Astroclimatology of ESO Sites	ATACAMA WORKSHOP, Ithaca NY M. Sarazin, June 15-17 2000, 3/14

Differential of the monthly fraction of photometric nights at Paranal (24 40'S) and La Silla (29 15'S) since the beginning of the VLT site survey. La Silla currently tops its 10-11 years cycle while Paranal experiences La Niña related weather degradation: the combination of both effects cancels the differences between the two sites.