This thesis describes a study of Forbush decrease events. These are rapid decreases in the cosmic ray intensity in the Earth’s atmosphere, which are caused by a temporary increased magnetic shielding at Earth due to solar eruptions. The aim is to investigate how these transient ionization phenomena in the atmosphere affect aerosol and cloud creation and whether it is realistic to observe Forbush decrease events in climate data. The thesis involves a theoretical examination of the ionization caused by Forbush decreases based on studies of hourly neutron monitor data and muon telescope data as proxies for cosmic rays. A list of the ionization change in the troposphere of the strongest Forbush decreases as compared to the ionization change over the solar cycle is calculated and indicates that only a few events induce ionization changes comparable to the solar cycle. Studies of recently available high resolution satellite data and aerosol ground based measurements are presented. Here it is observed that significant decreases in the angstrom exponent from AERONET aerosols and cloud liquid water from satellites take place after the largest Forbush decreases. The timescales of this indicate that the ionization decrease caused by the Forbush decreases diminishes the aerosol nucleation rate which, over time, affects first cloud condensation nuclei size aerosols and then clouds. As a part of the thesis, a model of the growth of neutral sulfuric acid aerosols has been developed. Assuming an initial distribution of stable nucleated clusters, the model takes condensation and coagulation into account and includes various loss mechanisms. This model is used to investigate the growth of aerosols into cloud condensation nuclei size particles and to study the influence of nucleation rates and background vapour gas concentration on aerosol and cloud optical properties over short time. The model is used to examine experimental efforts at DTU Space on the role of ions in nucleation, as well as it is used to investigate observational data on Forbush decreases in aerosols. The model confirms the existence of decreases in angstromexponents observed in AERONET aerosol data under assumption of realistic ion induced nucleation rates. The work presented in the thesis indicate that the largest Forbush decreases affect aerosol formation and in turn cloud cover on a global scale.