Picture this: you bolt a new wind sensor to the side of your house just under the gutter, and on the first stormy evening it reports a relaxed 3 m/s while the local forecast warns of gusts above 37 mph. In most cases the device isn’t broken. It is measuring the wind very accurately at exactly the spot where it hangs, which happens to be in the wind shadow of your own house. Good wind speed measurement starts with choosing the location and only then the hardware. The short answer: install the wind speed sensor in the most open position you can, ideally on a mast at about 10 meters (33 feet), with a distance from buildings and trees of at least ten times their height. On a roof, it belongs well above the ridge, not at the edge.
These rules of thumb come from the measurement guide of the World Meteorological Organization (WMO), which national weather services follow. In a backyard, on a company site or on a warehouse roof, you can rarely meet them in full. That is fine as long as you know which wind your site represents. This page covers how an anemometer works, which design fits which job, how to choose height and distance, what separates average wind from gusts, how to convert units and when a high-wind alert makes sense.
How does an anemometer measure wind speed?
An anemometer turns moving air into an electrical signal. The classic design is the cup anemometer: three or four hemispherical cups on a vertical shaft. Wind pushes harder on the open side of one cup than on the rounded back of the opposite one, so the cups spin, and the rotation rate is roughly proportional to wind speed. A magnetic or optical pulse generator counts the turns, and the electronics convert them to m/s using a calibration curve. With this design, wind direction needs a separate wind vane.
An ultrasonic anemometer has no moving parts. Its head holds several ultrasonic transducers at fixed distances from one another. They take turns sending short sound pulses back and forth. When wind blows along the measuring path, the pulse traveling with the wind arrives slightly earlier than the one traveling against it. From the difference in travel time, the sensor calculates the wind component along that path, and with two paths at right angles it gets both wind speed and wind direction at once. Because both directions are combined, the temperature-dependent speed of sound largely cancels out.
Ultrasonic vs cup anemometer: which is more accurate?
Under good conditions, both designs meet the accuracy the WMO considers sufficient for general weather observation: about 0.5 m/s below 5 m/s and better than 10 percent above that. The differences show up in gusty wind, in light wind and after a few years outdoors. A cup rotor speeds up faster than it slows down. In turbulent flow it therefore overestimates the average, and air arriving at an angle makes the effect worse. The WMO puts this so-called overspeeding at up to 10 percent for some designs. On top of that, cups have a starting threshold below which they don’t turn at all, and bearings that get stiff over time.
An ultrasonic anemometer responds almost instantly, has no starting threshold and no bearing to wear out. The WMO lists high durability and little loss of accuracy over time as its advantages. The limits: heavy rain, snow, ice on the transducers or a bird perching on the head can disturb individual readings. The table below sums up how the two designs differ in everyday use.
| Criterion | Cup anemometer | Ultrasonic anemometer |
|---|---|---|
| Measuring principle | Rotation rate of a cup rotor | Travel time of sound pulses |
| Moving parts | Yes, bearings and cups | None |
| Gusts and light wind | Inertia, starting threshold, overspeeding in turbulence | Very fast response, no starting threshold |
| Wind direction | Needs a separate wind vane | Measured in the same head |
| Typical problems | Bearing wear, ice on the shaft | Heavy precipitation, ice and dirt on the transducers |
For a permanent installation with remote data transmission, an ultrasonic sensor is usually the better choice, because nobody has to climb the mast to check bearings. One example is the compact RK900-09 weather probe, which works together with the RAKwireless RAK2560 sensor hub. It measures wind speed ultrasonically over a range of 0 to 40 m/s with a resolution of 0.01 m/s, plus wind direction, temperature, humidity and air pressure. The datasheet specifies wind speed accuracy as ±0.5 m/s plus 2% of full scale, an IP65 rating and an operating range of −40 to +80 °C (−40 to +176 °F).
How high should an anemometer be mounted?
Wind speed increases with height because friction with the ground slows the air down. So that readings from different stations can be compared, the WMO sets a standard height of 10 meters above level, open ground. The wind figures in weather reports refer to that height.
Does your reading have to be taken at 10 meters? Only if you want to compare it with weather services or other stations. For many jobs, what counts is the wind where it acts. If you want to protect an awning, a greenhouse or a solar tracker, mount the anemometer at a height similar to the object at risk, but where the wind reaches it freely. A sensor at 3 meters in an open area gives more honest numbers than one at 10 meters directly behind a warehouse roof.
What you should avoid is converting with a fixed factor. How fast wind increases with height depends on the roughness of the surroundings, and obstacles change the profile depending on wind direction. Instead, record the actual mounting height and the surroundings so that later comparisons stay traceable.
Can you mount a wind speed sensor on a roof?
Yes, but the roof is a tricky spot even though it is often the highest point you can reach. At the roof edge the airflow separates, above it the wind accelerates, and behind it a reverse flow forms. A wind sensor that sits only a couple of feet above the parapet will, depending on direction, read too high, too low or even report wind from the wrong direction.
The WMO recommends raising wind instruments on a building at least one building width above the roof. For a single-family home that is rarely realistic. The RAK2560 installation guide puts it differently and names 1.5 times the building height as the ideal mounting height. Either way, a sturdy mast above the highest point of the roof, preferably in the middle rather than at the edge, is far better than a short bracket on the gable wall. If you mount on the side of a mast, the boom should be at least three mast diameters long so the mast itself doesn’t distort the flow. RAK also recommends pointing the boom toward the prevailing wind.
Pay attention to details that have nothing to do with airflow as well. According to the manufacturer, the bracket in the RAK2560 kit fits masts with a diameter of 50 to 80 mm (about 2 to 3 inches). Keep at least 2 meters from radar equipment, and don’t place radio antennas, motors or transmitters right next to the probe. North alignment of the probe is a separate topic, covered in our post on setting up a weather station.
