Periodic mass transfer by baroclinic disturbances and periodic debris concentration at the tropopause, such as might reasonably be associated with annual variation of tropopause height, are introduced into a simple expression for debris transfer from stratosphere to troposphere. The resulting expression shows an annual oscillation of transport which, for plausible choices of parameters, includes e. spring maximum and an autumn minimum. The expression shows also a semiannual oscillation with a phase angle consistent with certain observations of surface debris concentrations. This mechanism of downward transport by baroclinic disturbances was directly tested by an atmospheric radioactivity sampling program. Beta activities were measured at several levels in the troposphere on 13 occasions, mostly when dry baroclinic zones with air of recent stratosphere origin were anticipated. For several reasons, including forecasting mishaps, dry baroclinic inversions did not always occur at the time of sampling, and sampling was not always done at levels below, within, and above the inversion. Of the total of 13 cases, a maximum of activity occurred within or in the vicinity of a dry stable layer on 6 occasions, in support of the proposed mechanism of downward transport by baroclinic disturbances. In another case a maximum occurred in air with measurable moisture. In 6 cases no maximum of activity was found, but on only two or three of these days did the sampling levels bracket a dry stable layer in such a way that a maximum of activity related to transport by baroclinic disturbances would be expected. In order to test further the mechanism of transport, isentropic trajectories were traced backward 24 to 48 hours on five sampling days when wind directions were such that the air had reasonably long trajectories over land. On two of these days when no maximum of activity occurred, trajectories for sampled levels remained in the troposphere. Maxima occurred on the three remaining days. On two of these latter days, trajectories at the levels of high activity were traced back to the lower stratosphere, while air at other levels traced back to the troposphere. On the remaining day, all trajectories traced back to the troposphere. The preponderance of observational evidence provided by radioactive, thermal, and moisture tracers shows that downward mass and radioactivity transport by individual baroclinic disturbances is the most important transport mechanism and one which allows prediction of specific regions of mass and radioactivity flow into the troposphere.