The risk that wind speed variations will reduce the volume of electricity generated by wind farms and therefore sales for wind power companies. The risk can be managed by a weather derivative based on wind power indices (WPIS). The average value of the index over any 12-month period is 100 in a normal year. An annual value of 95 indicates that the index is 5 per cent below normal for the region and leads to a payout. Wind farm developers, operators and financiers may all hedge against the wind risk.