Wind speed is a foundational measurement for meteorologists. Accurate wind data is essential for forecasting weather, ensuring safety in aviation and maritime transport, and predicting the movement of severe storms. Because the atmosphere is a three-dimensional environment, instruments are needed to gather measurements from the surface up to the highest layers of the atmosphere.
Mechanical Anemometers
The most common and easily recognizable tool for measuring surface wind speed is the cup anemometer. This device typically features three or four hemispherical cups mounted on arms that rotate around a vertical shaft. As the wind blows, the force difference between the concave and convex sides of the cups causes the shaft to spin at a rate directly proportional to the wind’s speed.
The rotation of the vertical shaft is translated into a digital reading through an internal mechanism. In modern units, a magnet attached to the rotating shaft passes over a sensor, generating an electrical pulse with each revolution. The frequency of these pulses is then calculated to determine the wind speed.
Another instrument is the propeller anemometer. This device measures wind speed based on the rotation of a multi-bladed propeller mounted on a horizontal axis. Because the propeller must face directly into the wind to function accurately, it is almost always paired with a wind vane that keeps it properly oriented. Propeller anemometers also translate their rotational motion into electrical signals.
Advanced Surface Measurement Technology
While mechanical tools rely on moving parts, modern meteorology increasingly uses devices that offer higher precision and responsiveness without rotation. The sonic anemometer measures wind speed by utilizing the time-of-flight principle of sound waves. This device uses pairs of transducers that emit ultrasonic pulses back and forth across a known distance.
Wind speeds are calculated by measuring the difference in the time it takes for the sound pulse to travel from one transducer to the other. A pulse traveling with the wind arrives slightly faster, while a pulse traveling against the wind is slightly delayed. This difference in transit time is directly proportional to the wind speed along that path, allowing the instrument to calculate wind velocity in multiple dimensions.
Another non-mechanical method involves the hot-wire anemometer, which works based on convective heat transfer. This instrument features an extremely thin, electrically heated wire, often made of tungsten, which is exposed to the airflow. As wind blows across the wire, it removes heat, causing the wire’s temperature, and consequently its electrical resistance, to drop.
The wind speed is determined by measuring the electrical current required to maintain the wire at a constant, elevated temperature, a method known as constant temperature anemometry. Faster moving air requires more current to keep the temperature stable. These instruments are highly sensitive and can capture rapid fluctuations in wind speed.
Measuring Wind in the Upper Atmosphere
To profile the wind in the troposphere and stratosphere, meteorologists rely on specialized remote sensing and tracking methods. The most common technique involves launching a weather balloon carrying a small instrument package called a radiosonde. As the balloon ascends, the radiosonde constantly transmits atmospheric data back to a ground station.
Wind speed and direction are calculated by tracking the radiosonde’s location over time, rather than being measured directly by a sensor. The instrument uses a Global Positioning System (GPS) receiver to determine its latitude, longitude, and altitude at frequent intervals. The change in the radiosonde’s position between successive GPS readings is used to calculate the speed and direction of the wind at that altitude.
Meteorologists also utilize Doppler Radar to remotely sense wind movement over large areas. This system transmits microwave pulses into the atmosphere and measures the frequency shift of the energy reflected back from precipitation or dust particles. This phenomenon, known as the Doppler effect, allows the radar to measure the radial velocity—the component of motion directly toward or away from the unit. By scanning at various angles, the system can map out the three-dimensional wind field, which is important for detecting the rotation associated with severe storms.