Anemometers
Last updated: February 5, 2009.
When you hear weather forecasters warning of how fast the wind is
going to blow, do you ever stop to think how they're going to measure it? Wind isn't
something you can see very easily, so you can hardly time it with a
stopwatch like you'd measure the speed of an
olympic sprinter or a race car! Fortunately, scientists are amazingly
inventive people and they've come up with some pretty clever ways of
measuring wind speed with gadgets called anemometers.
Let's take a closer look at how they work!
Photo: Measuring wind speed with a three-cup, handheld anemometer. The square plate at the back is a vane that aligns itself with the wind so you can measure wind direction too. This model, used by the US Navy, is an Ames RVM 96 B capable of measuring wind speeds up to about 50 m/s (180 km/h or 112 mph). Photo by Maebel Tinoko courtesy of US Navy.
Thinking about anemometers
Some people think wind turbines are
unsafe because gales and storms could make them
spin dangerously fast. That's not actually true: all large wind
turbines are fitted with brakes that stop
them rotating if the wind
blows too hard (and they have built-in anemometers to measure the speed
as well). But it's certainly true that wind turbines turn
faster—and generate more electricity—the
harder the wind is blowing. There you have a clue to how a basic
anemometer could work.
Suppose you build yourself a miniature, table-top wind turbine and
connect it to an electricity generator (effectively an electric
motor wired up backwards so it makes an electric current when you spin its central
axle around). The faster the rotor
blades turn, the quicker the generator spins, and the higher the electric current it will produce.
So if you measure the current, you have a basic way of measuring the
wind speed. You have to calibrate an
instrument like this before you use it, of course. In other words, you'd need to know how
much current is generated by a few winds of known speed. That would help
you figure out the mathematical relationship between wind speed and
electric current so you could figure out the speed of an unknown wind
simply by measuring the current.
Mechanical anemometers
Some of the simplest anemometers work in exactly this way. They're little more than an electricity generator mounted in a sealed-up metal cylinder with an axle protruding upward from it. On top of the axle, there are several large cups that catch the wind and make the generator spin around. Propeller anenometers work in much the same way. Like miniature wind turbines, they use small propellers to power their generators instead of spinning cups. Some anemometers have what looks like a small fan in place of the cups or propeller. As the wind blows, it spins the fan blades and a tiny
generator to which they're attached, which works a bit like a bicycle dynamo. The generator is connected to an electronic circuit that gives an instant readout of the wind speed on a digital display.
Photo: A handheld digital anemometer from La Crosse Technology. The fan at the top generates magnetic impulses, which electronic circuits inside convert into a precise wind speed. The display also indicates how strong the wind is on the Beaufort scale.
Some cup-style anemometers dispense with the electricity generator and, instead, count how many times
the cups or fan blades rotate each second. In one typical design, some of the fan blades have tiny magnets mounted on them and, each time they make a single rotation, they move past a magnetic detector called 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. This kind of
anemometer effectively makes a series of electric pulses at a rate that is proportional to the wind speed.
Count how often the pulses come in and you can figure out the wind speed from that.
In another design, known as optoelectronic, spinning cups turn a kind of paddle wheel inside the metal canister underneath. Each time the paddle wheel rotates, it breaks a light beam and generates a pulse of current. An electronic circuit times the pulses and uses them to calculate the wind speed. The anemometers shown in our photos up above,
made by Ames of Slovenia, work in roughly this way.
Photo: The main parts of the handheld, optoelectronic Ames anemometers used by the US Navy. The case is made of lightweight aluminum. Photo by Joe Painter courtesy of US Navy.
Ultrasound anemometers
You probably know that sound travels by making air molecules move
back and forth. It's fairly obvious that the speed of the wind affects
the speed at which sounds travels. If you're shouting to a friend who's down-wind
of where you're standing, you'll hear their voice slightly sooner
than they would if there were no wind at all. Similarly, if they
shout back, you'll hear their voice slightly later—because the
sound waves they generate have to fight against the wind to reach
you. The same idea is used in an ingenious way in ultrasound
anemometers, which measure wind speed using high-frequency sound
above the range humans can hear.
An ultrasound anemometer has two or three pairs of sound transmitters and
receivers mounted at right angles to one another. Stand it in the wind and each
transmitter constantly beams high-frequency sound to its respective
receiver. Electronic circuits inside measure the time it takes for
the sound to make its journey from each transmitter to the corresponding receiver. Depending on how the wind
blows, it will affect some of the sound beams more than the others,
slowing it down or speeding it up very slightly. The circuits measure
the difference in speeds of the beams and use that to figure out how
fast the wind is blowing.
Photo: This wind-measuring mast has several anemometers mounted on it. In the center, you can just make out an ultrasound anemometer. It sends its sound waves between those clawlike points mounted at angles. There are also a couple of propeller anemometers here, looking like tiny wind turbines. Photo by Warren Gretz courtesy of US Department of Energy/NREL.
Laser interferometer anemometers
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.
Photo: A laser interferometer anemometer being used by NASA.
Photo courtesy of NASA Glenn Research Center (NASA-GRC).
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.
Dopper laser anemometers
Given its high-precision nature, you'd use a laser interferometer
for making very precise measurements in a laboratory. But some laser anemometers
are robust enough for more general use outdoors. They send a safe,
infrared laser beam straight up into the air (which serves as the
reference beam) and detect the beam reflected back down from dust
particles, water droplets, and so on (which is the measurement beam).
Wind movements wobble those airborne particles around so the measurement
beam is slightly changed in frequency compared to the reference beam.
The change in frequency is called a Doppler shift and it's much like the
way a fire engine siren changes pitch from a high note to a low note as
it speeds past you. By measuring the frequency shift, you can precisely
measure the speed of whatever caused it (in this case, the wind speed).
A typical anemometer that works in this way is the ZephIR® from
Natural Power.
Why you can't always measure things precisely
Measuring things is the foundation of science, but it's important
not to get carried away. With brilliantly accurate scientific
instruments, you can make measurements that have no real meaning or
value if you don't think carefully about what you're doing.
A good anemometer will give you a wind speed reading accurate to about ±0.5 m/s (±2km/h or ±1mph), but that's often far more accurate than you need. Remember that the wind speed
isn't constant—it's varying all the time! So unless you're in a wind tunnel, where the
speed is constant and precise measurements count, any measurement you make is going to
be, at very best, a rough guide to how fast the air is actually moving.
You'll notice that weather forecasters often take
account of this in the figures they quote. They give you a basic
wind speed (in miles or km per hour) and also tell you how high the
gust (maximum) speed is going to be too. What you have is effectively a
range of speeds that the wind is likely to reach on a particular day, but
no-one can tell you exactly how fast the air will be moving in a
certain place, because it depends so much on the local geography—the
presence of things like hills, trees, houses, valleys that funnel the
wind, and so on.
