blowing constantly with this intensity and from RisDir wind direction angle. Below is the formula for
calculating the Resulting average speed.
RisVel =
( ) ( )
n
VelDirradCosVelDirradSin
22
))(())(( +
Direction’s standard deviation (sigma theta)
It is the standard deviation of the wind direction. It indicates the fluctuations of wind direction across its
average value. The formula for calculating the the Standard deviation of direction is as follows:
StDevDir =
( ) ( )
+
−
2
22
))(())((
1asingra
n
DirradCosDirradSin
Calm percentage
It indicates how many times, during the processing period, the wind intensity has remained below the relative
threshold set in the data logger (default: 0.3 m/s), and therefore how many times the wind direction has
been excluded from the calculations of the above indices, as they are not significant. In case of total absence
of wind during the processing period, CalmPerc assumes the value 100, while both PrevDir and RisDir assume
the conventional value 360 (wind angle to be considered "not significant"). The formula used to calculate the
Calm percentage is as follows:
CalmPerc =
Where:
Dir = instantaneous value of wind direction (0÷360°)
Vel = instantaneous value of wind velocity (m/s)
gra = conversion of an angle from radians to degrees
rad = conversion of an angle from degrees to radians
Calm = 0 in case of not calm wind velocity (< 0.3 m/s), otherwise 1
n = number of considered valid original data (no error)
Each measure can have different time basis.
If the Penman-Monteith evapo-transpiration calculation is enabled (§4.2.3.4), Alpha-log performs specific
processing for this type of calculation whose results are saved in specific files (§4.2.6).
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