572
GATZ AND DINGLE
In the case of the June 25 rain, the flux of Gulf air into the system
over southern Michigan appeared to be very poor, that of September 1
was excellent throughout the lower half of the atmosphere, and that of
September 23 was much less vigorous. This last case was dependent
mainly on a very weak divergent flow from the anticyclone just west of
the Appalachian chain.
Rainfall Intensity The rainfall-intensity graphs were formed by detailed
reduction of the tipping-bucket rain-gauge record. The results are
shown in Figs. 3 to 5. Here the September 1 rain is distinguished from
the other two in the persistence of rainfall rates of 20 mm/hr and
above. The two initial showers, which averaged about 70 and 60 mm/hr,
respectively, lasted for about 10 min each. This feature is quite similar
to the
12-min showers of 60 to
70 mm/hr average intensity which
initiated both the September 23 rain and the June 25 rain. As a result
of the sustained moderate intensity of the September 1 rainfall, it
yielded a total of 1.86 in., whereas the yield of the September 23 rain
was 0.70 in. and that of the June 25 rain was 0.64 in. Maximum intensities observed were 95, 114, and 133 mm/hr, respectively, showing an
inverse correlation of this characteristic with the total yield. Both
cold frontal rains exhibited suppressed showery activity toward the
end of the rainfall, that on June 25 reaching nearly 50 mm/hr and that
on September 23 nearly 20 mm/hr. Lightning and thunder were associated with all the showers, but data on the duration and intensity of the
electrical activity are lacking.
Interpretation of the rain~intensity graphs and data on rates of
motion of the respective weather systems suggeSts thatthe basic major
Shower units have a dimension of the order of 4 to 5 nautical miles
along the direction of motion. Those of the June 25 system appear to be
smaller. The fact that the initial shower in this case developed very
rapidly just prior to the onset of rain at the station suggests that ata
more mature state of development the basic elements would have
joined and produced a structure more nearly comparable to the other
systems,
Finer structure, of the order of 1 min or less in duration and
0.5 nautical mile or less in extent, is indicative of the the shaft-like
character of heavy rain which is seen in radar range—height—indicator
(RHI) presentations. The relation of these two scales of rain structure
to the cleansing action of the rain and to the entrainment of contaminated air is discussed.
Contaminant Concentrations
Graphs of the concentrations of pollen and of
radioactive materials are given in the upper part of Figs. 3 to 5. The
periods during which the samples of rainwater were collected are
shown in the middle of the figures. The samples whose number designa-
tions are modified by “P” were analyzed for their pollen content only.