Das Projekt "CERN-CLOUD project" wird vom Umweltbundesamt gefördert und von Paul Scherrer Institut, Labor für Atmosphärenchemie durchgeführt. CLOUD is an acronym for Cosmics Leaving OUtdoor Droplets. The scientific objective of CLOUD is to investigate the influence of galactic cosmic rays (GCRs) on ions, aerosols, cloud condensation nuclei (CCN) and clouds, with the CLOUD facility at CERN, and thereby to assess the significance of a possible 'solar indirect' contribution to climate change. Aerosols and clouds are recognised as representing the largest uncertainty in the current understanding of climate change. The Intergovernmental Panel on Climate Change (IPCC) estimates that changes of solar irradiance ('direct solar forcing') have made only a small (7Prozent) contribution to the observed warming. However, large uncertainties remain on other solar-related contributions, such as the effects of changes of galactic cosmic rays on aerosols and clouds. CLOUD aims to settle the important unanswered questions of the IPCC on possible cosmic ray effects on clouds and climate, and to help sharpen our understanding of the anthropogenic contribution to global warming. The scientific programme of CLOUD will involve the establishment of a central CLOUD facility in a beamline (T11) at the CERN Proton Synchrotron accelerator, comprising a large aerosol chamber, within which the atmosphere is recreated from ultra-pure air with added water vapour, trace gases under study and, for certain experiments, aerosols. The chamber will be equipped with a wide range of sensitive instruments to analyse their contents via optical ports or sampling probes. The accelerator provides an adjustable and precisely measurable beam of 'cosmic rays' that closely matches natural cosmic rays in ionisation density, uniformity and intensity, spanning the atmospheric range from ground level to the maximum around 15 km altitude. In contrast with experiments in the atmosphere, CLOUD will be able to compare processes when the cosmic ray beam is varied, and all experimental parameters can be precisely controlled and measured. More information is found at the CLOUD website http://cloud.web.cern.ch/cloud/.