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Why does rain always fall in drops?

Why does rain always fall in drops, rather than in a sudden burst or torrent, no matter how heavy the rainfall?
Mark Savage from Greater London (Age: 45-54)

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One Response

  1. Hello Mark,

    I believe the reason for this is what we often call “surface tension”. More correctly, it should be referred to as “surface energy”. It acts as a kind of skin – a bit like the skin of a balloon. You can also think of it this way: there are attractive forces between all particles (atoms, molecules etc). Some of these are pretty weak (such as Van der Waals), but they are nevertheless important. In water there is a relatively strong type, called the hydrogen bond, which causes the molecules to cluster together. Within the interior of a volume of liquid water, any given molecule will experience forces, due to its neighbours, pulling it in all directions and so there will be no net effect. However, a molecule sitting right at the surface will only “feel” forces pulling it in towards the bulk. In the absence of external influences this causes the liquid to take up a spherical form. Incidentally, it’s also the reason planets are spherical – albeit on a rather bigger scale than raindrops!

    Of course, raindrops are subject to gravity, wind etc, so are not entirely free. But you could say they are almost in free-fall, so they will tend to maintain a roughly spherical shape.

    I think the size of the drop will have something to do with the strength of the inter-molecular forces, but maybe someone else could comment on that.

    As an afterthought, you might ponder this alternative, “lateral-thinking” type question: since in a random system (eg the molecules making up a liquid) one direction in space is equivalent to any other; and that a sphere is the most symmetrical 3-d shape we can imagine, why should a volume of liquid take up any shape OTHER than a sphere?

    Hope this helps.

    All the best,

    Mike

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