  Knowledge of the Earth's atmospheric optical turbulence is critical for
astronomical instrumentation. Not only does it enable performance verification
and optimisation of existing systems but it is required for the design of
future instruments. As a minimum this includes integrated astro-atmospheric
parameters such as seeing, coherence time and isoplanatic angle, but for more
sophisticated systems such as wide field adaptive optics enabled
instrumentation the vertical structure of the turbulence is also required.
  Stereo-SCIDAR is a technique specifically designed to characterise the
Earth's atmospheric turbulence with high altitude resolution and high
sensitivity. Together with ESO, Durham University has commissioned a
Stereo-SCIDAR instrument at Cerro Paranal, Chile, the site of the Very Large
Telescope (VLT), and only 20~km from the site of the future Extremely Large
Telescope (ELT).
  Here we provide results from the first 18 months of operation at ESO Paranal
including instrument comparisons and atmospheric statistics. Based on a sample
of 83 nights spread over 22 months covering all seasons, we find the median
seeing to be 0.64" with 50% of the turbulence confined to an altitude below 2
km and 40% below 600 m. The median coherence time and isoplanatic angle are
found as 4.18 ms and 1.75" respectively.
  A substantial campaign of inter-instrument comparison was also undertaken to
assure the validity of the data. The Stereo-SCIDAR profiles (optical turbulence
strength and velocity as a function of altitude) have been compared with the
Surface-Layer SLODAR, MASS-DIMM and the ECMWF weather forecast model. The
correlation coefficients are between 0.61 (isoplanatic angle) and 0.84
(seeing).
