  Ice crystal clouds in the upper troposphere can generate polarisation signals
at the uK level. This signal can seriously affect very sensitive ground based
searches for E- and B-mode of Cosmic Microwave Background polarisation. In this
paper we estimate this effect within the ClOVER experiment observing bands (97,
150 and 220 GHz) for the selected observing site (Llano de Chajnantor, Atacama
desert, Chile). The results show that the polarisation signal from the clouds
can be of the order of or even bigger than the CMB expected polarisation.
Climatological data suggest that this signal is fairly constant over the whole
year in Antarctica. On the other hand the stronger seasonal variability in
Atacama allows for a 50% of clean observations during the dry season.
