John Woods: experiments on droplets in clouds

John Woods describes his experimental work on the behaviour of water droplets in clouds, conducted as part of PhD in Imperial College, London, early 1960s.

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I did my PhD – the project that John Mason gave me was a very good one, it was to try and understand the early stages of tropical rainfall. I mean there’d been a marvellous man, L F Richardson, he had studied turbulence and he had suggested, and he was right, that turbulence starts with great big eddies which are unstable and make smaller eddies and smaller eddies and smaller eddies and get – and there’s a great cascade through the spectrum, you know, wave number spectrum. But the question is as the eddies get smaller and smaller and smaller they begin to feel viscosity, the molecular viscosity, which suddenly, when they get small enough, soaks all the energy out of them and converts it to heat, the random motion of molecular rather than the random motion of the fluid dynamics. And the question is at what scale does that occur?  And the Russians had worked that out and I did the sum and worked out what the scale would be in the clouds. And it was amazing that the – I knew because I’d done lots of experiments about how far droplets fell as they rolled around each other and failed to coalesce and how far they went falling through the air, and it was less than the minimum scale of turbulence. Because the viscosity didn’t allow it to get to any smaller scale; it was pouring down from larger scales, smaller and smaller, smaller, and then at about a few millimetres it stops, it turns into heat. And so I realised that we can do an experiment in which you dodn’t have to have turbulence and I’d been making wind tunnels with turbulence in: very complicated [laughs] difficult things to use. I realised the whole thing was actually laminar flow. That if you sit on a droplet in the - imagine you’re sitting on a droplet in a cloud rather like Einstein sitting on a photon, there’s another droplet around, will it or will it not gobble you up and it’s coming down but the stream flow around it is pushing you sideways and you, 'oh got away with it': the predator didn’t get me. Because the predator’s own slipstream was pushing you around and the idea is maybe the shear from the turbulence, the little eddy is just countering that and pushing you together when you – the predator thought was going to miss, or the prey thought it was going to not be captured, it finds itself in a shear which is pushing it. And the shear, again I discovered from my reading of turbulence, gets bigger and bigger, the biggest shears of all are at the tiniest size. So it seems everything was right, so I made a wind tunnel with a laminar flow but in the middle the flow is steady and then there was a shear and it went down. And I had droplets go through so that they met just where there was a shear of the magnitude that I had calculated would exist in clouds. So I showed that if you had a shear of the right magnitude in the right orientation when the droplets were coming down, sure you could see them coalescing, by streak photography I showed it happening.  We can now extrapolate from this very simple laboratory experiment to the whole cloud. And it works a dream. But that’s what physics is about, it’s spotting something that allows you to do an incredibly simple experiment that costs almost nothing but actually hits the nail on the head; that’s why I love physics.

  • Interviewee John Woods
  • Duration 00:03:51
  • Copyright British Library Board
  • Interviewer Paul Merchant
  • Date of interview 2/16/2012
  • Shelfmark C1379/64

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