New experiment to investigate the effect of galactic cosmic rays on clouds and climate

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Jasper Kirkby in front of the CLOUD prototype. The sketch illustrates the possible link between galactic cosmic rays and cloud formation (Image: CERN)

Geneva, 19 October 2006. A novel experiment, known as CLOUD (Cosmics Leaving OUtdoor Droplets), begins taking its first data today with a prototype detector in a particle beam at CERN1, the world's largest laboratory for particle physics. The goal of the experiment is to investigate the possible influence of galactic cosmic rays on Earth's clouds and climate. This represents the first time a high energy physics accelerator has been used for atmospheric and climate science.

The CLOUD experiment is designed to explore the microphysical interactions between cosmic rays and clouds. Cosmic rays are charged particles that bombard the Earth's atmosphere from outer space. Studies suggest that cosmic rays may influence the amount of cloud cover through the formation of new aerosols (tiny particles suspended in the air that seed cloud droplets). Clouds exert a strong influence on the Earth's energy balance, and changes of only a few per cent have an important effect on the climate. The CLOUD prototype experiment aims to investigate the effect of cosmic rays on the formation of new aerosols.

Understanding the microphysics in controlled laboratory conditions is a key to unravelling the connection between cosmic rays and clouds. CLOUD will reproduce these interactions for the first time by sending a beam of particles – the "cosmic rays" - from CERN's Proton Synchrotron into a reaction chamber. The effect of the beam on aerosol production will be recorded and analysed.

The collaboration comprises an interdisciplinary team from 18 institutes and 9 countries in Europe, the United States and Russia. It brings together atmospheric physicists, solar physicists, and cosmic ray and particle physicists to address a key question in the understanding of clouds and climate change. "The experiment has attracted the leading aerosol, cloud and solar-terrestrial physicists from Europe; Austria, Denmark, Finland, Germany, Switzerland and the United Kingdom are especially strong in this area" says the CLOUD spokesperson, Jasper Kirkby of CERN. "CERN is a unique environment for this experiment. As well as our accelerators, we bring the specialist technologies, experimental techniques and experience in the integration of large, complex detectors that are required for CLOUD." An example in the present CLOUD prototype is the gas system, designed by CERN engineers, which produces ultra-pure air from the evaporation of liquid oxygen and liquid nitrogen. "It's probably the cleanest air anywhere in the world", says Kirkby.

The first results from the CLOUD prototype are expected by the summer of 2007. The full CLOUD experiment includes an advanced cloud chamber and reactor chamber equipped with a wide range of external instrumentation to monitor and analyse their contents. The temperature and pressure conditions anywhere in the atmosphere can be re-created within the chambers, and all experimental conditions can be controlled and measured - including the "cosmic ray" intensity and the contents of the chambers. The first beam data with the full CLOUD experiment is expected in 2010.


Further information:
James Gillies Press Office, CERN
Tel: +41 22 767 4101
Mobile: +41 76487 4555
Email: James.Gillies at


The CLOUD collaborators are:

  • University of Aarhus, Denmark
  • University of Bergen, Norway
  • California Institute of Technology, USA
  • CERN, Switzerland
  • Danish National Space Center, Denmark
  • Finnish Meteorological Institute, Finland
  • University of Helsinki, Finland
  • University of Kuopio, Finland
  • Lebedev Physical Institute, Russia
  • University of Leeds, United Kingdom
  • Leibniz Institute for Tropospheric Research, Leipzig, Germany
  • University of Mainz and Max-Planck Institute for Chemistry, Mainz, Germany
  • Max-Planck Institute for Nuclear Physics, Heidelberg, Germany
  • Paul Scherrer Institute, Switzerland
  • University of Reading, United Kingdom
  • Rutherford Appleton Laboratory, United Kingdom
  • Tampere University of Technology, Finland
  • University of Vienna, Austria


Scientific Background:
The roots of the CLOUD experiment can be traced back two centuries, when the British Astronomer Royal, William Herschel, noticed a correlation between sunspots and the price of wheat in England. This marked the first observation that Earth's climate may be affected by variations of the Sun. However solar-climate variability has remained a great puzzle since that time, despite an intensive scientific effort. The well-known Little Ice Age around the 17th and 18th centuries - when sunspots all but disappeared for 70 years, the cosmic ray flux increased and the climate cooled - seems to be merely the latest of around a dozen similar events over the last ten thousand years. However there is no established mechanism for the brightness of the Sun to fluctuate on these time scales. The possibility of a direct influence on the climate of galactic cosmic rays (which are modulated by changes of the solar wind) is therefore attracting the interest of scientists. Satellite measurements suggest that the mechanism may involve an influence of cosmic rays on the amount of low cloud cover, which is the focus of the CLOUD experiment.



1. CERN, the European Organization for Nuclear Research, is the world's leading laboratory for particle physics. It has its headquarters in Geneva. At present, its Member States are Austria, Belgium, Bulgaria, the Czech Republic, Denmark, Finland, France, Germany, Greece, Hungary, Italy, Netherlands, Norway, Poland, Portugal, Slovakia, Spain, Sweden, Switzerland and the United Kingdom. India, Israel, Japan, the Russian Federation, the United States of America, Turkey, the European Commission and UNESCO have Observer status.

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