Analysis of 1 week's data in August 1960 shows significant diurnal variations in surface geostrophic wind over the south-central United States. The oscillation in the southerly component (Vg) is driven by the response of the thermal wind to the diurnal temperature cycle over sloping terrain. A smaller oscillation in Ug derives from spatial variations in the amplitude of the diurnal pressure wave. The amplitude of the oscillation in Vg is about 3 to 5 m sec–1 at the surface, decaying exponentially with height to near 0 at 2 km. Examination of 11 yr of summertime rawinsonde data at Fort Worth, Tex., shows a very regular diurnal variation in boundary layer wind with maximum amplitude of about 3 m sec–1 at 600 m above the ground. This oscillation is forced by periodic variations in both eddy viscosity and geostrophic wind. Using a simplified model of the boundary layer, we obtain solutions for the diurnally periodic wind resulting from “reasonable” variations in eddy viscosity and “observed” variatio... Abstract Analysis of 1 week's data in August 1960 shows significant diurnal variations in surface geostrophic wind over the south-central United States. The oscillation in the southerly component (Vg) is driven by the response of the thermal wind to the diurnal temperature cycle over sloping terrain. A smaller oscillation in Ug derives from spatial variations in the amplitude of the diurnal pressure wave. The amplitude of the oscillation in Vg is about 3 to 5 m sec–1 at the surface, decaying exponentially with height to near 0 at 2 km. Examination of 11 yr of summertime rawinsonde data at Fort Worth, Tex., shows a very regular diurnal variation in boundary layer wind with maximum amplitude of about 3 m sec–1 at 600 m above the ground. This oscillation is forced by periodic variations in both eddy viscosity and geostrophic wind. Using a simplified model of the boundary layer, we obtain solutions for the diurnally periodic wind resulting from “reasonable” variations in eddy viscosity and “observed” variatio...