Experimental investigation of atmospheric coherence diameter is presented in order to examine relative effects of turbulence and aerosol forward scattering. The investigation includes measurements through the open atmosphere for path lengths of several kilometers. In addition to turbulence degradation of atmospheric coherence diameter, it is shown here that aerosol forward scattering also causes severe limitations, particularly for short exposure. Two methods, direct (spatial domain) and indirect (spatial frequency domain), for measuring atmospheric coherence diameter are presented. The methods are theoretically and experimentally independent. Results of both methods are in very good agreement, emphasizing measurement reliability. It is shown that, in contradiction to turbulence, aerosols affect light coherence identically for both short and long exposures. Experimental results during rather extreme atmospheric conditions such as fog are presented too. The results here are applicable to cost-effective imaging system design, and to predicting imaging system performance through the atmosphere.