Whether your interest extends from owning a home weather station, you are thinking about installing a wind generator turbine or you just have some unanswered questions, hopefully you will find some interesting information within this dummies guide to wind measurement.
We will show how wind speed and wind direction are measured and reported.
Let’s start with the basics:
What is Wind and Why Do We Have it?
Wind is simply the movement of air from one location to another. It is driven by pressure differences. Watch any weather forecast and you will be shown a chart that describes the location of pressure systems each of which is described as either low or high.
Weather chart, the lines superimposed (isobars) link all areas of equal pressure (similar to a contour map)
As you would expect air will move from an area of high pressure to one of low pressure. This would of course continue until (theoretically at least) a state of equilibrium was reached around the globe consisting of a single universal pressure all over the world and no more air movement.
What causes the pressure differences in the first place is an excellent question but beyond the scope of this article (fundamentally it is caused by heat differences around the globe).
As we have seen from the weather channel’s synoptic charts these pressure systems are large, broadly circular volumes of air.
You would probably expect that the air would rush from a high pressure area directly into a low pressure area in a nice straight line (it would certainly make it easy to work out the wind direction from a pressure chart). In reality though it is a little more complicated than that.
Why? Because the earth is rotating.
It is the result of a phenomenon known as the Coriolis Effect.
To explain it in an extremely simplistic way, a movement in a straight line appears to move in a curved line when seen from a rotating reference point. Imagine a toy globe.
Now imagine taking a penlight and describing a straight line on the surface of the globe with the illuminated dot from the North Pole down through Canada and the USA then straight down to the equator.
Do it again, with the torch in the same position and describing the same line but this time slowly rotate the globe as you do it. This time of course the line will follow a very different path of the face of the globe. If you were watching this giant penlight in the sky from the rotating surface of the globe it would describe a curve as you physically moved away from it. This is (at least in part) why winds appear to spiral around pressure systems from our earth-bound perspective.
The red arrow shows the path of the penlight as viewed from the ground
If you are in the northern hemisphere wind will rotate in an anticlockwise direction around a low pressure area.
Conversely wind will move in a clockwise direction around a high pressure system (they are opposite to each other as the wind is moving in to a low pressure system but out of a high pressure system).
If you are in the southern hemisphere these directions are reversed. This is because the apparent motion of a straight line is reversed below the equator.
If you go back to the imaginary globe, this time take the penlight in a straight line from the south pole to the equator.
Think about the direction that the penlight would appear to take to an observer on the globe surface in the southern hemisphere.
Once you get the hang of it it’s easy to get a feel for wind speed and direction from a weather chart:
The greater the difference in pressure between the high and low pressure areas the greater the wind speed.
This difference is indicated by the spacing of the isobar lines on a weather chart. An isobar is a line drawn on weather map linking all areas of equal pressure. Isobars will be plotted in fixed steps (for example an isobar drawn linking areas in which the pressure is 990mB, the next one for 994mB, the next one 998mB etc).
Therefore if the isobars are plotted very close together on a map then the pressure is changing quickly over a short geographical area and wind will be strong.
Conversely if the isobars are further apart the pressure difference over the same area is less and the wind will be weaker.
It can be seen now how a weather chart with isobars will tell you much about the wind speed and direction. The spacing of the isobars will dictate the speed of the wind and the position of the high and low pressure areas the direction.
How is Wind Measurement Reported?
The answer to this depends very much upon the application. For example climatologists studying long term trends in weather will have very different requirements from pilots making a final runway approach at an airport.
How would you report something as variable as wind speed?
You can of course measure the speed of air movement at any given moment in time, but that is just a snap shot and may not even be representative, you may have taken the measurement in a momentary lull.
OK so you can average it, but over how long? A second, a minute, an hour, a week?
You could pretty much get any average wind speed figure imaginable by choosing the period over which you average.
The point that I am making is that how the data is processed is at least as important as how it is measured. The way in which it will be processed will depend upon who needs the information and what they need it for.
The example earlier of the airline pilot is a good illustration:
Typically when flying en-route between airports and reviewing the weather ahead pilots will use 10 minute mean wind speed (that is to say all the individual measurements taken over the previous 10 minutes have been averaged).
10 minute mean data gives a good general idea of wind conditions and trends in the wind speeds (whether it is rising or falling for example).
When they are on final approach to an airport they will change to using 2 minute mean wind data. Why?
Because it will give a more immediate indication of what the wind is doing right now. Imagine a sudden step change in the wind speed, if you are reporting a 10 minute mean then it will take a while for the scale of that step change to become clear.
On the other hand a 2 minute mean will react much more quickly to that step change. This is important of course to a pilot who needs to know immediately if the wind changes significantly.
This might lead you to think – why average the reading at all? Just report it as soon as it has been measured. This takes us back to the intrinsically variable nature of the wind. A single measurement may not be representative, so in averaging it we are attempting better to represent it but at the potential cost of a slower reaction to a change in that wind.
