Why is some rain heavier than other rain?

12 August 2012

Part of the show Do Dogs Understand People?

RAIN

Credit:

CC0, via Unsplash

Question

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?

Answer

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.

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.