Wind usually gets stronger with height, because the ground slows the air through friction and that drag fades as you climb. Over open land in breezy conditions, the textbook profile puts the wind at 120 m (400 ft) about 1.4 times the 10 m value. On calm, clear nights, or over towns and forest, the ratio can reach 1.6–2.1 or more.
The boundary layer
The lowest part of the atmosphere, the boundary layer, feels the ground. The US National Weather Service describes its daily cycle:
- By day, sunshine heats the ground and rising air mixes the lowest few thousand feet. Wind is fairly even with height above the lowest layers, but gusty.
- At night, the ground cools and a shallow, stable layer forms. It is cut off from the air above, so winds can be calm at the surface and strong just above, with the change happening over 100 ft (30 m) or less.
Weather forecasts and airport reports quote wind at 10 m, the standard height for wind sensors. Drones fly in the layer where the wind changes fastest.
Friction and roughness
Rougher ground slows the air more and makes the wind increase more steeply with height. Meteorologists describe this with a roughness length, z0. The WMO lists typical values:
| Surface | z0 | Wind at 120 m ÷ wind at 10 m |
|---|---|---|
| Open sea | 0.0002 m | about 1.23 |
| Snow, mud flats | 0.005 m | about 1.33 |
| Open flat grass, few obstacles | 0.03 m | about 1.43 |
| Low crops | 0.10 m | about 1.54 |
| High crops, scattered obstacles | 0.25 m | about 1.67 |
| Parkland, bushes | 0.5 m | about 1.83 |
| Suburbs, forest | 1.0 m | roughly 2, but see below |
The ratios come from the logarithmic wind profile, u(z) = (u*/0.4) · ln(z/z0), for a well-mixed, breezy atmosphere. Over suburbs and forest, 10 m is close to rooftops and treetops, where the flow is patchy and turbulent and the simple formula is only a rough guide.
The power law
A simpler rule uses a power law: v(h) = v10 · (h/10)^α. Altiwind uses α = 0.14 over open land and 0.20–0.30 at night or over towns and forest.
Here is a 10 m wind of 5 m/s (18 km/h, 11 mph, 10 kt) carried up to drone heights:
| Height | α = 0.14 (open land, breezy day) | α = 0.20 | α = 0.30 (stable night, rough ground) |
|---|---|---|---|
| 10 m (33 ft) | 5.0 m/s | 5.0 m/s | 5.0 m/s |
| 30 m (100 ft) | 5.8 m/s | 6.2 m/s | 7.0 m/s |
| 60 m (200 ft) | 6.4 m/s | 7.2 m/s | 8.6 m/s |
| 90 m (300 ft) | 6.8 m/s | 7.8 m/s | 9.7 m/s |
| 120 m (400 ft) | 7.1 m/s | 8.2 m/s | 10.5 m/s |
Look at the last column. A gentle 5 m/s forecast at 10 m becomes 10.5 m/s (38 km/h, 24 mph) at 120 m. That is almost exactly the 10.7 m/s rating of many sub-250 g drones. See drone wind limits by class.
Day versus night
- Sunny afternoons: strong mixing spreads the wind fairly evenly through the boundary layer. The increase with height is modest, but gusts are larger. See gusts vs sustained wind.
- Clear, calm nights and early mornings: the stable layer near the ground decouples from the air above. Expect the steepest increase with height, and possible wind shear where the calm layer meets the faster air.
- Overcast, breezy days: closest to the textbook log profile.
This is why the calmest-looking hours on the ground, around dawn, can hide the biggest surprise for a drone at 120 m.
Terrain and buildings
- Hills and ridges: wind speeds up as it flows over a crest, and the downwind side can be turbulent.
- Valleys: wind is often channelled along the valley, and cold air can pool at the bottom on calm nights.
- Coasts: the sea is smooth, so wind off the water is stronger near the surface than wind that has crossed land.
- Tree lines: below the treetops it can feel sheltered; climb above them and the wind picks up quickly.
- Towns: buildings slow the average wind but add gusts, eddies and funnelling between buildings.
Models smooth all of this out over their grid boxes, so treat local terrain as a reason to expect more gustiness than the forecast shows.
How Altiwind estimates wind at your height
Weather models give wind at 10, 80, 120 and 180 m above ground. Altiwind interpolates between those levels on a log scale for 30, 60, 90 and 120 m (100, 200, 300 and 400 ft). For example, with 5 m/s at 10 m and 7 m/s at 80 m, the log interpolation gives about 6.1 m/s at 30 m and 6.7 m/s at 60 m.
When a model only has 10 m wind, Altiwind falls back to the power law above. The result feeds the sustained-wind check and the estimated gusts. See wind at altitude, or try the numbers yourself with the wind at altitude tool.
Sources
- WMO, Guide to Instruments and Methods of Observation (WMO-No. 8), Volume I, Chapter 5 and its annex (10 m exposure, log profile, roughness lengths after Davenport and Wieringa)
- NOAA/NWS, The Front, February 2014 (daily boundary-layer cycle, night-time decoupling)
- Open-Meteo, Weather forecast API documentation (wind at 10, 80, 120 and 180 m)
FAQ
How much stronger is the wind at 120 m than at ground level?
It depends on the surface and the time of day. The textbook profile for open land gives about 1.4 times the 10 m wind; over rougher ground or on calm nights it can be 1.6–2.1 times or more.
Why is it calm on the ground but windy at height at dawn?
Overnight cooling forms a stable layer that cuts the surface off from the faster air above. The change can happen within about 30 m, right where drones fly.
Does Altiwind measure the wind at my height?
No. It uses model forecasts at several heights and interpolates between them. The verified badge checks the model's 10 m wind against the nearest airport.
What is roughness length?
A measure of how much the surface slows the wind, in metres. It is about 0.0002 m over open sea, 0.03 m over open grassland and 1 m over suburbs and forest.