Diurnal Variation

Diurnal (daily) variation of wind is caused by strong surface heating during the day, which causes turbulence in the lower levels.  The result of this turbulence is that the direction and speed of the wind at the higher levels (e.g., 3,000 feet) are transferred to the surface.  Since the wind direction at the higher level is parallel to the isobars and its speed is greater than the surface wind, this transfer causes the surface wind to veer and increase in speed. At night, there is no surface heating and therefore less turbulence and the surface wind tends to resume its normal direction and speed – it backs and decreases.