Anemometers are a group of machines that measure wind velocity or pressure. They are very useful and provide a wealth of information to meteorologists, despite their extremely simplistic design. There is a variety of types of anemometers with their own special features and uses, from very old plate anemometers to high tech sonic and laser Doppler anemometers.
Anemometers generally consist of some form of wheel on a weather rod with hemisphere shaped cups on each spoke. These cups are designed to catch the wind and spin the wheel. A magnet built into one of the arms causes a pulse as it passes a reed switch at a certain position. These pulses are recorded and the time between them gives a relatively accurate estimate of the speed that the wind is moving.
The simplest form of anemometer is the cup anemometer, these have cups or other bowl shaped devices that move when wind pushes them. They are the original modern anemometer and were developed by Dr. John Robinson of the Armagh Observatory in 1846. Alterations of this model can measure both wind speed and direction by including tags that measure at which point in the spin each cup has the greatest pressure by the wind.
In wire anemometers a wire is heated up and allowed to cool by the wind. How quickly it cools relative to the temperature gives an accurate estimate of the speed of the wind. Sonic anemometers, on the other hand, use disturbances in sound waves to calculate the velocity of the wind. They are popular because they need little physical maintenance, because there are no moving components. Their accuracy and prediction of wind speed is tops, as well.
Another particular type uses laser beams to measure wind speed. These are known simply as laser Doppler anemometers. A split laser beam can track the speed of debris between two surfaces. Often times some kind of dust or seed material is introduced to give the lasers something to record.
The oldest form of anemometers are plate and tube anemometers, utilizing the pressure of wind on either a plate or a tube of liquid to determine wind speed. These forms are not very popular today, as other anemometers are more accurate and efficient. These anemometers are poor at evaluating very low wind speeds, as they do not provide enough force to push the spring that makes the measurement. It is important to note that the density of air affects how the wind can press on pressure-based devices. With this in mind, special allowances are made when considering wind speed at very high elevations.