Product manual

Appendix C. CSAT3 Measurement Theory
C-2
blowing perpendicular to the sonic path. No additional off-line corrections are
required as suggested by Liu et al., 2001.
c
d
tt
ob
=+
⎡
⎣
⎢
⎢
⎤
⎦
⎥
⎥
2
11
(5)
The speed of sound in moist air is a function of temperature and humidity and
is given by:
(
)
cP RTRT q
dv d
2
1061== = +γργ γ .
(6)
where γ is the ratio of specific heat of moist air at constant pressure to that at
constant volume, P is pressure, ρ is air density, R
d
is the gas constant for dry
air, T
v
is virtual temperature, T is the air temperature, and q is the specific
humidity defined as the ratio of the mass of water vapor to the total mass of air
(Kaimal and Gaynor, 1991; Wallace and Hobbs, 1977).
Note that γ is a function of specific humidity. It would be convenient if the
effects of humidity could be consolidated into one term.
The specific heats for moist air at constant pressure and volume are given by:
C
p
=
+
−
qC q C
pw pd
()1
=
+
Cq
pd
(.)1084
(7a)
C
v
=
+
−
qC q C
vw vd
()1
=
+
Cq
vd
(.)1093
(7b)
where C
p
and C
v
are the specific heats of moist air at constant volume and
pressure, C
pw
and C
vw
is the specific heat of water vapor, and C
pd
and C
vd
is the
specific heat of dry air, respectively (Fleagle and Businger, 1980).
Substitute Eq. (7a) and (7b) into (6) and ignore the higher order terms. This
yields
cRTRT q
dds dd
2
1051== +γγ(.)
(8)
where T
s
is sonic virtual temperature and γ
d
is the ratio of specific heat of dry
air at constant pressure to that at constant volume (Fleagle and Businger, 1980;
Kaimal and Gaynor, 1991; Kaimal and Businger, 1963; Schotanus et al., 1983).
With Eq. (8), the effect of humidity, on the speed of sound, is included in the
sonic virtual temperature.
The sonic virtual temperature, in degrees Celsius, is given by Eq. (9), where
γ
d
= 1.4 and R
d
= 287.04 JK
-1
kg
-1
.
T
c
R
s
dd
=−
2
273 15
γ
.
(9)