How the estimate works
Weather forecasts give the wind at 10 m above the ground, the height of standard wind instruments. Friction with the ground slows the air near the surface, and the effect fades as you climb, so the wind grows with height. Meteorologists estimate it with a power law:
wind at height h = wind at 10 m × (h / 10)^p
The exponent p depends on how rough the ground is and how stable the air is. The calculator uses the same values as our guide to wind at altitude: the ones the US EPA uses in its dispersion models for rural and urban ground in neutral air, and the value measured over the ocean.
| Terrain and time | Exponent p | Wind at 120 m vs 10 m |
|---|---|---|
| Open sea or a large lake | 0.11 | × 1.3 |
| Open flat country, daytime | 0.15 | × 1.45 |
| Suburbs, towns, forest | 0.25 | × 1.9 |
| Clear, calm night over open land | 0.35–0.55 | × 2.4–3.9 |
For the calm-night case the calculator shows a range, from the lower to the higher exponent, and judges your drone against the higher value.
Worked example
The forecast says 15 km/h at 10 m and you are flying over open country in the afternoon (p = 0.15). At 120 m: 15 × (120 / 10)^0.15 = 15 × 1.45 ≈ 22 km/h. Over a town the same forecast gives about 28 km/h, and on a calm, clear evening anywhere from 36 to 59 km/h. A DJI Mini 4 Pro (38 km/h limit) is green in both daytime cases; on that evening the upper end of the range is well beyond its limit.
When the estimate is wrong
- Calm nights and early mornings: the ground wind can drop to nothing while the wind a few hundred metres up keeps blowing (the nocturnal low-level jet). The calculator's night range is wide for that reason.
- Hills, cliffs and buildings: wind speeds up over ridges and between buildings and becomes turbulent behind them.
- Coasts: the sea breeze and the smooth water change the profile in the afternoon.
A model forecast at several heights beats any rule of thumb. Drone Weather Forecast Pro shows the forecast wind at 80, 120 and 180 m from the weather model itself, so you do not have to estimate it.
Frequently asked questions
How much stronger is the wind at 120 m?
Usually 1.3 to 1.9 times the 10 m forecast: about 1.3 over water, 1.45 over open country and 1.9 over towns and forests. On clear, calm nights it can be 2.4 to 3.9 times or more.
Why is the forecast given at 10 m?
Because 10 m above open ground is the international standard height for wind instruments, so forecasts and airport reports use it too.
Is the power law accurate?
It is a rule of thumb that works best in the lowest 50–100 m over flat ground in well-mixed air. Use it to know when to be careful, not as a forecast.