The strange behaviour of water

Now that the cold weather is with us there are still some hardy souls who keep fishing on days when most sensible people are indoors tying flies for the new season! For those addicted fishermen these notes may help.

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All of us recognise the old style thermometer where a line of coloured liquid moves up and down a narrow tube to indicate temperature. The liquid is usually mercury or dyed alcohol. This begs the question “Why is water not used?” Your answer may be that for temperatures below 0 deg. C the thermometer would freeze and break. This is correct but there is a more fundamental answer than that.

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Most substances expand when heated and contract when cooled. This is not always true for water. There is a range of temperatures for which water expands when cooled. When the water cools from 100 deg. C to 4 deg.C is behaving normally i.e. it contracts. However, below 4 deg.C it expands and continues to expand as it changes to ice.

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We normally expect warm water to rise and cool water to sink but below 4 deg. the opposite happens WARM WATER SINKS!

Without this strange behaviour of water, our deep oceans would be mostly ice at the bottom and life as we know it would not have evolved! On a more mundane level knowledge of the behaviour of cold water can help us with our early and late season fishing.