The conventional method of upper-wind evaluation involves projection of the sounding balloon's trajectory on the surface of a horizontal plane, tangent to the earth at the observation point. This method which fails to allow for the curvature of the earth is shown to be seriously in error in situations where the balloon is tracked by new electronic equipment to large horizontal distances. This is particularly significant under jet-stream conditions. By projecting the balloon's path on an appropriate spherical surface corresponding arc distances are obtained. Thus the effect of the earth's curvature is taken into account and more accurate wind evaluations result.

Two methods of upper-level wind computations, making proper allowance for the earth's curvature, are given. The first method involves the determination of a correction (ΔE)to one of the observed parameters (elevation angle, E) as an intermediate step to calculation of the required arc distances. The second method gives the corrected arc distances directly.

Examples are shown illustrating the effect of neglecting the curvature of the earth in computing upper-level wind speeds; positive errors of as much as 20 percent being possible under the conventional method.

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* Paper read at Baltimore Meeting, April, 1953.