In order to estimate the effect of horizontal advection on the change in direction of wind vectors with time, wind data from 71 observational stations in three areas: Oita City, California and Washington States, where the horizontal scale differs considerably from each other, are analyzed following the equation for the local rate of change of wind direction. The equation of rotation rate of wind vectors derived by Neumann (1977) is modified to a suitable form to discuss the direction of rotation of wind hodographs, because the shape of the wind hodograph is generally complicated and the origin of coordinate is outside the wind hodograph in most cases. Then, a daily weighted mean of the square of deviation from the daily mean wind vector is selected as a typical quantity of the direction of rotation of the wind hodograph. We can illustrate at once with this quantity the shape and the direction of rotation of wind hodographs. Furthermore, the daily mean of the rotation rate of wind vectors could be written in terms of the Coriolis parameter and the vertical vorticity which results from the daily mean wind field and the other terms.
As the result of data analysis for three areas, it is shown that the effect of horizontal advection in the modified equation of rotation rate of wind vectors is comparable to that of the Coriolis force, but those places where this condition is satisfied are limited to the local area.