The key parameters for determining the indirect effect are the effectiveness of an aerosol particle in acting as a cloud condensation nucleus, which is a function of size, chemical composition, of the mixing state and the ambient environment. In particular, aerosols act as nuclei on which water condenses. A high aerosol concentration therefore leads to clouds that contain many more, therefore smaller, water droplets, thus clouds with the same water content forming in cleaner areas. Since these clouds reflect more, this induces a negative radiative forcing which is called the first indirect effect, the cloud-albedo effect. The effect of aerosols on cloud height, duration and water content is classically referred to as the second indirect effect