Although evidence has shown that enhanced aerosol concentration reduces cloud drop size and increases cloud albedo, this phenomenon is not always observed by satellite. Satellite derived correlation between cloud drop size and aerosol concentration can be either negative, insignificant or even positive, depending on the location of the clouds. In this study, we propose an analytical model describing the coupled effects of aerosol concentration and cloud depth on cloud drop size and explain one major cause of the puzzling correlation. Marine stratocumulus observed over northeastern Pacific during summer is analyzed to support the proposed explanation. The result also reassures the aerosol indirect radiative effect being still at work even for the regions where it appears no negative correlation between aerosol concentration and cloud drop size observed by satellite.