Let us restrict for the moment to the 850 nm band that is more convenient for observing the 
intrinsic cloud polarization since molecular scattering is negligible. The second characteristic 
observation is that cloud polarization at 850 nm tends to vanish for scattering angles near from 
100°(see figure 4). For clarity, we only considered pixels corresponding to viewing directions 
within the solar incident plane. Because of the radiometric noise and the registration errors, 
zero polarization is not attained, but pp is minimum near © =90°-110°. 
3.2 Cloud Altimetry from Rayleigh scattering 
Let us now consider the spectral aspect of polarization measurements over cloud fields. We will 
limit our discussion to data acquired during the EUCREX campaign, because of the reasonable 
high altitude above the clouds permitted by the Falcon of DLR, thus providing sufficient effect 
of the molecular scattering layer between the cloud and the aircraft. The rainbow features are 
very similar in the 450 and 850 nm spectral bands of POLDER, but in other viewing directions, 
specially around ©=90°, the polarized reflectance is clearly larger at 450 nm than 850 nm. This 
effect is present in all sequences of all flights. It results from the increasing influence of the 
intervening molecular layer at shorter wavelengths. Of course, nothing similar does appear in 
the reflectance images where the spectral effect varies largely as a function of the underlying 
ground properties. On 10/21/91, because of the large extension of the cloud field, the Falcon 
aircraft was able to overfly the cloud at different altitudes, including a first run near the top, at 
about 3.5 km. Figure 5 shows p p vs © in the 450 nm band, derived from 3 pictures obtained at 
about 3.5 km (cloud top), 7.3 km and 11.3 km, with mean solar zenith angle 9 S =60°. For clarity, 
the p p measurements have been reported only for pixels near the solar incident plane. 
Scattering angle (degree) 
Figure 5:Polarized reflectances versus scattering angle at 450 nmfor various aircraft altitudes. 
Within the range of scattering angles 80°-110°, where polarization by molecular scattering is 
large, the polarized reflectance increases with the aircraft altitude, i.e. with the thickness of the 
intervening molecular atmosphere. By contrast, the polarized reflectance at 850 nm is 
practically unchanged, in accordance with the X." 4 dependence of the molecular scattering 
efficiency. These observations confirm that differences in the polarized reflectance observed at 
450 and 850 nm above clouds mainly correspond to molecular scattering, from which the cloud 
top altitude may be estimated. 
First attemps to estimate cloud altitude has been made and has led to an accuracy of 100 hPa in 
term of pressure. This means that cloud altimetry by means of polarization molecular scattering 
may be a valid method. There is, however, a large dispersion in the estimates mainly due to 
registration problems.