Why a forecast can read calm while your drone is fighting to hold position.
Surface wind observations and forecast fields are commonly referenced to 10 metres above open ground. Drone work can happen much higher. Wind often increases above the friction-slowed surface layer, but stability, terrain, buildings and weather systems can make the profile more complex.
A useful comparison keeps the model, location and valid time the same, then compares heights relevant to the mission. A 10 m field and a 120 m field can answer that question. A pressure-level reading hundreds of metres higher does not prove what the wind will be at 120 m.
Routine altitude limits also vary. Canada generally caps these flights at 122 m (400 ft) AGL, the EASA open category uses 120 m, and United States Part 107 uses 400 ft. Exceptions and local rules still apply.
In the surface layer, friction from terrain, buildings and trees often reduces wind near rough ground. That effect generally weakens with height, but local terrain and structures can also accelerate, redirect or disturb the flow. A real wind profile is not guaranteed to increase smoothly.
How steep that gradient is depends on the day. A well-mixed, gusty afternoon can be nearly uniform from the ground up. A calm, stable evening can be dead still at head height with a genuine breeze running a hundred metres above it. That second case is the one that catches people out, because conditions at the launch point feel perfect.
Available fixed heights depend on the weather model. The general Open-Meteo API lists wind at 10, 80, 120 and 180 m, while its GEM API lists native wind fields at 10, 40, 80 and 120 m. The provider and available levels can therefore change by location.
Pressure levels are surfaces of constant atmospheric pressure, not constant height. Flywhen reads each level's geopotential height, subtracts the model ground elevation, and calculates an estimated height above ground. Levels that resolve below ground are discarded rather than guessed at.
Where a model does not publish wind at the planned flight height, the app identifies the lower modeled height it uses. That reading is a proxy, not an equivalent measurement. It may understate or overstate the wind at the planned height.
A model's wind at height is still a model on a grid. For example, GEM HRDPS Continental is listed at 2.5 km resolution. Features smaller than a grid cell, such as a treeline, building wake or narrow valley, may not be resolved. Verify conditions at the site and keep control margins.