  In order to maximize the scientific return of the telescopes located at the
Paranal Observatory, we analyse the properties of the atmosphere above Paranal
in the near-infrared (NIR). We estimate atmospheric extinction in the spectral
range 1.10-2.30 um (J, Js, H, and Ks) using standard stars that were observed
during photometric and clear nights with ISAAC on the Very Large Telescope UT1
between 2000 and 2004. We have built a database consisting of hundreds of
observations, which allows us to examine how extinction varies with airmass and
the column density of water vapour. In addition, we use theoretical models of
the atmosphere to estimate Rayleigh scattering and molecular absorption in
order to quantify their fractional contribution to the total extinction in each
filter. Finally, we have observed 8 bright red standard stars to evaluate
filter color terms. We find that extinction coefficients are < 0.1 mag/airmass
in all the considered bands. The extinction coefficient in the J-band strongly
depends on the column density of water vapour. Molecular absorption dominates
the extinction in J, H and Ks, while Rayleigh scattering contributes most to
the extinction in Js. We have found negligible color terms for J, H and Ks and
a non-negligible color term for Js.
