In the light of the difference of microphysical characteristics between fogs and clouds, the spectrum characteristics of MODIS Channel 1, 6, 20, and 31 are simulated by utilizing the SBDART radiative transfer model. The results indicate that the measurements of these four channels all contain information of clouds and fogs and can be used to detect daytime fogs. A monitoring algorithm based on the simulations is presented and demonstrated by analyzing a fog case. The analysis shows that the monitoring method can improve the capability of daytime heavy fog and low cloud monitoring through satellites.