We have been measuring wind speed with anemometers for centuries, but recent advancements offer more reliable and accurate weather forecasts. Compared with traditional versions, sonic anemometers give fast, accurate measurements of wind speeds.
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These pieces of equipment are often used by the Centre for Atmospheric Science while making measurements routinely or for detailed investigations, helping to construct accurate weather forecasts in a range of locations. Some environmental conditions can restrict measurements, but specific adaptions can be made to overcome these challenges.
Anemometers date back to the 15th century and have been continually improved and advanced in recent years. First developed in the mid-19th century, traditional anemometers use cups in a circular arrangement and are connected to a data logger. They became three in the 1920s, giving a faster, constant response, which is helpful for measuring gusts of wind. To this day, the sonic anemometer is the next step up for weather forecasting, providing improved accuracy and resolution.
The sonic anemometer was developed in the 1970s, using ultrasonic sound waves to measure wind speeds instantly, indicating whether sound waves traveling between a pair of transducers are sped up or slowed down by wind.
Today, they are widely commercially available for different uses and sites. Two-dimensional (wind speed and wind direction) sonic anemometers have been used widely in weather stations, ship navigation, wind turbines, aviation, and even in the middle of the ocean, floating on weather buoys.
Sonic anemometers can take measurements with a very fine temporal resolution, typically from 20 Hz up to 100Hz, which makes them well suited for turbulence measurements. Speed and resolution within these ranges allow more accurate measurements to be achieved. Sonic anemometers are among the newest weather instruments in a weather station seen today, much more so than the weathervane that measures wind directions.
How Sonic Anemometers Work
Unlike the traditional version, sonic anemometers operate without moving parts. They measure the time taken for a sound pulse to move between two transducers. The time is determined by the distance between these transducers, where the speed of sound depends on the temperature, pressure, and airborne contaminants like pollution, salt, dust, or fog in the air.
To get the airspeed between the transducers, each one alternates as a transmitter and receiver, so pulses travel in both directions between them.
The airspeed results from the pulse times in each direction; this captures three-dimensional wind speed and direction and the angle by arranging three pairs of transducers on three different axes.
Company Windpower Engineering and Development states on their website: “A mechanical sensor is affected by the start-up torque of the moving cup and vane.
Observed differences occur in measured wind speed because of the time it takes a mechanical sensor to physically start up or register a change in wind direction. For example, if a storm blows through an area and the wind suddenly changes direction, the sensor must slow, stop, and restart with the change in wind direction. It will take some seconds to register and report the change.”
By contrast, they explain, a sonic anemometer (or an ultrasonic sensor) is not affected by its inertia. Instead, it will responsively measure a change in wind direction or a high gust instantaneously in real-time.
Obstacles Due to Sensitivity
Since these wind measurements are susceptible, they can be obscured by small flow distortions brought in by the flow of air moving past the transducers. However, some anemometers can be designed to reduce the effects of these airflows. Most manufacturers carefully calibrate and create a correction for these distortions that can occur.
The weather itself can cause limitations. The instruments can work in most atmospheric conditions, but heavy rain can affect the data quality since water droplets on the transducers can affect pulse times recorded. This can be mitigated by checking the signal quality of the sonic pulses received, eliminating those with poor signal quality. Ice can also build up on the transducers, so some are fitted with anti-ice heating mechanisms.
At the Centre for Atmospheric Science
The Centre for Atmospheric Science has sixteen sonic anemometers, including one with a capacity for 100Hz operations, two 50Hz operations, and the rest mainly with a capacity of 20Hz – fast enough for most operations.
Two of the tools are with anti-ice heating for use in icy conditions. Most have analog inputs allowing the connections of additional sensors to be added, like temperature, humidity, pressure, and trace gases.
Sonic anemometers have been used for projects such as NABMLEX to measure wind speed at varying heights about the heights and Cityflux, where different measurements were taken around various parts of the city.
The project team at CityFlux studies air pollution from cities, saying that “the essence of CityFlux is to investigate both of these issues simultaneously by measuring the rate at which turbulent gusts of wind remove particles from the network of street 'canyons' in a city centre where we live and breathe, into the air above, from where they can blow away on the wind.”
Sonic anemometers are the latest major development for wind speed measurements, advancing the accuracy of weather forecasts, untainted by harsh conditions like heavy rain that caused problems for traditional instruments.
More accurate wind speed data help us to understand what weather patterns to expect next, so preparations can be made across daily life and industries.
Continue reading: WeatherBug Lightning and Severe Weather Warning Systems.
References and Further Reading
The University of Manchester (2021) Sonic Anemometers (Centre for Atmospheric Science - The University of Manchester). [online] Available at: http://www.cas.manchester.ac.uk/restools/instruments/meteorology/sonic/
The University of Manchester (2021) CityFlux Project (Centre for Atmospheric Science - The University of Manchester). [online] Available at: http://www.cas.manchester.ac.uk/resprojects/cityflux/
(2021) Sonic anemometer [Online]. Science Direct. Available at: https://www.sciencedirect.com/topics/earth-and-planetary-sciences/sonic-anemometer
WINDCRANE (2021) How to measure wind speed [online]. Available at: https://www.windcrane.com/blog/windcrane-general/how-measure-wind-speed
KRemington (2010) Ultrasonic wind sensors or cup anemometers? That is the question.. [online] Available at: https://www.windpowerengineering.com/ultrasonic-wind-sensors-or-cup-anemometers/