How far should an anemometer be from buildings and trees?
Behind every obstacle there is a wake in which the airflow is slowed and turbulent. The WMO therefore defines open terrain as a site where the distance between the anemometer and any obstacle is at least ten times the obstacle’s height. A house 8 meters (26 feet) tall calls for 80 meters (260 feet) of clear ground. Even that is explicitly only the minimum, because wakes often extend 12 to 15 times the obstacle height downwind.
In a residential area you will practically never get distances like that. A simple check helps instead: go through every wind direction in your head, not just the most common one. A site that is fully exposed to westerly winds can sit in the shadow of a tree line when the wind comes from the east. The result isn’t a constant error you could subtract later, but one that depends on direction. Trees also grow, and they slow the wind less in winter than in summer.
The WMO’s second key piece of advice is surprisingly unglamorous: document everything. Write down the mounting height, photograph the surroundings in every direction and later record each change, such as a new building, a felled tree or scaffolding. If your readings suddenly drop one day, you’ll be able to tell whether the weather, the surroundings or the sensor is responsible.
Average wind speed vs gusts: what does the sensor report?
Wind is never constant. A number on a display can therefore mean three very different things: an instantaneous value, an average over a time interval or a gust. Following WMO guidance, weather services report the average wind over 10 minutes. For gust measurement, the WMO recommends the 3-second peak gust, the highest running 3-second average within the observation period. Conditions count as calm when the average wind is below 1 knot.
A wireless sensor that sends one number every ten minutes may only deliver an instantaneous value at the moment of transmission. A strong gust two minutes earlier won’t show up in the data at all. The highest value in your chart is then not the strongest gust of the day, just the highest moment that happened to be captured. On the RAK2560, the uplink interval can be set anywhere from 60 to 86,400 seconds. Whether the probe sends an average, a peak or an instantaneous value isn’t clearly stated in the manufacturer documentation. Check the payload description before you treat the readings as gusts.
A weather app often shows the 10-minute average from the nearest station, while your wind sensor may show an instantaneous value at a different place and height. Only compare values with the same statistic, the same unit and similar exposure.
How do you convert wind speed between m/s, km/h, mph and knots?
Meteorologists and sensors usually work in meters per second, US weather apps in mph, most of the world in km/h, and sailors and pilots in knots. Conversion is easy once you know the factors: 1 m/s equals 3.6 km/h, about 2.24 mph and about 1.94 knots. One knot is 1.852 km/h, or roughly 1.15 mph and 0.51 m/s. One mph is about 1.61 km/h or 0.45 m/s. Beaufort, by contrast, isn’t a unit but a set of force classes, each covering a range. The table shows the levels typically relevant for a wind alert.
| Beaufort | Description | m/s | km/h | mph | Knots |
|---|---|---|---|---|---|
| 4 | Moderate breeze | 5.5–7.9 | 20–28 | 13–18 | 11–16 |
| 5 | Fresh breeze | 8.0–10.7 | 29–38 | 19–24 | 17–21 |
| 6 | Strong breeze | 10.8–13.8 | 39–49 | 25–31 | 22–27 |
| 7 | Near gale | 13.9–17.1 | 50–61 | 32–38 | 28–33 |
| 8 | Gale | 17.2–20.7 | 62–74 | 39–46 | 34–40 |
| 9 | Strong gale | 20.8–24.4 | 75–88 | 47–54 | 41–47 |
| 10 | Storm | 24.5–28.4 | 89–102 | 55–63 | 48–55 |
The table shows how easy it is to get a threshold wrong. If you enter “20” in a dashboard thinking of mph while the system works in m/s, you’ve moved your alert from Beaufort 5 to Beaufort 8. Always check the raw value, the axis label and the rule field together. An export should carry the unit and the type of value along with the number, for example “10-min average, m/s”, so it stays clear later what was actually measured.
What wind speed should trigger a high-wind alert?
There is no universal number, because the limit depends on what you are protecting. The most reliable source is almost always the manual of the thing at risk. Awnings, for example, are rated under the European standard EN 13561 in wind resistance classes, and a class 2 awning is designed for up to roughly Beaufort 5. Greenhouses, tents, aerial work platforms and solar trackers come with their own limits from the manufacturer.
Set the alert somewhat below that limit so there is enough time to retract or secure things. Also decide deliberately whether the average wind or the gust should trigger it. For damage to lightweight structures, gusts usually matter most; for longer outdoor work, sustained average wind is the better guide. Hysteresis keeps the alert from switching on and off every minute when the wind hovers around the threshold. Besides the fixed uplink interval, the RAK2560 can be configured to send event-based messages when a value rises above or falls below a threshold or stays within a range.
Finally, plan for what happens when no data arrives. If the radio link or power supply fails, the last low reading must not stay on screen as current calm. A stale-data alert therefore belongs with every wind alert. And a monitoring sensor doesn’t replace a certified safety shutdown where one is required.
Where can you buy a wind speed sensor?
Before you pick a device, settle the location question first. Do you need a reading comparable to the weather service, or the wind right at your building, field or scaffolding? That determines mounting height, mast length and distance from obstacles. For a permanent installation, a lot speaks for an ultrasonic anemometer with no moving parts that captures wind speed and direction in one head. Look at measuring range, accuracy, IP rating and temperature range, and at whether the system reports averages, gusts or instantaneous values. Check which unit the readings arrive in and whether thresholds and uplink interval can be set the way your wind alert needs. Also check that your wireless network reaches the mounting point and how the sensor is powered. If you want wind speed measurement that lets you react in time, what you need most is an honestly described site and readings whose meaning is clear. If you are looking for a suitable wind sensor, you can find our models in the wind speed sensors category.