PowerPoint Presentation

• Anemometry objectives:

– Learn how the wind speed and

direction is measured

– Know the limitations of these

measurements

– Appreciate the WMO

standards for wind
measurements

ATMS 320 –

Meteorological

Instrumentation

http://www.msc-smc.ec.gc.ca/education/imres/42_instruments_e.cfm

• Wind measurements are

usually related to the

horizontal component

• It is a vector, requiring

both a magnitude

(speed) and direction

(the direction from

which the wind is

blowing)

• Wind velocity is

turbulent; it is often

reported as a mean and

variation about the

mean (gustiness)

ATMS 320 – Anemometry

http://www.ci.chi.il.us/LiveShots/stillshots_slideshow020.htm

• Ideal (perfect) wind

instrument

– Respond to slight

breezes

– Rugged enough to

withstand hurricane-
force winds

– Respond rapidly to

turbulence

– Have linear output
– Exhibit simple dynamic

performance
characteristics

ATMS 320 – Anemometry

http://www.banzai-institute.com/0102PTad.html

• Wind force

One design of wind

sensors responds to
the drag force, or
the closely related
lift force.

ATMS 320 – Anemometry





• Drag or lift force

anemometers – cup and
propeller

– Cup wheel – responds to the

differential drag force

– Propeller – responds to both

the drag and lift forces

– Raw output is the

mechanical rotation rate of
the cup wheel

– Shaft is coupled to an

electric transducer which
produces an electrical
output signal (dc voltage)

ATMS 320 – Anemometry

• Drag or lift force

anemometers – cup and
propeller (cont.)

– Linear over most wind

speeds (except at the lower
end of the range)

– At rest, little wind force

available to overcome
internal friction of the shaft

– Starting threshold much

higher than stopping
threshold (because running
friction is much less than
static friction)

ATMS 320 – Anemometry

•

Drag or lift force
anemometers – cup and
propeller (cont.)

– Static performance

specifications; range and
threshold

– Nonlinear threshold

effects are usually ignored

– Threshold speed – a

function of vibration and
of bearing friction, which
increases as the
anemometer ages.

ATMS 320 – Anemometry

http://www.skfsport.com/friction.htm

•

Drag or lift force
anemometers – cup and
propeller (cont.)

– Ideally, indicated speed

would be proportional to
the cosine of the angle of
the wind vector with
respect to the horizontal

– Actual response is

sometimes greater in
turbulent flow or when
the vertical component is
not zero (in complex
terrain or near buildings)

ATMS 320 – Anemometry

wind

elevation angle (<0)

• Drag or lift force

anemometers – cup
and propeller (cont.)

– Dynamic performance

of the cup
anemometer is
reasonably
approximated with a
first-order linear
differential equation

ATMS 320 – Anemometry

http://www.swatch.com/





where

















• Drag or lift force

anemometers – cup
and propeller (cont.)

– Want to minimize

distance constant (



);

make m

small and A

large

– Dynamic performance

specification (not



)

ATMS 320 – Anemometry

http://www.swatch.com/







• Drag or lift force

anemometers – cup and
propeller (cont.)

– Time “constant” is not truly

constant;

– Simplify solution by

assuming wind speed
fluctuations are small
compared to the mean wind
speed. Substitute mean
wind speed for V

above

ATMS 320 – Anemometry

http://www.swatch.com/









• Drag or lift force

anemometers – cup and
propeller (cont.)

– Sinusoidal response is

applicable

Recall that when 2





= 1,

= 0.707; that is, the

amplitude of the response
has been reduced to
approximately 70% of the
input amplitude

ATMS 320 – Anemometry

http://www.novalynx.com/200-27005.html













gust wavelength

• Drag or lift force

anemometers – cup and
propeller (cont.)

– Example, if a typical

anemometer has a
distance constant of 3 m,
it will attenuate the
amplitude of all gusts
whose wavelength is less
than 2



= 19 m to less

than 70% of the input
amplitude

– What about a distance

constant of 1 m?

ATMS 320 – Anemometry

http://www.novalynx.com/200-27005.html

• Drag or lift force

anemometers – cup
and propeller (cont.)

– What if time

“constant” is not
constant? We have a
non-linear problem

ATMS 320 – Anemometry















small for high wind speed,
large for small wind speed

anemometer responds more rapidly to an increasing step  overspeeding error

• Drag or lift force

anemometers – cup and
propeller (cont.)

– Overestimation problem of

cup anemometers

• Static overestimation error

due to the lack of cosine
response

• Dynamic effect that

increases for anemometers
having larger distance
constants

– Overestimation problem of

propeller anemometers

• Dynamic effect

ATMS 320 – Anemometry

**threshold speed and distance
(and time) constant are inversely
related to the air density

• Drag or lift force wind vane

– Transducer is a pot

mounted concentrically
with the vertical shaft to
convert azimuth angle to a
voltage proportional to that
angle. A dead zone of 3 to
5

exists and is usually

oriented toward North.

