The speed of wind, i.e. the flow velocity of air, is naturally changing rapidly over time and space. Even if assumed to be constant over time, its value is highly depending on where you measure it. You will measure a higher wind speed higher up in the air than close to the ground or behind obstracles. To be able to deal with this fluctuating quantity of wind speed in calculations, it is standardized in a profile where its amplitude is depending on the height above ground. This profile is often referred to as the atmospheric boundary layer. To define this profile, the following quantities need to be given:
- A reference velocity Uref, which is simply the wind speed at some specific height above the ground.
- The reference height Zref, at which Uref is measured.Usually, in international standards as well in weather forecasts, Zref is 10m (32ft) above the ground.
- The roughness of the landscape by choosing one of a few landcape types.
