Tuesday, 19 April 2011

Where does the water come from?

This is the first of, inshallah, a series of posts that look at "paradigm shifts" in our understanding of the world around us.

Sometimes, it is easy to forget how much of the scientific understanding that we now have - and take for granted - was only discovered in the 300 hundred years (or even in the last 100 years).

This particular post was provoked by a comment on a BBC4 programme on the relationship of the British people to rain. The programme mentioned that, during the European Enlightenment, natural scientists were trying to understand where the water in rivers and clouds came from.

The French scientist Pierre Perrault provided the first piece of the puzzle by analysing the rainfall over an area and showing that it was sufficient to account for the water flowing in the river.

But where was the rainfall in the clouds coming from?

Edmund Halley (yes, the comet one) turns out to be something of a polymath and spent some time considering this issue. He performed an experiment in which he placed a 8" bowl of water on a set of scales and warmed it to the temperature of a hot summers day. After 2hrs, he found that the weight of water in the bowl had unsurprisingly, reduced by some 15grams. Critially, Halley then considered how much water evaporation this would equate to in a sea, such as the Mediterranean.

The answer is 5.3 billion tons every 12 hrs !


He then demonstrated that this was about three times the amount needed to provide the water for the largest rivers flowing into the Atlantic.

So suddenly, mans view of the natural world was changed, the scale of the evaporation from the oceans revealed and the broad outline of the water cycle revealed.

One can only imagine what was going through Halleys mind as he obtained the result and realised its implications. . .

"So let us compute the calculation. . . 3 add 9, so that's 2 carry over 1. . now the millions column.. . .and finally we have. . .. Gadzooks ! I am astounded at the enormity of the result, my flabber is truly gasted, my gob completely smacked and my dum is utterly founded. One can scarcely credit it, that such a volume of water can evaporate into the atmosphere, as though a thousand Niagras are silently and invisibly flowing up into the clouds every day. I think I need a sit down and a cup of tea. . . .

Incidentally, the breadth of Halleys achievements is incredible, why not check him out on Wikipedia?

Download an article on this topic here :

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