CHARACTERIZING THE OBSERVED WIND RESOURCE - Wind Resource Assessment: A Practical Guide to Developing a Wind Project - page 130

more complete data coverage. And if the data cover more than 1 year but not an
integer number of years, some calendar months may occur more often than others,
also possibly resulting in a bias in the estimated mean speed.
As an example, suppose the data record spans 14 months, with January and Febru-
ary appearing twice. If the winter months are typically the windiest, a simple average
of the wind speeds for the entire period will probably overestimate the true annual
average speed. Or suppose the period of record spans exactly 12 months, but half
the data in the winter months are lost because of icing. In that case, it is likely the
calculated mean speed will understate the true annual average.
The annualized mean wind speed attempts to correct for these problems. Note
that this is not the long-term historical mean speed, rather it is a seasonal correction
for the observed period of data. (The long-term adjustment process is explained in
Chapter 12.) The annualized mean can be estimated in a variety of ways but is usually
found by calculating, first, the mean for each calendar month in the record, and second,
the mean of the monthly means weighted by the number of days in each month. In
equation form,

1
N m

N m

12 12

1
365

1
365

v annual = D m v m = D m v im (10.4)

. 25 . 25

m = 1 m = 1 i = 1

The outer sum is over the 12 calendar months, with D m being the average number
of days in month m (28.25 for February, counting leap years). The inner sum is over
those speeds that fall within a particular calendar month. The calculation is illustrated
in Table 10-2. Here, the data record spans 17 months, from January 2008 to May 2009,
with January through May repeated. The straight average of the speeds (taking into

Table 10-2. Sample monthly data record for a station, illustrating the difference
between period of record and annualized mean speeds

Month No. of days in month 2008 2009 Average

January 31 8.94 8.68 8.81

February 28.25 8.35 7.37 7.86

March 31 7.63 8.13 7.88

April 30 6.79 7.00 6.90

May 31 6.56 6.85 6.71

June 30 6.58 — 6.58

July 31 5.81 — 5.81

August 31 6.25 — 6.25

September 30 7.50 — 7.50

October 31 7.85 — 7.85

November 30 8.26 — 8.26

December 31 8.36 — 8.36

Annualized average 7.39