Geoscience Reference
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electromagnetic by the molecules under consideration. Temperature can either be
measured with a fast-response thermocouple, or using the temperature that can be
derived from the sonic anemometer (see later).
The sonic anemometer (see
Figure 3.9
) measures the wind speed by determining
the travel time of a (ultrasonic) sound pulse. If the pulse travels in the same direction
as the wind is blowing (tail wind) the pulse will travel faster than in still air and hence
the travel time will be shorter (
∆
x
cu
∆
=
+
, with
Δ
x
the distance between source and
receiver of the sound pulse and
c
the speed of sound). With headwind the situation
will be reversed: a longer travel time. If the travel time is measured in both directions
simultaneously, both the wind speed and the speed of sound can be determined, as
one has two travel times and two unknowns.
9
Because the speed of sound - mainly -
depends on temperature, a sonic anemometer can also be used as a fast response ther-
mometer using the following relationship (with
T
in K):
t
2
273 15
1051
.
.
To obtain the air temperature the information of the sonic anemometer needs to be
combined with information on
q
from a fast response hygrometer. Note, that if the
correction for humidity is not made, the resulting temperature (sometimes called
'sonic temperature') is close to the virtual temperature.
=
c
+
T
331 3
.
q
Question 3.9:
Consider an idealized sonic anemometer with the path between sound
source and receiver pointing vertical. The distance between the sound source and the
receiver is
Δx
=10 cm.
a) First a sound pulse is ired in only one direction, upward. The travel time is observed
to be
Δt
= 0.310 ms. A speed of sound in air is assumed of 330 m s
-1
. What is the
magnitude and direction of the wind speed?
b) On another occasion, two sound pulses are ired, one upward with a travel time of
Δt
up
= 0.295 ms and one downward with a travel time of 0.302 ms. What is the mag-
nitude and direction of the wind speed, and what is the speed of sound?
c) Compute the temperature of the air from the speed of sound determined under (b)
(assuming dry air).
A gas analyser uses the absorption of electromagnetic radiation by molecules to mea-
sure the concentrations of water vapour and CO
2
. Some sensors use infrared radiation
(around 4 µm and 2.5 µm for CO
2
and H
2
O, respectively; see
Figure 2.2
), and others
ultraviolet radiation (the so-called Lymann-
α
line around 0.12 µm). The amount of
absorption of radiation is related to the number of molecules between source and
x
cu
∆
x
cu
∆
9
The set of equations is: ∆
2
=
−
. With
Δt
1
and
Δt
2
measured and
Δx
known (it is a property of the
instrument) one can solve for both
c
and
u
.
t
1
=
+
and ∆
t
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