Wind Measurement
From Wind wiki
An anemometer is an instrument used to measure the speed of the wind and in turn, deciding if a wind turbine is going to viable in a particular location. There are several types of anemometers, ranging in complexity; the most basic models measure the wind speed, or more complex ones can measure wind speed, wind direction and air pressure.
A wind measuring instrument is composed of two parts: the sensor, and a means for displaying the data it measures. The sensor generates an electrical signal that’s proportional to wind speed.
The spinning cup anemometer measures wind speed only. It is the most common type of anemometer and is also the most basic model. The cup anemometer consists of three or four cups (one with a magnet attached to it) positioned on a 45 degree angle and mounted to a vertical pole. As the wind blows, it catches in the hollow of one or more of the cups, and forces the anemometer to spin on the pole. Each time the anemometer completes a full rotation, the magnet on the cup is detected by a reed switch. When a magnet is nearby, the reed switch closes and generates a brief pulse of electric current, before opening again when the magnet goes away. The number of pulses is counted over a period of time, and converted into an average wind speed that is recorded on a display or weather station.
Another basic anemometer is the windmill anemometer. The windmill anemometer consists of shaft with a tail fin and a propeller positioned on opposite ends and mounted to a vertical pole. Wind blowing against the tail end causes the anemometer to rotate until it faces into the wind (indicating wind direction), which in turn causes the blades of the propeller to rotate. The wind speed is calculated using a magnet and reed switch just like on the spinning cup anemometer.
A slightly more complex type of anemometer is the sonic anemometer. The sonic anemometer uses ultrasonic sound waves to measure the wind speed, as well as the direction. Two pairs of ultrasound transducers (devices that both emit and receive sound waves), are positioned approximately 4 to 8 inches apart, facing one another. In turn, each transducer emits an ultrasonic sound wave into the wind, towards the opposite transducer. If the wind is behind the sound wave, it will cross the gap faster, or if the wind is in front of the sound wave, it will cross the gap more slowly. By calculating the time it takes for the sound wave to cross back and forward across the gap we can tell the speed of the wind - a weather station makes this calculation for us and displays it as wind speed. The wind direction is also determined by examining which angle the wind passed through the ultrasonic waves. Sonic anemometers are popular because they can take very accurate readings and require no moving parts.
You can make a similar—but much more precise—measurement using beams of light instead of ultrasound. The basic principle is called interferometry, and it can be used to measure all kinds of different things with incredible precision. How does it work? You take a laser beam and split it in half using a semi-silvered mirror (a mirror partly coated with silver so it allows half the light to pass through and reflects the rest away). You keep one part of the beam intact (let's call it the reference beam) and allow the other part of the beam (let's call it the measurement beam) to be affected by the thing you want to measure. Whatever it is will slightly alter the phase (pattern of vibration) of the light waves in the measurement beam, but it won't affect the waves in the reference beam (which travel along a separate path). Now you recombine the two laser beams. The measurement beam will be slightly out of step with the reference beam, causing a strange light pattern to form where they meet and overlap, known as a set of interference fringes. By measuring the spacing of the fringes, you can calculate how much the measurement beam was affected.
When it comes to measuring air speed, you simply allow your measurement beam to pass through a chamber where the air is moving. You could fire it through part of a wind tunnel, for example, or through a pipe or tube where you're studying air flow. You need to calibrate the setup first, of course, so you know the relationship between wind speed and the changes you observe in the interference fringes. Once you've done that, you can use your laser anemometer to measure the speed of any unknown air current.
After the actual measurement of the wind, it needs to be logged via some sort of data logger. Data loggers consist of multiple accumulators (similar to odometers) that tally the data falling into each accumulator’s domain. For example, each accumulator could represent a given wind speed range. Wind speeds of 0-1 meters per second could be recorded into one accumulator, winds 2-3 would fall into the second, and so on. After the end of the observational period, the contents of each accumulator can be used to plot the speed distribution.