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We present an extinction map of the Polaris molecular cirrus cloud derivedfrom star counts and compare it with the Schlegel et al. (1998) extinction mapderived from the far--infrared dust opacity. We find that, within the Polariscloud, the Schlegel et al. Av values are a factor 2 to 3 higher than the starcount values. We propose that this discrepancy results from a difference in$\tau_{FIR}/ A_V$ between the diffuse atomic medium and the Polaris cloud. Weuse the difference in spectral energy distribution, warm for the diffuse atomicmedium, cold for the Polaris cloud, to separate their respective contributionto the line of sight integrated infrared emission and find that the$\tau_{FIR}/ A_V$ of cold dust in Polaris is on average 4 times higher than theSchlegel et al. value for dust in atomic cirrus. This change in dust propertycould be interpreted by a growth of fluffy particles within low opacitymolecular cirrus clouds such as Polaris. Our work suggests that variations indust emissivity must be taken into account to estimate Av from dust emissionwherever cold infrared emission is present (i.e. molecular clouds).Comment: 9 pages, 7 figures, accepted in A&

Far-infrared dust opacity and visible extinction in the Polaris Flare