In the normal use of pilot-balloon reports as an aid to drawing upper-air charts, these reports are usually assumed to represent the mean air flow undisturbed by turbulence. This assumption needs to be justified if the pibals are used in the preparation of wind convergence charts.

The failure of the pibal to represent the mean undisturbed wind is due to (1) inaccuracy and (2) unrepresentativeness of the pibal observation. Most of the inaccuracy arises from incorrectly prescribed heights of the balloon based on a constant rate of ascent. In the cases studied, the average error (Table I) was 4.1° in direction and 3.3 miles/hour in speed. The unrepresentativeness due to turbulence averaged 1.3° and 2.5 miles/hour. The total inaccuracy and unrepresentativeness together averaged 5.6° and 4.1 miles/hour.

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