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pause level to the ground. Table 2 suggests that potential temperatures
of 295 to 305°K, when found within the stable layer underneath the jet
core, could carry such a rapid downward transport. (In the special case
of a warmand low stratosphere or a jet stream in relatively high latitudes, potential temperatures as low as 290°K may produce the same
effect.)
Potential temperatures lower than 295°K characterize cold tropospheric air masses, Air masses traveling at higher potential tempera-
tures (especially in the case of the subtropical jet stream) must
undergo diabatic cooling before they can reach the ground. Such diabatic
processes have a tendency to diffuse radioactive debris over a larger
area, thus decreasing the observed concentrations.
Cases of cyclogenesis associated with jet streams with potential
temperatures between 295 and 305°K in their stable layer should be
viewed as potential fallout-weather situations. The possible implications of dynamic instability in the development of such weather patterns have not yet been studied in detail. However, this is a promising
area for future research.
OUTLOOK FOR FUTURE RESEARCH
If jet maximums of moderate intensity are capable of transporting
air masses equivalent to the mass of the stratosphere poleward of 45°
latitude into the troposphere within one year, a rather powerful mechanism of return flow of tropospheric air into the stratosphere must
exist. Such a flow may in part be moist adiabatic and associated with
precipitation. Thus computations of air trajectories become difficult
since potential vorticity and specific humidity no longer will be conservative quantities,
The geographic distribution and magnitude of return flow will
drastically influence the contamination level of stratospheric layers
in which originally tropospheric air mixes with stratospheric air. The
understanding of these mixing processes, as well as of the exchange
mechanisms across the tropopause, would be enhanced greatly by detailed measurements of atmospheric properties at low stratospheric
levels. These measurements might be of such characteristics of tropospheric air as ozone and water-vapor concentrations and chemical and
radioactive tracers.
REFERENCES
1. D. O, Staley, Evaluation of Potential Vorticity Changes near the Tropopause
and the Related Vertical Motions, Vertical Advectionof Vorticity and Transport of Radioactive Debris from Stratosphere to Troposphere, J. Meteoror.,
17: 591-620 (1960).