Another key characteristic of wind speed which has not yet been addressed is its gusty nature. We have all experienced being out walking in steady breeze but being buffeted occasionally by strong gusts.
How can this be reported?
Going back to pilots again this is key information for them, they not only need to know the mean wind speed but also the gust. To do this someone needs to define what constitutes a gust. In the U.K. this is the Meteorological Office.
A gust is defined as any measured wind speed that exceeds the average wind speed by at least 10 Knots (1 knot = 1.15 miles per hour). A gust is always therefore associated with an average wind speed.
For example a 10 minute gust figure will be the highest measured wind speed in the last 10 minutes that exceeds the 10 minute average speed by at least 10 knots.
The wind speed may therefore be reported as “26 knots gusting 39″ (meaning that the mean wind speed is 26 knots but a maximum measurement of 39 knots was recorded). If that maximum had been 35 knots then no gust would have been reported as the maximum was not at least 10 knots higher than the mean.
Between the reported average and gust figures you can see how it is possible to convey a lot about the strength and behaviour of a wind.
Wind direction is usually reported in degrees :
Cardinal points (north, east, south and west) and their corresponding value in degrees- note that in meteorology the 000 degrees value is only used when there is no wind to report, a wind straight out of the north would be reported as 360 degrees
The direction states where the wind is moving from (an easterly is a movement of air from the east heading to the west). It is worth noting that the direction is usually reported in terms of either ‘magnetic’ or ‘true’.
This refers to whether the north used as the reference is magnetic north or true north. The former moves both over time and with respect to where on the globe you are located.
The difference between the two can be significant (again depending upon where you are). The latter does not vary with time or location.
Wind is usually reported to the nearest 10 degrees (for example if the measured wind direction was 272 degrees it would be reported as 270 degrees and similarly if the measured direction was 186 it would be reported as 190 degrees).
All wind measurement equipment (on shore or on fixed installations offshore) should be aligned to true north.
The same considerations about the processing of wind speed measurements also apply to wind direction.
Typically a 2 or 10 minute mean wind speed will be reported in association with the corresponding 2 or 10 minute mean wind direction. Unless otherwise stated it should be assumed that any wind direction reported is relative to true north.
Wind direction will often include an arc of measurements, for example a mean wind direction may be reported along with the minimum and maximum direction recorded over the mean interval.
This is certainly the case for reporting weather in aviation applications and again is an attempt succinctly to convey the behaviour of the wind.
How Do You Measure Wind Speed and Wind Direction?
At the simplest level with a wind sock. To the skilled eye this can convey pretty much everything that you need to know for a particular application and of course in terms of cost and maintenance requirements it is hard to beat.
A wind sock with aviation colour markings
For immediacy, conveying the wind conditions at a pinpoint location and with doing so with high visibility there is nothing else to beat it.
This is why despite the wide range of far more sophisticated equipment installed at any airport, you will also see a wind sock or two around.
An anemometer is the name given to an instrument that measures wind speed, they vary in their design and method of measurement.
Rotating Wind Speed and Direction Sensors:
The most common is still the widely recognised spinning cup variety. A set of cups attached to a rotating shaft are oriented to be propelled by the wind. The cups, driven by the wind cause the shaft to rotate at a rate proportional to the wind speed.
The classic RW Munro IM124 generator type anemometer, found all over the world
Different types of spinning cup anemometer use different methods to determine the rate of rotation (and hence the wind speed).
The most widely used is probably the dynamo (or generator) type.
Permanent magnets are mounted on the rotating shaft. The shaft rotates within a chassis lined with electrical windings.
As any senior school physics student will tell you, if you move a magnet next to an electrical winding you will generate an electrical signal.
The magnitude (and also the frequency) of the electrical signal generated by the shaft-mounted magnets spinning is directly proportional to the wind speed.
The beauty of this type of wind speed measurement is that it requires no power at the sensor, the signal is generated by the instrument itself.
Typically the spinning cup anemometer would be connected to a wind speed dial display:
A selection of RW Munro wind speed dials, the dial is powered entirely by the anemometer
Often it will also be connected to a chart recorder (a paper chart on which a pen records the wind speed), this maintains a record of the wind speed over time rather than the simple snapshot offered by the wind dial display.
An RW Munro wind speed chart recorder
These days of course it is very likely that the signal from the anemometer will be taken to a computer at some point, however the dynamo type of sensor does not lend itself to computer logging as its output signal requires considerable modification before it can be acquired by digital equipment.
The dynamo type is essentially a low-tech creation and has therefore found favour all over the world for its low cost and ease of maintenance and repair.
Another common variant of the spinning cup type of anemometer is the optical encoder disk type.
