Figure 1 shows a dataset employed for a statistical approach of the magnitude of aerosol-cloud interactions. The cloud droplet number concentration (CDNC) in the lower part of the cloud is plotted in dependence of the aerosol extinction coefficient below the cloud. The scatter of the data is due to the multitude of effects, which have an impact on cloud microphysical properties. This multitude makes it very challenging to isolate and quantify the relevant effects. However, the data show a trend. Higher aerosol loads lead to an increased CDNC at cloud base. This finding is in accordance with the predictions of the Twomey effect. Aerosols from below the cloud are carried into the cloud with updrafts and get activated, forming new cloud droplet. The magnitude of this effect is obtained from the slope of the linear fit of the displayed data, denoted as ACI, which is common measure in climate science (cf. McComiskey et al., JGR, 114, 2009).