You’ve likely seen anemometers and wind vanes many times around where you live but haven’t associated them with the wind speeds you hear reported on the news. This lesson looks into how we determine wind speed and direction with modern technology.
Introduction to Wind Measurement
While most winds are benign, winds associated with destructive storms have the ability to cause property damage and other problems. Therefore, being able to measure and predict wind is important to our society in understanding possible wind damages and weather patterns. In this lesson, we’re going to briefly cover the history of wind measurement and then delve into the technology we use to measure wind today.There are two different devices for measuring the wind: wind vanes and anemometers.
Wind vanes are an instrument for showing the direction of the wind. Wind vanes are much older technology than anemometers. The oldest known wind vane was on top of the Tower of the Winds in Athens, dating back to 48 B.C. Although the wind vane did not survive, the Tower of the Winds is still in present-day Athens.
There are writings that suggest that wind vanes may have existed in ancient Mesopotamia and ancient China at even earlier periods. Wind vanes became common in ninth century A.D. when the Catholic Church decreed that all churches should have a rooster as a symbol of a passage in the Bible. Since these churches already had wind vanes, the rooster was combined with the wind vane to create the extremely common rooster wind vanes still seen today.An anemometer is a device used to measure wind speed. It is derived from anemos, which is the Greek word for ‘wind.
‘ Even though the word is derived from ancient Greek, we were not able to measure wind speed until 1450 when Leon Battista Alberti invented the first anemometer, which used a flat disk that would swing back and momentarily show the force of the wind perpendicular to that disk.A more familiar anemometer is the cup anemometer, which was invented in 1846 by Irish scientist Thomas Romney Robinson. The wind caused the cups to rotate horizontally, which moved a series of wheels to record the rotations. The most recent anemometer technology is the sonic anemometer, which was invented in 1994. We’ll delve further into both of these anemometers in this lesson because they’re methods that are still commonly used.
Modern Velocity Anemometers
Modern anemometers can be classified as either a velocity anemometer or pressure anemometer based on the technology used to measure wind speed and direction. We’re going to examine some of the common anemometers in both of these categories, starting with velocity anemometers.
Cup anemometers are still the most common wind measurement device at weather stations. They consist of three arms ending in hemispheric cups all at equal angles to each other. As the cups rotate in the wind, a copper wire in the vertical shaft causes a minor electrical field. The output voltage from the electrical field is directly correlated to wind speed.
Cup anemometers are preferred because they are cheap, easy to operate, and simple. However, the moving parts mean that they need to be replaced. Also, the cup anemometer won’t tell you wind direction.Hot wire anemometers are another style of anemometer that give you wind speed but very little directional information.
In these instruments, a very thin wire is heated to a set temperature. The wind across the wire drops the temperature. The relationship between the temperature drop and wind speed is dependent on the type of wire and the starting temperature. These anemometers are preferred when you are measuring wind speed in a very tight space.
Users must be wary to keep these clean at all times because they will become inefficient when the wire becomes dirty.Sonic anemometers use ultrasonic sound waves to measure wind speed and direction. They work by measuring the speed at which sonic pulses travel between pairs of transducers. Depending on the configuration, these anemometers can measure either two or three dimensions. The 2D sonic anemometers can provide 2-dimensional direction and the 3D version can provide 3D direction.
Finally, the propeller style anemometer combines both anemometer and wind vane technology with a propeller in front and a wind vane in the rear. They are mounted on a movable vertical axis that allows the wind vane to rotate and record dominant wind direction, while the speed of the propeller marks the speed of the wind.
Modern Pressure Anemometers
All of these anemometers mentioned thus far are velocity anemometers. The other type of anemometer are pressure anemometers, which use pressure as a proxy for wind speed and direction.
These pressure anemometers are similar in theory to the original plate style anemometer developed by Leon Battista Alberti. There is still a modern plate anemometer that has a plate attached to a spring. The compression of the spring directly relates to the force of the wind.The more advanced tube anemometer determines wind speed using a glass tube closed at one end and pressure transducers both inside and outside the tube.
The pressure differential between the inside and outside of the tube is directly related to wind speed.A special type of tube anemometer is the pitot tube. These instruments contain two separate tubes running alongside each other. One is open to the wind and one is closed at the upstream end with small holes somewhere downstream. Pressure will increase in the open tube proportionally to the wind speed, while the pressure in the closed tube will remain relatively undisturbed. Pressure sensors in the bottom of each tube monitor the pressure in the tube and the differences between these two measurements is used to calculate wind speed. Next time you are on an airplane, see if you can identify a pitot tube on the body, as they’re commonly used to measure air speed on aircraft.
Any of these modern anemometers may be used at weather stations to monitor wind speed and possibly direction, depending on the technology. The choice of anemometer is dependent on the situation, cost, and accuracy. Many scientists and meteorologists will choose velocity anemometers, such as the cup and the sonic anemometers. Sonic anemometers are the newest technology and are popular among users who want to collect both wind speed and direction in one instrument; however, they are costly.Propeller style anemometers are also quite popular due to their combination of velocity and wind vane type technologies to provide wind speed and 2-dimensional direction in one instrument. Next time you are outside, see if you can notice one of these anemometer types near your home.