Wind speed at height calculator
Wind forecasts are for 10 m above the ground. Estimate how much stronger the wind is at the height of a crane jib, an elevating work platform or the top of a structure.
36 km/h
10.1 m/s · 19.7 knots · 1.21 times the 10 m wind
Why wind is stronger higher up
Near the ground, friction from everything the wind passes over (scrub, buildings, stockpiles, pit walls) slows it down. Higher up there is less in the way, so the wind is stronger. How quickly it picks up depends on the ground: over open country the wind is already fairly strong near the ground, while over a town it starts weaker and gains more with height.
The formula
The calculator uses the power law, a standard way to estimate wind at one height from wind at another:
wind at height h = wind at 10 m × (h ÷ 10)α
The exponent α depends on the ground. The calculator uses typical values of 0.14 for open ground, which suits most mine sites, 0.22 for scattered buildings or trees, and 0.30 for suburbs and town centres.
| Height | Open (0.14) | Scattered (0.22) | Built-up (0.30) |
|---|---|---|---|
| 20 m | 1.10 | 1.16 | 1.23 |
| 30 m | 1.17 | 1.27 | 1.39 |
| 40 m | 1.21 | 1.36 | 1.52 |
| 50 m | 1.25 | 1.42 | 1.62 |
| 60 m | 1.29 | 1.48 | 1.71 |
| 80 m | 1.34 | 1.58 | 1.87 |
| 100 m | 1.38 | 1.66 | 2.00 |
Gusts, not averages
Forecasts give a sustained wind speed and a gust speed, the strongest few seconds. Lifting and work-at-height limits are usually about gusts, so plan with the gust forecast. Gusts gain a little less with height than the average wind does, so scaling a 10 m gust with this formula tends to overstate it slightly, which errs on the safe side.
On clear, calm nights the difference with height can be far bigger than the formula suggests. The air near the ground goes still while the wind some way above keeps blowing, so a calm pad at dawn does not mean a calm jib.
Limits for cranes and work platforms
- Elevating work platforms: many models rated for outdoor use are designed for wind up to 12.5 m/s (45 km/h), and some are rated for indoor use only. The data plate and the operator’s manual give the limit for your machine.
- Cranes: the limit comes from the manufacturer’s manual and load chart and depends on the lift. Loads with a large surface, such as panels, formwork and roof sheets, catch the wind and can need a much lower limit. Crane wind limits often refer to the wind at the top of the boom or jib, which is why the wind at height matters.
- On the day: use an anemometer at height for the lift itself, and the forecast to choose the hours to schedule it in.
Plan lifts around the gusts
Lumen Sites forecasts gusts for every hour of the next 48 hours at each site, against a caution and a stop limit you set to match your crane, platform or scaffolding limits, so lifts can be booked into the calmer hours.
Units
1 m/s = 3.6 km/h, and 1 knot = 1.852 km/h. So 45 km/h is 12.5 m/s or about 24 knots.
Related: WBGT heat stress calculator · Lightning distance calculator · Gust forecasts for cranes and work at height