Your quoted nominal 1050 mbar will exert 15.2 psi of pressure

Every foot of water exerts .43 psi pressure (so 10ft = 4.3 psi, and so on)

So, at 2m (6.56 feet) the water pressure would be 6.56 x .43 psi = 2.82 psi

at 1m (3.28 feet) the water pressure would be 3.28 x .43 psi = 1.41 psi

(Actually that Im depth information isn't strictly relevant to your maths, as you ended up stating only what the difference would be between 950 mbar of air pressure and 1050 mbar, given a depth of 2m )

So,

with air pressure at 950 mbar, with a 2m depth, the total pressure = 2.82 psi +13.8 psi = 16.62 psi. So, the air pressure is (13.8 over 16.62) x 100 = 83% of that total pressure.

with air pressure 1050 mbar, and a 2m depth, the total pressure is = 2.82 psi + 15.2 psi = 18.02 psi. So, the air pressure is (15.2 over 18.02) x 100 = 84.4% of that total pressure. (as you say, about 85%)

However, you were saying 'How much of a difference that will be'. The actual pressure difference felt by the fish at 1050 mbar air pressure, compared to that at 950 mbar, with a depth of 2m....is an increase of only 1.4% of the total....not exactly the huge difference you were implying methinks?

Oh...and water is effectively NOT compressible, as Jim stated.

All good fun though Sam, lol.

Cheers, Dave.
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That's a cracking post, Dave. I had intended to reply to Sam but you've done the job and much better than I could have.
 
Lets assume for practical purposes water is not compressible then any pressure difference in air pressure will be transmitted directly to the fish (which certainly is compressible) and detected by its swim bladder. What it does with that information is the important thing and what we should be discussing.
 
Adrian Williams said:
Lets assume for practical purposes water is not compressible then any pressure difference in air pressure will be transmitted directly to the fish (which certainly is compressible) and detected by its swim bladder. What it does with that information is the important thing and what we should be discussing.
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Absolutely spot on
 
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