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 “coast” when slowing down, so true average (dotted) is lower than measured average (red line). Cups may sense a component of the vertical wind as if it was horizontal wind. (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. Total 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) / z 0 d = 0.65 x grass height (zero plane displacement) z 0 = 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

Beware the falling barometer The aneroid barometer.

An aneriod barograph records the pressure over time Balancing spring Pen on pointer Rotating drum Pressure capsule

(Figure from Brock and Richardson) 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 or expands with ambient pressure changes. 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.