The Wind Index (WI) aims to quantify the monthly electric energy production fluctuations coming from the wind resource around the historical average value. It is based upon real wind farm production results, properly corrected in order to dissipate any operational unavailability effects. More than represent the produced wind resource variation on the produced electricity, it is intended to describe regional patterns. It is a very useful analysis tool for all the players in the wind energy market, allowing them to properly evaluate their monthly results regarding the long term patterns of fluctuation. The WI is published every trimester by MEGAJOULE, together with APREN, and with the support of a great number of some of the larger promoters at national level.