The two wind-speed ranges are for a nominal height of 80 metres above ground and correspond loosely to the International Electrotechnical Commission (IEC) Class III.
The speeds ignore air density, the speed frequency distribution and other factors that influence production, but serve as a reasonable guide to world potential. The map makes no concession for any other development suitability criteria, nor does it show small areas.
So far, most of the areas developed for wind power on a large scale - including in the central US and Europe - have good or moderate resource, says Michael Brower, chief technical officer at AWS Truepower, and there is more potential there. But the map shows that there are suitable low-wind resources in China, south-east Asia, Brazil and other populous regions with rapidly growing economies, where much of wind power's future lies, he says.