  The low wind effect is a phenomenon disturbing the phase of the wavefront in
the pupil of a large telescope obstructed by spiders, in the absence of wind.
It can be explained by the radiative cooling of the spiders, creating air
temperature inhomogeneities across the pupil. Because it is unseen by
traditional adaptive optics (AO) systems, thus uncorrected, it significantly
degrades the quality of AO-corrected images. We provide a statistical analysis
of the strength of this effect as seen by VLT/SPHERE after 4 years of
operations. We analyse its dependence upon the wind and temperature conditions.
We describe the mitigation strategy implemented in 2017: a specific coating
with low thermal emissivity in the mid-infrared was applied on the spiders of
Unit Telescope 3. We quantify the improvement in terms of image quality,
contrast and wave front error using both focal plane images and measured phase
maps.
