Hodographs of Slowly Rotating Winds in Midlatitudes

Trend analysis of hourly wind direction angle recorded at nine meteorological stations across southern Canada (in the 43 degrees-53 degrees N belt) identified wind direction rotation periods in the range of 7-9 days. Rotation persists during the summertime season from May to mid-October during 1953-2001. Rotation with a 7.5-day period was also established in the 850-hPa geostrophic summertime wind over the Canadian province of Saskatchewan in 2000. Hodographs built from wind vectors corresponding to consecutive days of the 7.5-day period (summertime average vectors of winds binned into separate days of the period) formed nearly elliptical loops centered around the end point of the net westerly transport vector that ranged from 0.5 to 1.3 m s(-1) and was either smaller than or of comparable magnitude to the zonal and meridional oscillations of wind. The observed rotations appear quasi periodic rather than purely periodic, because nearly elliptical loops of relatively large amplitude were present at different hodographs constructed at each location for sampling periods of 6.5, 7.0, 7.5, and 8.0 days. For sampling periods outside the similar to 6.5-8.0-day range, the rotational magnitude in a hodograph appears diminished because of partial cancellation of wind vectors in corresponding hodograph bins. Quasi-periodical rotation occurs in the clockwise direction.