Thereafter, it dropped to 2.19 by the 14th, 2.52 by the 15th, 2.77 by the 17th, 2.92 by the 19th and 3.45 by the 20th. This variation seems to support Hruska's study. Observing in darker skies than the Spanish group, Roggemans detected 1315 Perseids and gave the average magnitude of the shower as 3.10. However, there were two exceptions. This set of observations seems to support Ceplecha's study.
Filamentary structure seems the best explanation.
During some years, the filaments are encountered in rapid succession by Earth's passage through the Perseid stream, thus accounting for the consistent magnitude estimates followed by a steady decline. In other years, the filaments are spread out across the stream's width, thus causing the consistent average magnitude estimates to be disrupted by periods of activity from primarily brighter or fainter meteors. Another statistic that has been brought forward during the last 30 to 40 years has been the percentage of Perseids that exhibit persistent trains. This is a major factor long noted in the separation of Perseids from other active showers occurring during the first half of August. The variations could not be correlated to sunspot numbers.