21
The wind measurement requires 42 ms to complete. An additional 60 ms are required for
the computations to determine phase differences. The anemometer samples every 10 s (or
more often if requested). The gust speed reported is the highest instantaneous wind speed
measured during the selected averaging interval (must be >20 s or gusts will equal speed).
NOTE: Cup anemometers average over a much longer interval than 42 ms, so the gusts measured with a sonic
anemometer will have a larger peak-to-mean ratio than one would see with a cup anemometer.
3.12.2 TEMPERATURE SENSOR
The ATMOS 41 uses an energy balance correction to adjust measured temperature to actual
air temperature according to
Equation 11
T
T
c k
d
u
S
corr
uncorr
P
s
t
a
=
-
f
p
where:
α
s
=
the absorptivity of the surface to solar radiation
S
t
=
the total solar radiation measured
c
p
=
29.3 J mol
-1
C
-1
, k is a constant
u
=
the wind speed
d
=
the characteristic dimension
Although these values can be assumed, some (
α
s
and
k
) were optimized using a Levenberg
Marquardt Least Squares analysis. Optimal air temperature was obtained using an Apogee
TS-100 Aspirated Radiation Shield. Data were collected over several weeks and final values
are shown in
.
NOTE: A maximum value optimization for
S
t
was added because radiation values higher than that cause the corrected
temperature to deviate from actual values more than when a maximum
S
t
was used.
Table 2 Optimized values for air temperature correction
α
s
d (m)
k
c
p
(J mol
-1
K
–1
)
Max
S
t
(W/m
2
)
0.295
0.00083
0.0984
29.3
352.3
