This is a post about experiments at CERN. But I am not going to mention the name of a proposed subatomic particle named after a certain British physicist whose name rhymes with figs. Not even once. You can check.

I am going to talk about something much more important: rain. Specifically, how rain drops start their life high up in Earth’s atmosphere. Before ending their life falling on me, as many have done this August.

A rain droplet say 1 mm across has lots of molecules, maybe 100,000,000,000,000,000,000 of them. But it starts off as a tiny droplet of more like 10 water molecules, before growing to a 100 molecules, then 1,000, etc. Now, when the droplet only has say 10 water molecules, the energy per molecule holding them together is lower than when you have a droplet of a million or a billion molecules. In practice this means that this tiny droplet tends to evaporate, not grow.

This tiny water droplet needs help to form and to grow. Anything that really likes to dissolve in water can do this. One example is salt (sodium chloride), just a few salt ions can dramatically stabilise a tiny water droplet as the salt ions far prefer being in water to being in air, so they fight against evaporation of the droplet. Sulphuric acid and ammonia can also help.

So often rain droplets start off as tiny nano-droplets not of pure water but of acid or salt water. But I guess that almost any ion will do here, as any ion will prefer water to air. And cosmic rays, i.e., high energy radiation from space, creates ions. The rays easily have enough energy to knock electrons off neutral molecules so creating charged ions.

The chain of events is then as follows, ray comes from space, knocks out electron so creating an ion, water droplet grows around ions, and then falls on my head. Simple.

The scientists at CERN, Kirkby et al., are using an accelerator to generate rays like cosmic rays (but with the advantage that they can be turned on and off) and then study how water droplets form with and without these rays. Their Nature paper is here. They do indeed find that their simulated cosmic rays increase the rate at which droplets form.

Now the fact that cosmic rays can create water droplets does not necessarily mean that they are important, other ions such as sulphuric acid, salt etc., may be dominant, leaving cosmic rays a minor role. For a discussion of this see the blog post of a real climate scientist. Also there is a short video of the collaboration head Jasper Kirkby discussing the experiment (but not really the results as they were not published when it was filmed).

But for me at least the most interesting thing concerns dirt. Simply speaking it seems if a major achievement of the CERN experiment is to make a super-clean chamber for studying droplet formation. They can then observe that even if they then add in the right amounts of sulphuric acid back in, droplets form at only the tenth of the rate they do in the atmosphere. It appears that some, as yet uncharacterised dirt, is having a significant effect on clouds and rain, and maybe even some effect on our climate.