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An attempt is made to determine whether or not the effectiveness of cloud seeding in increasing the amount of natural rainfall during the years 1951 and 1952 over western New Mexico and southeastern Arizona can be established. Only the months of July and August are considered since these are the months during which the rainfall is greatest and is mostly from non-frontal convective thunderstorms or showers. The method used consists of estimating the amount of natural rainfall that might be expected from the mixing ratios at the 700-mb and 500-mb levels and the temperature difference between these levels, the difference between the estimate of the natural rainfall and the actual rainfall containing the part that might be attributed to the effectiveness of cloud seeding. It is found that the effectiveness of cloud seeding cannot be established by this method on the basis of the data accumulated to date at an acceptable level of significance (1% or at most 5%). An interesting dilemma develops in that if effectiveness is acceptable at the 9% level for only those days on which seeding occurred, it is not possible to escape the conclusion that since seeding did not occur on all days, the suspension of operations on these few days reduces the effectiveness of the whole operation to nearly zero. It seems evident from the data analyzed that if there is additional rainfall due to cloud seeding it can, at most, not exceed an amount of about 10%. Statistical methods to establish so small an amount must be very refined.

An Evaluation of the Results of Cloud Seeding in Western New Mexico and Southeastern Arizona During July and August, 1951 and 1952