The European Organization for Nuclear Research, CERN, is creating an atmospheric research facility at its particle physics laboratory in Geneva. Called CLOUD, it will consist of a special cloud chamber exposed to pulses of high-energy particles from one of CERN’s particle accelerators, the Proton Synchrotron. Conditions prevailing in the Earth’s atmosphere will be recreated in CLOUD, and the incoming particles will simulate the action of cosmic rays. An elaborate set of instruments will trace the physical and chemical effects of the particle pulses.
The project was first proposed by Jasper Kirkby of CERN in 1998, in response to the discovery in Copenhagen of an apparent link between cosmic rays and clouds. The name CLOUD is an acronym for ‘cosmics leaving outdoor droplets’. More than fifty atmospheric scientists, solar-terrestrial physicists and particle physicists from seventeen institutes in Europe and the USA joined Kirkby’s team, including Henrik Svensmark and colleagues at the Danish Space Research Institute (now called the National Space Institute).
Financial problems at CERN left the project 'on ice' until its eventual approval in March 2006. By then, the simpler SKY experiment in Copenhagen was beginning to give results. The multinational team decided that the first use of the experimental site should be a re-run of SKY, in the autumn of 2006. The Danish team has built a duplicate of the apparatus, SKY-2, which was tested successfully at CERN and provided important inputs for the design of the CLOUD facility. Other scientists from the CLOUD team contributed additional instruments.
The main cloud chamber for the CLOUD facility is expected to begin operating in 2010. Apart from the use of controllable accelerated particles rather than natural cosmic rays, and more extensive instrumentation, the main difference from the SKY experiment is that CLOUD will be able to use air at low pressures and low temperatures. It will thus reproduce conditions high in the atmosphere, while SKY explores cosmic-ray action only in warm, dense air close to the ground.