Characteristics of the Wind - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 475

of estimating long-term average wind speeds from short-term data samples showed that the
accuracy of more-sophisticated methods (including principal component analysis and
weather pattern classification ) was not significantly higher than that of simpler, linear
statistical methods [Barchet and Davis 1983]. Accuracy was measured by the degree of
correlation between each of the estimates and actual long-term wind speed data.
Seasonal and Monthly Variability
Significant variations in wind speeds from season to season are common over most of
the world. The degree of seasonal variation in the wind at a given site depends on latitude
and position with respect to specific topographic features such as land masses and water.
In general, mid-latitude continental locations that are well-exposed will experience higher
winds in winter and spring, primarily because of large-scale storm activity. However,
mountain passes in coastal areas may experience strong winds in the summer when cool
maritime air moves into a hot interior valley [Elliott 1979].
Within a given season, time variations in the wind over periods of one to several days
can be caused by disturbances in the overall flow pattern such as cyclonic storms (in
temperate latitudes) and traveling wave systems (in the tropics). These disturbances are
quite capable of causing the output of a wind power station to cycle between zero and rated
power several times a month. This type of wind variability is illustrated in Figure 8-5 for
a mid-latitude site. Here the fluctuations in wind speed have several different time periods,
but three storms approximately 10 days apart are dominant.
In tropical latitudes, pronounced wind changes from season to season are well
recognized. A belt of feeble winds named the Intertropical Convergence Zone or ITCZ
Figure 8-5. Example of wind speed variations caused by a series of cyclonic storm dis-
turbances at a mid-latitude site. ( Courtesy of the U.S. Department of Energy )