Cosmics Leaving Outdoor Droplets or CLOUD is an experiment being run at CERN by a group of researchers lead by Jasper Kirkby to investigate the microphysics between galactic cosmic rays (GCRs) and aerosols under controlled conditions. The experiment began operation in November 2009.

In meteorology, a cloud is an aerosol comprising a visible mass of liquid droplets or frozen crystals made of water or various chemicals. The droplets or particles are suspended in the atmosphere above the surface of a planetary body. On Earth clouds are formed by the saturation of air in the homosphere when air cools or gains water vapor.