[thedoc (OP)]

Rain falls from clouds as we know, the thing that attracts it is gravity.  We get light rain and we get heavy rain.  The distance of the clouds above us is the same, droplets will be slightly bigger and so on, but why do we get heavier rain that beats incessantly and bounces off the roads?  Where does the energy come from that gives it our impetus and the energy that we feel down below?
Asked by Glynn, Cambridge


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[thedoc (OP)]

We answered this question on the show...



Chris -   If you're a regular Naked Scientists listener, you might  remember we interviewed a gentleman earlier this year who had actually  found fossilised raindrops.</link>  He had gone to South Africa and found some  volcanic ash, and the volcanic ash had all these little dots in it, and  he realised that these are fossilised raindrops.  And he then went and  got some Hawaiian volcanic ash and dropped droplets down a stairwell  into this ash to work out what dot pattern he could get, and he was able  to try and work out what the atmosphere of Earth would’ve been like  when those rocks were fossilised millions of years ago, based on the  impact pattern on the Hawaiian ash, extrapolating it back to the South  African fossilised raindrops.  And the assumption he was making in doing  that work is that the raindrops are going to have a maximum size and  therefore velocity in air because as the air changes its density – so if  the atmosphere was as dense in the past as it is today - then you would  expect the raindrop’s maximum velocity and size, and so on, all to be  the same.  It’s quite an interesting way of going about the study.

Basically, when you have a rain cloud, you’ve got lots of little nucleation sites.  These are sites which will allow a droplet to form from the vapour in the cloud, and those droplets will attract water to  them, and they will reach a certain size.  They’ll be held up in the  cloud by updrafts - so air rushing upwards - and the stronger the  updraft, the bigger the particles that it can support in the cloud  before that’s then dropped down.  So, if you have a fairly weak updraft,  then the particles are not going to get very large before their  attraction down to Earth’s surface under the force or attraction of  gravity is going to overwhelm the updraft and they’ll fall.  But if you  have some very strong updrafts which in big storm clouds can be really  strong, they will support fairly bulky droplets for quite a long time  before the droplets get really very large, and then they come cannoning  down.

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[neilep]

I love big fat rain !!...so much nicer than little thin rain !!