As reported in the 27 Dec 2010 issue of the Scotsman newspaper (http://thescotsman.scotsman.com/news/39Green39-Scotland-relying-on-French.6672024.jp)
Output from major wind farms fell to as low as 2.5 per cent of their potential generation capacity during the cold snap as power demand rose to close to the highest level yet recorded, new figures have revealed.
Meteorologists say extremely cold temperatures can occur only when there is little or no wind and icy pockets of air are trapped close to the ground, prompting accusations from anti- wind-farm campaigners that wind power cannot be relied on to meet Scotland’s electricity needs in the depths of winter.
and (my emphasis added)
Over the past ten days, when temperatures have plunged across Scotland, the average power generation from Britain’s wind developments – the majority of which are in Scotland – was 261 megawatts (MW), just 10.75 per cent of the total possible of 2,430MW.