The Cosmics Leaving Outdoor Droplets (CLOUD) experiment uses a special cloud chamber to study the possible link between galactic cosmic rays and cloud formation. Based at the Proton Synchrotron (PS) at CERN, this is the first time a high-energy physics accelerator has been used to study atmospheric and climate science. The results should contribute much to our fundamental understanding of aerosols and clouds, and their affect on climate.

What can cosmic rays tell us about climate?

Cosmic rays are charged particles that bombard the Earth's atmosphere from outer space. Studies suggest they may influence cloud cover either through the formation of new aerosols (tiny particles suspended in the air that can grow to form seeds for cloud droplets) or by directly affecting clouds themselves. 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. However, despite its importance for climate, aerosol formation is poorly understood. Measuring the underlying microphysics in controlled laboratory conditions is important for a better understanding of atmospheric aerosol and is the key to unravelling the possible connection between cosmic rays and clouds.

What does the CLOUD experiment do?

A brief tour of the CLOUD experiment at CERN, and its scientific aims (Video: Noemi Caraban/CERN)

The CLOUD experiment involves an interdisciplinary team of scientists from 17 institutes in nine countries, comprising atmospheric physicists and chemists, and cosmic-ray and particle physicists. The Proton Synchrotron provides an artificial source of “cosmic rays” that simulates natural conditions between ground level and the stratosphere. A beam of particles is passed through the cloud chamber and its effects on aerosol production or on liquid or ice clouds inside the chamber are recorded and analysed.

The experiment includes an advanced cloud chamber equipped with a wide range of external instrumentation to monitor and analyse its contents. The temperature conditions anywhere in the atmosphere can be recreated within the chamber. All experimental conditions can be controlled and measured, including the “cosmic ray” intensity and the trace atmospheric vapours in the chamber, which are set to levels of only a few molecules per trillion.

What has CLOUD shown us about our world?

In 2014 CERN’s CLOUD experiment made a huge discovery when it showed that biogenic vapours emitted by trees and oxidised in the atmosphere have a significant impact on the formation of clouds, thus helping to cool the planet.