For about a decade now, cosmic rays have been thought to affect the formation of clouds, but no one had come up with any evidence for an exact mechanism. Now, a Danish experiment has shown that subatomic debris created when these high-energy particles collide with the atmosphere could be behind some of our cloud cover.

Henrik Svensmark of the Danish National Space Center in Copenhangen first noticed a connection between cosmic-ray intensity and cloudiness in the mid-1990s. He found that when the sun's activity was at its lowest, during which time about 25 per cent more cosmic rays reach Earth, the planet was 3 per cent cloudier than during solar maxima. The link was greeted with scepticism, but evidence continued to mount (New Scientist, 28 January, p 17).

Svensmark's team decided to try and nail the mechanism behind this link. They built a transparent 7-cubic-metre Mylar box, which they then flushed continuously with purified air. The concentrations of sulphur dioxide, ozone and water vapour were controlled to mimic conditions in the atmosphere. The idea was to find out what happens when natural muons and antimuons interact with this mixture. These charged particles are formed when cosmic rays hit the upper atmosphere.

The researchers found that when the muons knocked electrons off the molecules in the box, in the presence of the ozone, sulphur dioxide and water, plus artificial ultraviolet light, vast numbers of fine droplets of sulphuric acid appeared. These fine aerosols are the building blocks of larger aerosols - the nuclei around which water vapour can condense and form clouds.

To check that the electrons freed by the muons were doing this, the team bombarded the box with gamma rays to produce more free electrons, and found that the production of fine aerosols increased. Conversely, when they applied a strong electric field across the box to sweep away the free electrons, the level of aerosols dropped. The researchers believe this shows the electrons were aiding the formation of fine aerosols. "The catalytic behaviour of the electrons was a big surprise," says Svensmark. The work will be published in Proceedings of the Royal Society A.

A similar but more elaborate experiment named CLOUD is being set up at the CERN particle physics lab near Geneva, Switzerland. Starting this month, it will use a beam of accelerated particles rather than muon and gamma-ray sources to replicate the Copenhagen box. Later, it will explore the action of cosmic rays at all altitudes in the atmosphere.

Robert Bingham of the UK's Rutherford Appleton Laboratory near Oxford, who has been involved with the CLOUD project, does not consider the Danish experiment conclusive. "This means that more experiments under controlled conditions are badly needed."

From issue 2572 of New Scientist magazine, 10 October 2006, page 13