  SPHERE is the high-contrast exoplanet imager and spectrograph installed at
the Unit Telescope 3 of the Very Large Telescope. After more than two years of
regular operations, we analyse statistically the performance of the adaptive
optics system and its dependence on the atmospheric conditions above the
Paranal observatory, as measured by the suite of dedicated instruments which
are part of the Astronomical Site Monitor and as estimated by the SPHERE
real-time calculator. We also explain how this information can be used to
schedule the observations in order to yield the best data quality and to guide
the astronomer when processing his/her data.
