CONVECTIVE STORMS AND SCAVENGING
525
self relative to air in response to certain dynamical, as well as radiative, controls. It is also known that a break in the tropopause occurs
near the polar-front jet stream and that with large amplitude of the
thermal pattern of the troposphere the geographic position of this
break has larger amplitude than do the streamlines, and winds there
are stronger than the horizontal displacement of the pattern, producing
some systematic horizontal movement of below-tropopause air to
above the tropopause and conversely. All these differential and relative movements present some added difficulties in establishing the
pattern of composition which enters into and which is modified by the
storm circulation. An answer to this is to acquire more samples of
atmospheric radioactivity in the air affecting the storm. We obtained
samplings by aircraft this season and would like to see such sampling
expanded.
It is pertinent to provide a background for the work at the University of Oklahoma before proceeding to describe the facilities for performing this research and to discuss the results obtained so far.
Thunderstorms occur everywhere in the contiguous United States,
though very infrequently along the west coast. They are more common
in the southern than in the northern states. Annual maximum fre-
quencies occur in Florida and in the southern Rocky Mountains, but the
‘large central area of the United States experiences an annual average
of 40 or more thunderstorm days. The severe thunderstorms which
are attended by hail or tornadoes and have great vertical depth are
most common in the Central Plains area, as suggested by Fig. 1, a
Weather Bureau Chart which shows the 40-year total of tornadoes observed in two-degree squares of latitude and longitude. “Tornado
Alley” extends from north-central Texas through central Oklahoma,
eastern Kansas, and into Iowa, The peak of activity in Oklahoma is in
the spring season. Norman and Oklahoma City are located in central
Oklahoma, It is evident why the public in that area is conscious of
severe thunderstorms
and why university meteorologists there are
interested in the whole thunderstorm mechanism. There is good rea-
son for the U. S, Weather Bureau locating its National Severe Storms
Laboratory
(NSSL) in Norman to carry on research in all phases of
these storms, The area of stippled overlay in Oklahoma in Fig. 1 de-
notes an attempt to depict the Beta Surface Network of observation
stations maintained by NSSL. It should be corrected by removing approximately the northern half.
The spring season in Oklahoma includes both the nonconvective
rainfall systems, which are of entirely tropospheric character and
attend the usual extratropical cyclones, and the thunderstorm rain sys-
tems, most of which are tropospheric but the most violent of which
penetrate through the tropopause, The rainfall maximum in May and
early June reflects the peak of convective activity. Consequently, in