EN 1991-1-4 terrain categories
Wind pressure is not one number for a site. It depends on how rough the ground upwind is and how far above it you are.
Terrain categories and resulting pressure
| Category | z0 (m) | zmin (m) | kr | 5 m | 10 m | 20 m | 30 m | 50 m |
|---|---|---|---|---|---|---|---|---|
| 0 — Open sea or coastal, exposed | 0.003 | 1 | 0.1560 | 1,100 | 1,261 | 1,431 | 1,536 | 1,672 |
| I — Lakes, flat ground, no obstacles | 0.010 | 1 | 0.1698 | 1,000 | 1,170 | 1,351 | 1,463 | 1,609 |
| II — Low vegetation, isolated obstacles | 0.050 | 2 | 0.1900 | 815 | 994 | 1,187 | 1,307 | 1,465 |
| III — Villages, suburban, forest | 0.300 | 5 | 0.2154 | 541 | 722 | 922 | 1,048 | 1,215 |
| IV — Urban, >15 % built, buildings over 15 m | 1.000 | 10 | 0.2343 | 497 | 497 | 695 | 821 | 990 |
Peak velocity pressure qp(z) in N/m² for a basic wind velocity vb = 26 m/s on flat terrain (co = 1). Scale by (vb/26)² for another wind speed.
How pressure is built up
The basic velocity pressure is what the wind would do in clean air, with the recommended air density of 1.25 kg/m³:
The roughness factor then scales the mean wind for terrain and height, and below a terrain's minimum height the profile is frozen rather than extrapolated down:
Finally turbulence is added, because gusts break things, not means:
A check the standard runs on itself
The basic wind velocity is defined as the 10-minute mean at 10 m over terrain category II. So the roughness factor at exactly that point has to come out as 1, and it does: 0.19 · ln(10/0.05) = 1.007. That only works if the terrain table and the terrain factor expression are both right, which makes it a useful way to verify any implementation.
Height costs you twice
Raising an antenna reaches faster wind and lengthens the lever arm. Doubling the height of a given antenna typically more than triples its contribution to the base moment, which is how a mast that worked fine last year stops working after somebody adds half a metre of pole.
Your basic wind velocity vb is a local climate figure from your National Annex wind map. Related tool: Antenna mast wind load.