The EPA recommends hourly standard deviation of horizontal wind direction
(sigma theta) be computed from four fifteen-minute sub-intervals.
7.9.22.2.1 Measured Raw Data
• S
i
: horizontal wind speed
• Θ
i
: horizontal wind direction
• Ue
i
: east-west component of wind
• Un
i
: north-south component of wind
• N: number of samples
7.9.22.2.2 Calculations
Input Sample Vectors
Figure 76. Input Sample Vectors
In figure Input Sample Vectors (p. 298), the short, head-to-tail vectors are the input
sample vectors described by s
i
and Θ
i
, the sample speed and direction, or by Ue
i
and Un
i
, the east and north components of the sample vector. At the end of data-
storage interval T, the sum of the sample vectors is described by a vector of
magnitude U and direction Θu. If the input sample interval is t, the number of
samples in data-storage interval T is N = T / t. The mean vector magnitude is Ū =
U / N.
Scalar mean horizontal wind speed, S:
where in the case of orthogonal sensors:
Unit vector mean wind direction,
298
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