In order to clarify variability in wind energy over a long-term period of 30 years, an investigation was carried out on Jeju Island in South Korea, which has complex terrain. The selected areas for this study were Jeju, Seogwipo and Seongsan, which have wind data for 30 years from 1978 to 2007. The long-term wind data measured from automated synoptic observation system (ASOS) of meteorological observatories was analyzed in accordance with inter-annual, annual and diurnal time scales. Wind variations in various time scales were quantified by both the coefficient of variation and the range of variation. As a result, the yearly average wind speed tended to slightly decrease at the Jeju site, while the other two sites have random trends. The regional deviation for wind variations was significant on Jeju Island, which may result from its complex terrain. It was found that when wind data is measured and analyzed over a longer-term period, more reliable results can be obtained. Besides these, a statistically meaningful result will be presented in this paper. (C) 2010 Elsevier Ltd. All rights reserved.