Anemometer is a type of weather instrument which is used for measuring the wind speed.
The direction of the wind and its velocity that is measured by the anemometer helps the meteorologists in detecting the natural disasters caused by wind.
Anemometers measure several aspects of wind:
- Velocity
- Pressure
On the basis of these aspects, Anemometers are broadly divided into two categories:
- Pressure Anemometers
- Velocity Anemometers
As there is a close relation between these two aspects of wind (velocity and pressure), so either anemometer can give the adequate information about both.
Pressure Anemometers; are typically divided into two classes plate and tube anemometers.
Plate anemometers are constructed of a square or circular flat plate with a precision spring resting behind it. A wind vane keeps the plate directed against the wind, and the force of the plate against the spring can accurately measure the pressure of the wind. The majority of plate anemometers are not suited to record abrupt fluctuations in wind pressure, mainly because the fundamental design prohibits it. They are generally used to record average wind pressures.
Tube anemometers are of slightly more complex design than plate anemometers. These devices use a vessel with an opening at the top to record wind pressure. The pressure exerted through the mouth of the opening causes changes to the internal pressure of the chamber, which then allows calculations to be made concerning wind pressure. One advantage of tube anemometers is their self-sufficiency; they can be mounted atop a high pole and left unattended for long periods.
Velocity Anemometers; Velocity anemometers are divided into two classes: those that require some sort of wind vane and those that do not.
Cup anemometers are perhaps the most widely used anemometers. It contains four hemispherical cups that form a cross if viewed from above; the wind is captured in the cups, causing the anemometer to spin. A mechanism at the axis records and calculates the number of revolutions, which then allows wind velocity to be observed. Cup anemometers are simple and do not require intricacies such as a weather vane, but they do not record velocity at any given moment as accurately as other types do. For example, a sudden gust of wind may pass by unrecorded.
Windmill anemometers are based on the design of wind vanes, which simply illustrate wind direction. A windmill anemometer is more accurately described as an aero vane, which is essentially a wind vane with a propeller on it. The tailfin of the wind vane accurately displays wind direction, while the propeller at the front can record wind velocity by calculating revolutions per minute.
Other Types of velocity Anemometer are:
Hot-wire Anemometers; Hot wire anemometers use a very fine wire (on the order of several micrometres) electrically heated up to some temperature above the ambient. Air flowing past the wire has a cooling effect on the wire. As the electrical resistance of most metals is dependent upon the temperature of the metal (tungsten is a popular choice for hot-wires), a relationship can be obtained between the resistance of the wire and the flow speed. Hot-wire anemometers, while extremely delicate, have extremely high frequency-response and fine spatial resolution compared to other measurement methods, and as such are almost universally employed for the detailed study of turbulent flows, or any flow in which rapid velocity fluctuations are of interest.
Laser Doppler Anemometers; Laser Doppler anemometers use a beam of light from a laser that is divided into two beams, with one propagated out of the anemometer. particulates (or deliberately introduced seed material) flowing along with air molecules near where the beam exits reflect, or backscatter, the light back into a detector, where it is measured relative to the original laser beam. When the particles are in great motion, they produce a Doppler shift for measuring wind speed in the laser light, which is used to calculate the speed of the particles, and therefore the air around the anemometer.
Ping-pong ball Anemometers; A common anemometer for basic use is constructed from a ping-pong ball attached to a string. When the wind blows horizontally, it presses on and moves the ball; because ping-pong balls are very lightweight, they move easily in light winds. Measuring the angle between the string-ball apparatus and the line normal to the ground gives an estimate of the wind speed. This type of anemometer is mostly used for middle-school level instruction which most students make themselves.
Using Anemometers
i) Measuring Variations in Wind Speed
Anemometers can be used to measure wind speed at various positions around buildings (e.g. the school), over a transect, above different types of terrain and ground cover and at various points under variable meteorological conditions. Ideally, a number of anemometers should be employed for simultaneous measurements in different positions. Wind direction using a vane and compass should be recorded alongside wind speed.
ii) Measuring Vertical Changes in Wind Speed and Direction
Professional meteorologists can measure vertical changes in wind speed and direction using instruments bearing balloons which are released into the atmosphere. The various high profile attempts by balloonists in late 1996 and 1997 to fly around the world highlighted the vertical changes in weed speed and direction which the balloonists were attempting to exploit. Fieldworkers with their feet more firmly planted on the ground must be less ambitious but can, nevertheless, make some interesting investigations using one or more anemometers.
iii) Measuring Wind Speed Variations in Complex Terrains – Sand Dunes
Sand dune areas offer great opportunities for fieldwork using anemometers. Dunes develop as accumulations of wind blown sand grains up to 1mm in diameter. Of particular interest are the ways in which the geomorphology of the sand accumulation changes in response to variations in wind force and direction. Anemometers can be used to measure these variations and to assist in the measurement of sediment transport within dune areas.
Applications
- Anemometers are used for measurement of wind speed, wind velocity, wind pressure and wind direction and other information like the largest gust of wind during a particular amount of time. These instruments can be used in a backyard weather station or on a well-equipped scientific research vehicle.
- Industrial applications for anemometers include re-vegetation projects, projects taken on by NASA, research into shipboard wind speed, and measurements of the wind velocity on Venus.
- Sonic or ultrasonic anemometers, calibrated in a wind tunnel, and containing no moving parts, are really useful for measuring weather conditions that are very turbulent.
- Other applications are wind-tunnels and gas pipe technology.
- It helps the meteorologists in detecting the natural disasters caused by wind.
Safety Guidelines
- Clean the cup regularly in case of cup anemometer. Heavy contamination in cup such as the bird droplets and ice can disrupt the accuracy of anemometer.
- Temperature range should not extend the operational range of the instrument.
- This instrument is NOT designed for gas mixtures other than air. Its use with corrosive or other dangerous gas mixtures can destroy the accuracy of instrument.
- High winds can be dangerous – be alert.
- Do not work close to cliff edges in windy weather.
- Beware of flying debris and falling objects in very windy weather.