Precipitation data from the Canadian prairies have been examined for the presence of persistent rainfall singularities. The meteor theory suggests that these might be caused by increases in concentration of efficient ice nuclei, originating from annual meteor shower occurrences during the preceding month. The present investigations have not tended to support this contention. There is no correlation apparent between the persistence, magnitude, or sharpness of a particular rainfall anomaly and the equivalent properties of the meteor influx presumed to cause it. Furthermore, although a number of peaks correspond, with 31 days delay time, to the mean dates of meteor showers, it is shown that, due to the nature of the rainfall pattern, other delay times can provide equally good apparent correlation between the two sets of events. This would indicate that the rainfall amounts are not truly influenced by periodic increases in the influx of meteoritic material. An alternative explanation for rainfall singularities is advanced which suggests that they are simply the result of day-to-day inhomogeneities in the long-term, average pressure pattern. Thus, low pressure areas would be, on the average, more prevalent or intense on certain days of the year than on others, and under such circumstances, an increase in mean precipitation could be expected. This proposed theory of persistent synoptic instability has been tested by a comparison of precipitation forecasts based on long-term mean pressure maps with actual precipitation amounts. While further critical tests are required, the results are promising, and there is indication that anomalies in mean temperature and cirrus cloud prevalence might be attributed to the same mechanism. The synoptic instability theory may thus provide a more satisfactory explanation of precipitation peaks, from the physical point of view at least, than does the meteor hypothesis.
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Nov 1, 1969
A H Auer + 2
Open Access