These ions are thought to help create new particles called aerosols and, when aerosols grow above a certain size, they become the seeds around which clouddroplets form.
Using a particle accelerator, he demonstrated that cosmic rays colliding with molecules in the atmosphere can, in fact, cause gaseous water vapor to condense into cloud-forming droplets.
These are able to measure the spectrum of the light reflected from a cloud so accurately that Dr Rosenfeld could deduce the average size of its water droplets from the amount of light that they reflected.