Wind and Pressure
-Cup, propeller & sonic
anemometers
- Exposure of
anemometers
- Mechanical and
electronic barometers
- Reduction to sea-level
pressure
Cup anemometers:
- wind drag on open face is much stronger
than on tapered face, so cups spin in the
wind.
Starting speed is an important specification for cup anemometers
- Starting speed is usually higher than stalling speed
Speed of response is often specified as the distance constant.
Time constant x wind velocity = constant = distance constant
So… distance constant is the time constant for a 1 m/s wind.
Cup anemometer sources of error.
Cups may sense a
component of the
vertical wind as if it
was horizontal wind.
Cups “coast” when slowing
down, so true average (dotted)
is lower than measured
average (red line).
(Figures from Brock and Richardson)
What is meant by the “cosine response” of a wind sensor?
Air parcels typically
have both horizontal
and vertical components
to their motion
When we wish to know the average horizontal wind speed, the sensor
should always respond only to the horizontal component of the wind.
Horiz. component
h
For good “cosine response”, wind
sensor should obey:
Measured wind = Total wind x cos(h) .
Propeller and vane
(Figure from Brock and Richardson)
Methods of recording anemometers and
vanes
• Pulses for wind speed – push a switch, pull a
magnet, interrupt a light beam
• Voltage for wind speed – turn a small generator.
• Voltage for direction – Vane controls slider on a
variable resistor.
Sonic anemometer
Sound pulses shot
back and forth
between sensor
pairs.
Difference in two
pulse travel times
depends on wind
speed – very fast
response sensor.
Three axes to
measure all
components of
wind.
Standard exposure of wind
sensors is at 10m above a
grass surface.
• Extrapolation to other heights:
Wind1 / Wind2 = ln(A1) / ln(A2)
A = (Height – d) / z0
d = 0.65 x grass height
(zero plane displacement)
z0 = 0.15 x grass height
(roughness length)
Advantages of choosing 10m as standard wind height
- Small obstacles do not interfere
- Stronger wind speeds allow robust sensors
- Fewer “calm” periods below the starting speed of sensor
- Reduced vandalism
1, What specifications would you list for a good wind
measurement site?
2. Why is it not acceptable to use the roof of a building to
help achieve the standard 10 m exposure height?
3. Suppose you wanted to check an anemometer you have
installed at 2 m elevation against data from an
anemometer installed at the standard 10 m elevation,
both above 5 cm tall grass. What is the correction
factor in the following equation?
Wind at 2 m = (Correction factor) x (Wind at 10 m)
A “standard” mercury barometer
The aneroid
barometer.
Beware the falling
barometer
An aneriod barograph records the
pressure over time
Balancing spring
Pen on pointer
Rotating drum
Pressure capsule
An “Electronic Barometer”
In the above sensor, a change in capacitance is measured when
the plates move closer together or further apart as the evacuated
capsule is squeezed
expands
with
ambient pressure changes.
(Figureorfrom
Brock and
Richardson)
Other electronic pressure sensors have small diaphragms made of
silicon, quartz or ceramic which flex with changes in pressure.
Motion is sensed with a strain gauge, or a change in diaphragm
capacitance or resistance is measured,
“Reduction” of station pressures to mean sea level datum is
necessary to remove the influence of elevation on the
pressure data. This is the pressure reported by the Weather
Channel or the Meteorological Service.
Correction is approximately 1 hPa per 10 m of elevation change.