682 
dépendance of the polarized reflectance around 140 degrees scattering angle or, more 
generally, scattering angles for rainbows. 
5-CONCLUSIONS 
The photopolarimetric observations made by the POLDER instrument show that polarized 
reflectance may be used also for cloud altimetry. They indicate that snow/ice and clouds 
have very similar spectral and bidirectional properties, making it difficult, even impossible 
to use only those properties to detect the presence of clouds over snow-ice surfaces. The 
polarization state of the reflected and scattered light by the two types of targets, in the 
contrary, exhibits distinct characteristics at scattering angles for rainbows, when the 
clouds contain water droplets. We conclude that polarization measurements provide an 
efficient means to screen liquid water clouds over snow/ice surfaces. 
AKNOWLEDGEMENTS 
This work has been supported in part by the National Aeronautics and Space administration 
under grant NAGQ-2774 to Robert Frouin 
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