Higher level wind speeds are traditionally predicted either on the basis of the widely used logarithmic equation or using the Hellmann equation. In both equations, a coefficient that assesses the average wind shear curvature is usually used. This coefficient has been considered to be a constant, depending on the roughness of the surrounding landscape. In this paper, on the basis of wind speeds, recorded at different heights, we show that this coefficient is not a constant but it varies periodically with a period close to one year.