Roughly half of absorbing aerosol is due to biomass burning, much of it due to boreal forest fires, yet the effects of smoke on clouds have mainly been studied in the tropics. In this study 3 summers of MODIS atmosphere products are used to compare this relationship for Canada and Siberia during the forest fire season. To minimize shadowing and illumination effects, the experiment geometry is kept within a limited range for both sites. Meteorological variables such as total precipitable water, surface pressure and temperature are also restricted. A local binning procedure is used to address changes in cloud base.

The results show that the drop size of Canadian clouds is more sensitive to changes in aerosol loading than in Siberia, where the average drop size is also smaller for the same aerosol optical depth. Cloud liquid water in Siberia is more sensitive to changes in total precipitable water. In both regions the cloud liquid water initially increases with aerosol loading then decreases. It is not yet known which of several possible mechanisms is responsible for these these trends and differences between the regions.

Satellite retrieval biases of cloud variables are so significant that only comparison studies of this type may be meaningful; unknown but equal biases will cancel in the comparison.