The Cosmics Leaving Outdoor Droplets experiment, also known as CLOUD, is an ongoing project that studies the connection between galactic cosmic rays and Earth climate using the Proton Synchrotron at CERN. It may be difficult to imagine the link between cosmic rays and the Earth's climate. The cosmic ray bombardment of the Earth's atmosphere affects cloud formation and clouds have a strong influence on the Earth’s energy balance. The Proton Synchrotron beams artificial “cosmic rays” into a cloud chamber to mimic the interaction under natural conditions. Within the chamber, the chemical and physical properties of cloud forming materials are being monitored and analyzed to tease out the effects of the high energy particles on aerosol production.
In 2016, an observation from the CLOUD experiment revealed that before the industrial revolution the Earth's atmosphere was much cloudier than previously thought. The majority of sulphuric acid in the atmosphere is the result of fossil fuel combustion, and it was deemed essential to initiate the formation of aerosol particles according to earlier observation. Therefore, industrialization around the globe should have brought in cloudier weather. But CLOUD shows that under the influence of cosmic rays, organic vapors emitted by trees produce lots of aerosol particles even without the presence of sulphuric acid. These vapor molecules help aerosol grow in size, which eventually allows them to seed clouds.