The sensor uses the same cups-mounted-on-a-spinning-shaft arrangement but instead of having permanent magnets mounted on the shaft there is a thin disk. The wind spins the cups which in turn spins the encoder disk.
Mounted in fixed positions on either side of the disk are an infra-red transmitter diode (on one side of the disk) and directly above it but on the opposite side of the encoder disk an infra red receiver diode.
The receiver diode switches on whenever it receives a signal from the transmitter. The hole in the encoder disk is located such that it will pass between the diodes once per revolution of the shaft.
By monitoring the switch rate of the receiver diode the detecting equipment can calculate the speed at which the shaft is rotating and therefore the wind speed.
There are many other ways of converting the rotation of anemometer cups into a measurable wind speed but all operate along the same lines.
The wind turns the cups, the cups are fixed to a rotating shaft so the shaft rotates, the speed of shaft rotation is measured and the wind speed then deduced.
The classic handheld wind speed anemometer
As you might imagine wind speed sensors come in all shapes and sizes as well as being made from a wide range of materials from heavy brass to lightweight plastic.
This variation necessarily gives rise to a range of responses to the same wind conditions. Different sensors will be quicker or slower to respond to changes in air flow. This gives rise a range of defining characteristics used in conjunction with each senor. An example is the distance constant.
Imagine how different anemometers will respond to a sudden increase in wind speed, a light plastic instrument will reach the new speed almost instantly but a larger brass sensor will take much longer.
The distance constant is used to define this difference in response time. If you consider the wind as a column of air that moves through the anemometer then, broadly speaking, the distance constant for any given sensor is the length of column of air that will have to pass through the sensor before it will reach the true wind speed rotation rate.
A fast response plastic sensor will have a short distance constant and conversely a heavier, slower response sensor will have a longer distance constant.
While there are statistical methods which can even out some of the differences between the data measured by wind speed sensors with very different distance constants, it is generally the case that weather monitoring organisations try to stick to using sensors with broadly comparable characteristics for their official measurements.
When using a spinning cup anemometer you also need a way of measuring wind direction. This is almost universally done using a direction vane.
Just as you might see on a church roof, the wind direction sensor utilises a vane which will rotate to align itself into the wind.
The vane is fixed to a rotating shaft and so the movement of the vane is transmitted to the movement of the shaft which typically will move the wiper on a circular potentiometer (a variable resistor).
This results in a signal voltage which directly corresponds to the physical position of the wind vane.
A combined wind speed (top section) and direction sensor, the IM146 from RW Munro.
Non-Rotating Wind Measurement Instrumentation:
In the past the requirement for wind speed sensors that do not use spinning cups or similar designs had been limited to a few applications where such designs were not practicable, notably in regions with extremely low temperatures which would cause moving parts to ice-up or in remote locations where access was difficult and / or expensive.
Instruments with moving parts inevitably require significant maintenance due to the physical strain (on bearings for example) of running 24 hours per day.
However such equipment is now becoming more widely used partly because of the reduced maintenance implications and in some instances because it can perform measurements that are not possible with the spinning cup type of sensor.
Ultrasonic Anemometer:
This sensor has no moving parts at all. It consists of an array of ultrasonic emitter and receivers.
The basic principle involved is that the instrument measures the time taken for an ultrasonic pulse to pass from an emitter to a receiver. It will then reverse the direction and measure the time taken for a pulse travelling in the opposite direction.
If there was no wind then the time taken in each direction would be identical. The difference in the measured times taken between directions is a function of the air flow rate (wind speed) in between the emitters and receivers.
There are usually 2 sets of transceivers (combined ultrasound transmitters and receivers), one pair pointed along the north – south axis and the other along the east – west. The results can then be vectored to produce a wind speed and direction.
An ultrasonic wind sensor made by Gill instruments. Each of the 4 heads is a combined ultrasound emitter and receiver
In reality the air temperature will also have an influence on ultrasound propagation time and so the sensor will factor this in.
Wind Profilers:
Profilers are devices used to measure wind speed and directions at various elevations from ground level up to high altitudes (even as high as 15Km above sea level depending upon the type of instrument) and hence produce a wind “profile” up through the atmosphere.
Profilers are typically weather radar.
This is usually Doppler radar. By tracking the shift in frequency between emitted radar pulses and their reflected signals received back at the radar it is possible to deduce the speed and direction of air movement. The radar waves are reflected off tiny airborne particles.
There are also devices which use sound wave pulses to achieve the same ends (although their range is much less than a weather radar’s).
Needless to say the cost of wind profiling equipment is several orders of magnitude greater than any other method.
Hopefully you have found this brief and admittedly very basic guide to wind measurement interesting.
If so then I can heartily recommend getting a home weather station, they are an inexpensive and endlessly fascinating addition to a home and these days the external sensors (including the wind measurement instruments of course) tend to be wireless and so the installation is easy.
