The wind direction and speed can be varied by macroscopic and local conditions. All the parameters must be measured and monitored so that not only is a mean value known, but its variances and statistical uncertain­ties are known also. The main characteristics are: prevailing direction that is largely a macroscopic effect dependent on the site proximity to large mountain ranges and the sea; persistence; wind shear or variation of speed and direction with height caused by the pressure gradient; local circulations due to the surface roughness in the site locality; and turbulence caused by a disparity of temperatures during the day and night.