ON THE COMPUTATION OF WIND FROM PRESSURE DATA

Abstract

From a statistical analysis of geostrophic- and gradient-wind computations for all stations reporting winds on two 700-mb charts, it is shown that there is a variability of about 25 per cent in the computations by different individuals, and that the computed values differed by about 35 per cent from the observed wind speed on the average. The percentual deviation was smaller for strong winds. The gradient wind computed. using approximate trajectory curvature was only slightly better than the geostrophic, and using contour curvature it was worse. The geostrophic wind thus appears to be the best approximation which can be computed from the pressure field alone. Theoretical expressions for the deviations of the computed from the observed speeds and velocities are derived. These show that the gradient speed is always greater than the true speed. The gradient speed is of shown to be a better approximation than the geostrophic for most cases, but for some cases of curved cross-isobaric flow the geostrophic speed is closer to the true speed. The vector deviation of the gradient wind is, of course, larger than the scalar for cross-isobaric flow.