– Only source of static error

is misalignment of the vane

– Uses a combination of the

lift and drag forces on the
vane to align itself with the
wind vector

ATMS 320 – Anemometry

• Drag or lift force

wind vane – dynamic
misalignment error

– Moment of inertia
– Aerodynamic

damping

due to the changing

wind direction,



ATMS 320 – Anemometry

http://www.vh1.com/artists/az/simon_carly/flipbooks.jhtml

























• Drag force – drag

cylinder or sphere

– Sensor that measures

wind velocity by
measuring the drag
force on an object in
the flow

– Cylinder measures 2D

flow, sphere measures
3D flow

ATMS 320 – Anemometry





The drag force acting on the sphere is given by

(vectors are three-dimensional)

The dynamic response is determined
by the spring torque of the supporting
members used to hold the cylinder or
sphere in position

• Drag or lift force

anemometers – pitot-
static tube

– A pair of concentric tubes
– Stagnation port is a blunt

obstacle to airflow (drag
coefficient is unity)

– Static port is located at a

point far enough back along
the tube to have no dynamic
flow effects at all

– Must be oriented into the

airflow (virtually unsuitable
for atmospheric work)

– Ideal for wind tunnels

ATMS 320 – Anemometry

• Drag or lift force

anemometers – pitot-
static tube

– Calibration equation

ATMS 320 – Anemometry

transfer equation plot













 



static

stagnation











The pitot-static probe is inexpensive but requires a high-quality
differential pressure sensor to convert the p to a usable signal.

It is insensitive to light winds as the static sensitivity goes to zero
As the wind speed goes to zero (see Fig 7-10)

• Heat dissipation – hot-

wire and hot-film
anemometers

– Infer wind speed from

the cooling of a heated
wire or film (dependent
on the mass flow rate
{speed and density of
flow} past the sensing
element)

– Response speed is

dependent on thermal
mass of element

ATMS 320 – Anemometry

http://www.efunda.com/designstandards/sensors/hot_wires/hot_wires_intro.cfm

• Heat dissipation –

hot-wire and hot-film
anemometers (cont.)

– Hot-wire; fast

response, small
diameter (5 m)

– Hot-film; thin film

deposited on a
cylindrical core and
insulated, slower
response, large
diameter (50 m)

ATMS 320 – Anemometry





King’s law:

http://www.aoe.vt.edu/~simpson/aoe4154/hotwirelab.pdf

• Hot-wire and hot-film

anemometers - issues

– Susceptible to atmospheric

contamination

– Rain contamination

through spikes in the data

– Expensive
– Large power requirements
– Susceptible to drift
– Difficult to resolve low

wind speeds (static
sensitivity becomes very
large at low wind speeds)

ATMS 320 – Anemometry

transfer equation plot

ATMS 320 – Anemometry

• Speed of sound –

sonic anemometers

– Measures the time

required to transmit
an acoustic signal
across a fixed path to
determine the wind
velocity component
along that path





cos

















cos

;

cos

if cos



~ 1

ATMS 320 – Anemometry

• Speed of sound –

sonic anemometers
(cont.)

– Measures the time

required to transmit
an acoustic signal
across a fixed path to
determine the wind
velocity component
along that path





















cos



















ATMS 320 – Anemometry

• Speed of sound – sonic

anemometers (cont.) -
issues

– Space resolution limitation

imposed by the path
length d (much better
resolution than cup and
propeller anemometers)

– Expensive
– Requires considerable

power

– Signal loss due to heavy

rain or wet snow

http://www.mountwashington.org/research/sonic.html

ATMS 320 – Anemometry

• World Meteorological

Organization (WMO) wind
standards

– Exposure height for surface

winds is 10 meters

– Averaging time is 10

minutes

– Gust definitions:

•Gust peak speed (p) = wind speed associated with a positive
gust amplitude
•Gust duration (t

) = time interval from the beginning of a gust

to its end

•Gust magnitude (m) = the scalar difference between a gust
peak speed and lull speed

•Gust frequency = number of positive gusts which occur per
unit time
•Gust amplitude (g

) = maximum scalar of the gust from the

mean wind speed

•Gust lull speed (L) = wind speed associated with a negative
gust amplitude

ATMS 320 – Anemometry

• Anemometry exposure issues

– Good exposure in all directions within

about 3 km

– No obstruction to wind flow should be

more than 3

above the horizon

– Distance from the anemometer to an

obstruction should be at least 20 times the
height of the obstruction

• General exposure issues

– Site characterization is important
– Snow accumulation, icing, extreme low

and high temperatures, high wind gusts,
deterioration of plastic parts due to UV
radiation, breakage due to fatigue failure
in turbulent wind, corrosion due to rain
and high humidity, wind loading, and
lightning-induced power surges

http://www.cdc.gov/nasd/docs/d000901-d001000/d000925/d000925.html

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