Why does the number change when I move the height?

Wind speed is not constant at all heights above the ground. It generally increases with height. This is due to friction from the ground surface, which slows the wind closer to the ground. As you move higher, this friction effect reduces, allowing the wind to move faster. The instrument shows the wind at your chosen working Height. When you change this Height, the displayed wind speed will change to reflect this natural phenomenon. The big readout is labelled Primary wind at your height in metres, so you can always see which height it describes. For more details on how to set your working height, see How to set your working height.

The instrument uses wind data from weather models. These models provide wind speed and direction at specific, fixed heights above the ground, and the instrument also interpolates the direction. These are called native model levels. For this instrument, the native model levels are at 10, 80, 120, and 180 metres. If your chosen Height matches one of these levels, the instrument uses that direct model output. If your chosen Height is between two native model levels, the instrument calculates the wind speed for that height. This process is called interpolation. It uses a logarithmic height scale for this calculation. This ensures that the wind speed presented is relevant to your specific operational height, even if it is not a primary model level.

Log-height interpolation means the instrument estimates wind speed between two native model levels using the logarithm of height, not a straight-line average. The speed is weighted by where your height sits between the two levels on a log scale, which fits the way wind speed rises quickly near the ground and more slowly higher up. On a desktop screen, the Working height block next to the wind profile reads "Interpolated between native levels (log-height)" when this method is used. For example, a height of 45 metres is interpolated between the 10 m and 80 m levels, which suits a crane jib at 45 metres, and 100 metres is interpolated between 80 m and 120 m, which suits a drone. The agreement spine tick named Height fidelity holds when you are within 10 m of a model level and turns hollow when you are further away.

If you set a Height below 10 metres or above 180 metres, the instrument must estimate the wind beyond its native levels. It scales the nearest model level with a neutral log law and a roughness length of 0.03 metres. The Working height block then reads EXTRAPOLATED with a note that the height is outside 10 to 180 m and uses a log law with z0 of 0.03 m, and the Height fidelity tick on the agreement spine fails. The value is an estimate from a standard profile, since there is no model output at that height. Extrapolated values carry more uncertainty than interpolated or native ones.

While mean wind speed changes with height, the gust figure on the instrument is always a 10-metre quantity. It is labelled GUST 10 m and does not change when you adjust the Height. The models supply gust at 10 metres only, so the instrument does not invent a gust at your working height. For the same reason, a limit set on gust is compared only at exactly 10 m, and at any other height the comparison uses the mean speed at your height. The mean speed shown in the big readout is the one worked out for your height. For more on the main number, see How to read the hero number.

If the number changes sharply and you did not touch the Height box, check three things. First, the persona menu: choosing a persona overwrites the height, unit and limit with that preset's values. Second, the time: you may have moved the forecast hour on the time strip, and the wind changes hour by hour. Third, the place: choosing a new place reloads the forecast. If the readout shows a dash with UNKNOWN, data is missing for that hour or height, which does not mean zero wind. See Why does it say UNKNOWN instead of zero?. If you suspect a fault, use the /contact page.