• Categories

  • Most Popular Questions

  • Recently Viewed Questions

  • Recent Answers

    How To Make a Digita… on What does a frequency of 100 H…
    Daigrepont on Can an earthquake cause air tu…
    Benedict on How did God come into exi…
    joshua on How does the human body g…
    Ian on How did God come into exi…
  • Recent Questions

  • Blog Stats

    • 2,227,362 hits
  • Visitors since 11-3-08

    counter create hit
  • Terms and Conditions

  • Warning

    We are doing maintenance on this site, so some posts may disappear for a short time. Sorry. Normal service will soon be resumed...
  • Pages

  • March 2008
    M T W T F S S
    « Feb   Apr »
  • Archives

  • Meta

  • Advertisements

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)


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,


Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s

%d bloggers like this: