CLOUD – Cosmics Leaving OUtdoor Droplets

Cosmic rays and cloud formation

CLOUD is an experiment that uses a cloud chamber to study the possible link between galactic cosmic rays and cloud formation. Based at the Proton Synchrotron at CERN, this is the first time a high-energy physics accelerator has been used to study atmospheric and climate science; the results could greatly modify our understanding of clouds and climate.

Cosmic rays are charged particles that bombard the Earth's atmosphere from outer space. Studies suggest they may have an influence on the amount of cloud cover through the formation of new aerosols (tiny particles suspended in the air that seed cloud droplets). This is supported by satellite measurements, which show a possible correlation between cosmic-ray intensity and the amount of low cloud cover. Clouds exert a strong influence on the Earth’s energy balance; changes of only a few per cent have an important effect on the climate. Understanding the underlying microphysics in controlled laboratory conditions is a key to unravelling the connection between cosmic rays and clouds.

The CLOUD experiment involves an interdisciplinary team of scientists from 18 institutes in 9 countries, comprised of atmospheric physicists, solar physicists, and cosmic-ray and particle physicists. The PS provides an artificial source of ‘cosmic rays’ that simulates natural conditions as closely as possible. A beam of particles is sent into a reaction chamber and its effects on aerosol production are recorded and analysed.

The initial stage of the experiment uses a prototype detector, but the full CLOUD experiment will include an advanced cloud chamber and a 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.