During the past four decades much important information on Northern Hemisphere snow extent has been provided by the NOAA weekly snow extent charts derived from visible-band satellite imagery. Passive microwave satellite remote sensing can enhance snow measurements based on visible data alone because of the ability to penetrate clouds, provide data during darkness and the potential to provide an index of snow depth or water equivalent. We compare the fluctuation of Northern Hemisphere snow cover over the past twenty years using these two satellite remote sensing techniques. Results show comparable inter-annual variability with similar long-term hemispheric-scale trends indicating decreases in snow extent of approximately 0.2 percent per year. The passive microwave snow algorithm applied in this study indicates less snow-covered area than the visible data during fall and early winter when the snow is shallow. New algorithms designed to reduce this apparent error are being developed and tested. ¿ 2001 American Geophysical Union