If you look up at most weather masts, you'll see one of two very different instruments measuring the wind. Either a familiar trio of spinning cups and a wind vane, or a compact cylindrical head with no moving parts at all: the ultrasonic anemometer.
Both measure wind speed and direction, both are widely used by meteorological services and industry, and both have strengths and weaknesses. A key reference is the KNMI report "Comparison of parallel wind measurement with Sonic and Cup‑Vane at nine locations for climatological applications" (IR2011‑01), which analysed years of parallel measurements across the Netherlands.
How cup and vane anemometers measure the wind
A traditional wind set consists of two parts: a cup anemometer for speed, and a vane for direction.
- The cup anemometer has three (sometimes four) hemispherical cups mounted on a rotor. As the wind blows, the rotor spins faster or slower depending on wind speed. Electronics count the rotations per unit time and convert that to a wind speed using a calibration curve.
- The wind vane is a fin that aligns itself with the wind. A potentiometer, encoder or magnetic sensor converts its angle into a wind direction.
This design is simple and robust, which is why cup and vane systems have been a standard for decades. But they are mechanical devices:
- At very low wind speeds, friction in the bearings prevents the cups from turning freely, leading to underestimation of light winds and a "threshold" speed below which the instrument reports near zero.
- At high wind speeds and in gusty conditions, inertia and turbulence can cause under‑response, so short, sharp gusts may not be fully captured.
- Over time, bearings wear, lubrication degrades, and the cups themselves can age, all of which change the calibration and increase maintenance needs.
How ultrasonic anemometers measure the wind
Ultrasonic (sonic) anemometers take a different approach: instead of moving parts, they use sound. A typical 2D sonic has pairs of ultrasonic transducers arranged along two perpendicular paths. Each pair alternately sends and receives short pulses of ultrasound. The time a pulse takes to travel from one transducer to the other depends on the wind speed along that path.
By comparing travel times in both directions along each axis, the instrument's electronics can solve for the horizontal wind components (u and v), and from there compute wind speed and direction. Because there are no rotating parts:
- Response is effectively instantaneous, limited only by sampling rate.
- There is no startup threshold due to friction, so low‑wind sensitivity is very good.
- Long‑term performance is dominated by electronic stability and contamination (e.g. dirt, ice), not mechanical wear.
Accuracy and reliability: what the KNMI comparison found
The KNMI IR2011‑01 study compared parallel wind measurements at nine locations for climatological applications. Several headline conclusions are especially relevant:
| Aspect | Cup & vane | Ultrasonic |
|---|---|---|
| Low-wind response | Underestimates due to bearing friction and starting threshold | Very good; no friction threshold, responds to very light winds |
| High winds & gusts | Can underestimate peak gusts due to inertia | Fast response captures gust structure more faithfully |
| Maintenance | Regular bearing/vane checks, lubrication, calibration; cups can wear or ice | No moving parts, but needs cleaning; sensitive to icing and electronic faults |
| Long-term stability | Calibration drifts with wear; needs periodic lab checks | Good electronic stability; main risks are contamination and sensor drift |
| Cost | Lower upfront hardware cost | Higher upfront cost, lower mechanical maintenance |
For long‑term climate records, consistency over decades is crucial. Many climate series are based on cup‑and‑vane instruments, so any transition to sonic sensors must be carefully managed and homogenised. KNMI's work shows that with parallel measurements and appropriate adjustments, sonic anemometers can be integrated into climatological networks without compromising continuity.
MeteoA deploys ultrasonic anemometers across our network precisely because of their superior low-wind response, lower maintenance burden, and ability to capture gust structure that matters for agricultural spray timing, construction safety windows, and heat-stress WBGT calculations.
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