Cosmics Leaving Outdoor Droplets or CLOUD is an experiment being run at CERN by Jasper Kirkby to investigate the microphysics between galactic cosmic rays (GCRs) and aerosols under controlled conditions. The experiment began operation in November 2009. The group hope to definitively answer the question first posited by Henrik Svensmark and colleagues in 1997 of whether solar modulated cosmic rays have a significant effect on cloud cover and therefore the global climate.The experiment consists of a 3 m diameter aerosol chamber which is exposed to an adjustable particle beam that simulates GCRs at any altitude or latitude. The chamber is filled with air, water vapour, selected trace gases and can be operated at any temperature or pressure found in the atmosphere. UV illumination allows photolytic reaction. The chamber contains an electric field cage to control the drift of small ions and charged aerosols.CERN posted a 2009 progress report on the CLOUD project. J. Kirkby (2009) reviews developments in the CERN CLOUD project and planned tests. He describes cloud nucleation mechanisms which appear energetically favourable and depend on GCRs.On 24 August 2011, preliminary research published in the journal Nature showed there was a connection between Cosmic Rays and aerosol nucleation. Kirkby went on to say in the definitive CERN press Release "Ion‐enhancement is particularly pronounced in the cool temperatures of the mid‐troposphere and above, where CLOUD has found that sulphuric acid and water vapour can nucleate without the need for additional vapours. This result leaves open the possibility that cosmic rays could also influence climate. However, it is premature to conclude that cosmic rays have a significant influence on climate until the additional nucleating vapours have been identified, their ion enhancement measured, and the ultimate effects on clouds have been confirmed.