  The fraction of available telescope time is one of the most important
requirements for selecting astronomical sites affecting the performance of
ground based telescopes. A quantitative survey of clouds coverage at La Palma
and Mt.Graham is presented using both ground and satellite based data. The aim
of this work is deriving clear nights for the satellite infrared channels and
verifying the results using ground based observations. At La Palma we found a
mean percentage of clear nights of 62.6% from ground and 71.9% from satellite.
Taking into account the fraction of common nights we found a concordance of
80.7% clear nights from ground and satellite.
  At Mt.Graham we found a 97% of agreement between Columbine heliograph and
night time observing log. From Columbine heliograph and TOMS-OMI satellite we
found about 45% of clear nights, while satellite data (GOES, TOMS) are much
more dispersed than those ones of La Palma. Setting a statistical threshold we
retried a comparable seasonal trend between heliograph and satellite.
