Field Studies - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 183

Fig. 16.12 Using a vehicle to drag a GPR system across a linear dune at Quattaniya, Egypt. Note this GPR is a longer wavelength antenna than
that shown in Fig. 16.8 and thus is physically rather larger. Photo R. Lorenz

temperature that depends on how much heat is applied, and
how efficiently that heat is removed by the medium around
the wire—and thus on wind speed. Thus, for example, when
a wire is driven by a constant current, the voltage across it
will depend on wind speed, albeit in a nonlinear way.
Because the sensing element can simply be a short length of
narrow wire, these sensors can be very compact and are
often used in arrays in boundary layer studies in wind
tunnels. Furthermore, because the thin wire (often just a few
tens of microns across) can change temperature rapidly,
these sensors are very good for characterizing fluctuations
in windspeed and thus for measuring turbulence. An
important consideration in using these otherwise attractive
devices in the field is their modest robustness—the wire
may be easily broken.
There are variants of the principle, using metallic films
deposited on a surface rather than wires, and with different
heating and sensing elements. One approach is to have a
central heater, whose temperature gives the wind speed, and
a set of temperature sensors around it, such that one will be
in the plume of warm air from the heater and can thus
indicate the direction. Usually hot wires are mounted such
that they respond to wind from some range of directions and
comparison between differently-mounted sensors is needed
to recover direction.
Hot wire anemometers have been used on the Viking and
Pathfinder Mars landers (although results from the latter are
in

afternoon defeating the calibration assumptions). A hot film
anemometer was developed for the Beagle 2 lander, and is
on Mars today on the Mars Science Laboratory Curiosity.

16.3.3 Pitot Tubes

Measuring the 'ram' pressure of air is a convenient way to
infer windspeed. This method, wherein the difference
between the static pressure and the total pressure (dynamic
plus static) is measured by pointing a 'Pitot' tube upwind, is
the basis of airspeed indicators on aircraft. While modern
systems use semiconductor pressure transducers, pitot tubes
allow 'analog' measurement, in that the ram pressure can be
balanced by the weight of a column of liquid in a glass tube
(i.e., a manometer), and thus the length of the liquid column
(usually colored with a dye) can simply be read off a scale
next to the tube. Such systems are readily scaled up to
display several pressures simultaneously, such as those
corresponding to winds at different heights. Before the
advent of electronic data acquisition, an effective way to
record wind data was to take a photograph of the manom-
eter array. It is apt that the cover photograph of Bagnold's
autobiography, Sand, Wind and War (1991) is of the man
doing exactly this while a sand trap is measuring the
transport flux. The remarks by Bagnold (1941, p. 77) on the
design of his field manometer array (with which he made
the first quantitative measurements of wind velocity and some doubt, fluctuations in air temperature in the